The astronomy glossary aims to help you understand some of the terms used in this website. It is by no means a comprehensive astronomy dictionary.
Aphelion is the furthest point that a body gets to the Sun in its orbit. The word comes from the Latin prefix ‘ap’ meaning away from and Helios which is the Greek god and personification of the Sun.
The image shows the exaggerated elliptical orbit of the Earth around the Sun. All planetary orbits around the Sun are elliptical. Ellipses are oval shaped – like a stretched out circle and the Sun is not at the centre of the ellipse, as it would be if the orbit were circular. This is why the planet is not always the same distance from the Sun. Instead, the Sun is at one of two points called "foci" (which is the plural form of "focus") that are offset from the centre. This means that each planet moves closer towards and further away from the Sun during the course of each orbit. Not all planets have the same shaped oval orbit; some are more elliptical than others. The Earth’s orbit is almost circular but not quite! See Elliptical Orbit for more information.
At the moment the Earth reaches aphelion in early July approximately 14 days after the summer solstice. This means that in the northern hemisphere the Earth is furthest from the Sun in summer while in the southern hemisphere this corresponds to winter. At aphelion the centre of the Earth is about 1.02 astronomical units or 152,097,700 km (94,509,100 miles) from the centre of the Sun. The date of aphelion changes over time due to precession and other orbital factors which follow cyclical patterns. Because aphelion falls in the summer months in the northern hemisphere and the winter months in the southern hemisphere, the summer is slightly longer (93 days) in the northern than in the southern hemisphere (89 days). See Elliptical Orbit for more information.
When looking up at the night sky, stars are often grouped into recognised patterns. These recognised patterns may be a group of the brighter stars within a larger constellation or may indeed form a pattern which includes more than one constellation. As a result, asterisms are visually obvious collections of stars and the lines used to mentally connect them. Unlike constellations which are officially recognised areas of the sky, asterisms do not have officially determined boundaries and are therefore a more general concept which may refer to any identified pattern of stars.
A popularly recognised asterism is the Plough (Saucepan or Big Dipper) within the International Astronomical Union's officially recognised constellation Ursa Major. Other popular asterisms include the Summer Triangle and the Winter Triangle. The Summer Triangle is made from the brightest stars in the constellations of Lyra, Cygnus and Aquila: the stars are Vega, Deneb and Altair. The Winter Triangle joins the stars Betelgeuse in Orion, Procyon in Canis Minor and Sirius in Canis Major.
Asteroid 3200 Phaethon was discovered in October 1983. This unusual Near Earth Asteroid (NEA) may be an extinct comet. It measures 5.1 Km in diameter and its orbit crosses the orbits of Mars, Earth, Venus and Mercury. It was the first asteroid to be discovered by a spacecraft. Its orbit is tilted at an angle of 22.2 degrees to the ECLIPTIC. It also has a highly elongated orbit; with an ECCENTRICITY of 0.88, which is more like a comet than an asteroid.
Phaethon's (pronounced FAY-a-thon) most remarkable distinction is that it approaches the Sun closer than any other numbered asteroid and comes even closer to the Sun than Mercury; it gets within 20.9 million km [13 million miles]. The surface temperature at its closest (perihelion) could reach approximately 1025 Kelvin. This is why it was named after the Greek myth of Phaëton, son of the Sun god Helios. As Phaethon approaches the Sun the dust is literally cooked off its surface. It then makes its way back to far beyond the orbit of Mars some 223 million miles from the Sun (359 million Km) in just 262 days. This far away from the Sun Phaethon cools down to very low temperatures. This constant periodical cooling and heating cycle cracks its mineralogical surface into small dusty particles. Each December, when Earth passes close to the orbit of Phaethon, the small grains swept from Phaethon by the radiation pressure (of sunlight) enter our atmosphere as the Geminids METEOR SHOWER.
The Geminids meteor shower is observed between the 4th-17th December (maximum is usually December 14th). Along with the Quadrantids, observed between 1st-6th January, they are the only meteor showers originating from an asteroid rather than a comet. It will approach relatively close to the Earth on December 14, 2093, passing within 0.0198 AU (Astronomical Units).
3200 Phaethon is a rocky object with a strange blue hue (artist impression).
Astronomical twilight is defined to begin in the morning, and to end in the evening when the centre of the Sun is geometrically 18 degrees below the horizon. From the end of astronomical twilight in the evening to the beginning of astronomical twilight in the morning, the sky (away from urban light pollution) is dark enough for all astronomical observations. Most casual observers would consider the entire sky fully dark even when astronomical twilight is just beginning in the evening or just ending in the morning, and astronomers can easily make observations of point sources such as stars, but faint diffuse items such as nebulae and galaxies can be properly observed only beyond the limit of astronomical twilight. In some places, especially those with sky glow, astronomical twilight may be almost indistinguishable from night.
It takes 29.53 days for one lunation i.e. the time taken to go from, for example, New Moon back to New Moon. There are 365.24 days in a year so there are usually 12.37 lunations in one year and therefore usually 3 full Moons every season. However, because there are 12.37 lunations every year it means there are roughly 11 days more than the number of days in 12 lunar cycles. The extra days accumulate so every 2-3 years there is an 'extra' 13th full moon. This extra full moon means that one of the seasons ends up having 4 full moons and not 3. Originally the 3rd full moon in the season with 4 full moons was called a Blue Moon.
Modern Day Definition
In 1946 an article was printed which misinterpreted this traditional definition; it stated that if there is a second full moon in one month then this second full moon is called a Blue Moon. This misinterpretation is far simpler than the traditional definition and has now been widely adopted to describe a Blue Moon.
The Moon of course is not blue. This would be a rare event indeed and only occurs in certain atmospheric conditions; e.g., when there are volcanic eruptions or when exceptionally large fires leave particles in the atmosphere. The image you see here has been taken using a blue filter.
The stars that you see above your head on a clear night form part of the celestial sphere. The whole sphere can be thought of as surrounding the globe. It is really a practical tool allowing astronomers to specify apparent positions of objects in the night sky using co-ordinates.
All objects on the celestial sphere appear to be at equal distances, because casual observation cannot give us information about their actual distance from Earth. However, the sphere does not have a fixed radius and individual stars and other objects are certainly not at the same distance from Earth. Some objects are just a few LIGHT YEARS away while others are thousands of LIGHT YEARS away. In fact the celestial sphere can be considered to be infinite in radius.
As Earth orbits from west to east the celestial sphere appears to rotate east to west. For example on a daily basis the Moon rises in the east and sets in the west.
The celestial sphere has imaginary poles; the north and south celestial poles. Here the Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The north and south celestial poles appear permanently directly overhead to observers at the Earth's North Pole and South Pole, respectively. As the Earth spins on its axis, the two celestial poles remain fixed in the sky, and all other points appear to rotate around them, completing one circuit per day (strictly, per SIDEREAL DAY).
The celestial sphere also has an imaginary equator; a great circle around the celestial sphere which is on the same plane as the equator of Earth (the equator divides the Earth into the northern and the southern hemispheres). In other words, the celestial equator is an abstract projection of the terrestrial equator onto outer space and can be used to divide the celestial sphere into the northern hemisphere and the southern hemisphere. Due to Earth's axial tilt, the celestial equator is currently inclined by about 23.44° with respect to the ecliptic (the plane of Earth's orbit). The inclination has varied from about 22.0° to 24.5° over the past 5 million years.
The ecliptic is the apparent path that the Sun follows across the sky as seen from Earth (see the diagram). It is coplanar with Earth's orbit around the Sun (and hence the Sun's apparent path around Earth - but of course the Sun does not orbit Earth, the Earth orbits the Sun). Look at the diagram, if the Earth were not tilted then the celestial equator and the ecliptic would be the same.
Comets are cosmic snowballs or snowy dirt balls. They are mostly made up of ices mixed with smaller amounts of dust and rock. Most comets are no larger than a few kilometres across. The main body of the comet is called the nucleus, and it can contain water, methane, nitrogen and other ices. Comets orbit the Sun and when they come closer to it they begin to heat up and release gases in a process called outgassing. This produces a visible atmosphere around the nucleus of the comet called a coma. This can end up being 15 times the diameter of the Earth! The solar wind blows the gas and dust away from the comet creating a pair of tails that can be millions of miles long. We usually see the dust tail when we look at comets from Earth. Recent evidence has shown there is a third tail.
Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star.
The Material that streams away from the comet is left in its orbital path and if Earth passes through this debris trail then we see a meteor shower or shooting stars.
In the past, comets were named for their discoverers, such as Comet Halley for Sir Edmond Halley. In modern times, comet names are governed by rules set forth by the International Astronomical Union (IAU). A comet is given an official designation, and can also be identified by the last names of up to three independent discoverers.
Here’s how it works. Once a comet has been confirmed, the following naming rules are followed. First, if the comet is a periodic comet, then it is indicated with a P/ followed by the year of its discovery, a letter indicating the half-month in which it was discovered, followed by a number indicating its order of discovery. So, for example, the second periodic comet found in the first half of January, 2015 would be called P/2015 A2.
A non-periodic comet would be indicated with a C/ followed by the year of its discovery, a letter indicating the half-month in which it was discovered, followed by a number indicating its order of discovery.
The periodic Comet Halley (1P/Halley) is the most famous in history. It returns to the inner solar system once every 76 years and gives rise to the Orionids meteor shower. Other well-known periodic comets include 2P/Encke, which appears every 3.3 years and gives rise to the Taurids meteor shower and 9P/Tempel (Tempel 2), which was visited by the Deep Impact and Stardust probes, and comes closest to the Sun every 5.5 years. Other comets associated with well known meteor showers includes: Tempel-Tuttle which is associated with the Leonids; Swift-Tuttle which is associated with the Perseids.
Conjunction is when two or more objects appear to meet each other from our line of site on Earth. Technically speaking they have the same right ascension (that is an astronomical coordinate similar to longitude) on the celestial sphere. Practically speaking, objects in conjunction will likely be visible near each other for some days.
The word conjunction comes from Latin, meaning to join together.
In the example shown, conjunction is the position of the outer planet (in orange) in relation to the Sun and the Earth when the outer planet is at the opposite side of the Sun to the Earth. At this point in the orbits of the 2 bodies the outer planet is the furthest it will be from the Earth and appear to be in conjunction (close to) the Sun. However because of the elliptical nature of planetary orbits the actual distance between the Earth and the outer planet will be different at each conjunction. In this type of conjunction which is known as superior conjunction (sometimes referred to as solar conjunction) the planet will disappear behind the Sun and cannot be seen. The diagram is simplified to show opposition and conjunction. The ELLIPTICAL ORBITS are very exagerated.
The planets Mercury and Venus have 2 conjunctions with the Sun: one at superior conjunction when it disappears behind the Sun and another when it passes in between the Earth and the Sun. This is called inferior conjunction and is again lost in the glare of the Sun (see ELONGATION for a diagram showing superior and inferior conjunction of an inner planet).
The most common type of conjunction, though, doesn’t involve the sun. Any time two objects pass each other on the sky’s dome, they’re said to be at conjunction. These sorts of conjunctions happen multiple times every month and maybe between two planets, or a planet and a star, or a planet or star and the moon.
Conjunctions involve either two objects in the Solar System or one object in the Solar System and a more distant object, such as a star. A conjunction is an apparent phenomenon caused by the observer's perspective: the two objects involved are not actually close to one another in space. Conjunctions between two bright objects close to the ECLIPTIC, such as two bright planets, can be seen with the naked eye.
A constellation is a group of stars that forms an imaginary outline or meaningful pattern on the CELESTIAL SPHERE. Imagine the night sky as a huge dot to dot puzzle. If you joined some of the stars by imaginary lines you would make patterns.
In ancient times poets, farmers and astronomers began to pick out patterns in the stars and named them after heroes and fabled animals. In Ptolemy's time 48 constellations were recognised by the Eastern Mediteraneans and many mythological stories were associated with them. Nowadays because we have included the whole of the celestial sphere and not just the ones that Ptolemy could see, the tally has gone up to 88.
The origins of some constellations extend back to prehistory, while others have changed. While the patterns of old seem quite specific to the brightest stars, the constellations of today make up a block of sky encompassing not only the recognised pattern of stars but all the stars within that block which are not necessarily visible to the unaided eye. Each block forms a border with another block much like the county boundaries in the UK. These specific boundaries were put in place by the International Astronomical Union in 1922 and the 88 constellations have remained since then.
We still recognise the patterns that the brighter stars make within the whole of the constellation but assigning a region of sky to one constellation has helped us to locate specific deep sky objects. The yellow dashed line marks the constellation boundaries.
A double star is a pair of stars that appear close to each other as viewed from Earth, especially with the aid of an optical telescope.
There are three different types of double stars:
Optical doubles are unrelated stars that appear close together through chance alignment with Earth. An example of an optical double are the two stars Mizar and Alcor in the constellation Ursa Major. Look at the tail of the bear, Mizar and Alcor are the second stars in from the tip of the tail (see constellation).
Visual binaries are in mutual orbit and gravitationally bound to each other. They are seen as one star with the unaided eye but can be resolved into two with a telescope.
Non-visual binaries cannot be resolved through a telescope but they can be split into their respective components by spectroscopy (spectroscopic binaries), or more esoteric means, such as occultation (eclipsing binaries), or anomalies in proper motion (astrometric binaries).
Multiple star systems also exist but these are more complex than binary stars.
Improvements in telescopes can shift previously non-visual binaries into visual binaries, as happened with Polaris A in 2006. It is only the inability to telescopically observe two separate stars that distinguish non-visual and visual binaries.
Mizar and Alcor optical double in Ursa Major
Algieba binary star in Leo
An eclipse occurs when one celestial object moves into the shadow of another celestial object along the line of sight of an observer. The observer sees either a total eclipse where the first body is completely obscured or a partial eclipse when only a portion of the first body is obscured. Examples of eclipses include a Lunar eclipse and a Solar eclipse. Satellites of planets can also eclipse each other as they orbit around their parent planet.
Johannes Kepler was a German astronomer, mathematician and astrologer. He is best known for his three laws of planetary motion. Kepler’s first law states that each planet’s orbit around the Sun is an ellipse. An ellipse is a squashed or elongated circle which has two focus points or foci (see diagram 1). The Sun is at one focus point and the planet follows the ellipse as it orbits the Sun. This means that the distance from the Sun to the planet is constantly changing. There isn’t anything sitting at the other focus point; it is empty.
Not all orbits are ‘squashed’ to the same extent; some are more elliptical than others. This is called the eccentricity of the orbit. The more oval the orbit then the higher the eccentricity. A circle has an eccentricity of 0 and the more ‘squashed’ the circle the greater the value is up to 1. Most planets in the Solar System have very small eccentricities and they look almost circular. However, take a look at the list below and work out which planets have the most and least elliptical orbits (note Pluto has also been added to the list but this is of course a Dwarf Planet):
| Planet | Eccentricity |
| Mercury | 0.2056 |
| Venus | 0.0068 |
| Earth | 0.0167 |
| Mars | 0.0934 |
| Jupiter | 0.0484 |
| Saturn | 0.0549 |
| Uranus | 0.0472 |
| Neptune | 0.0086 |
| Pluto | 0.2488 |
Kepler’s second law of planetary motion explains how the speed of the planet changes as it orbits around the Sun. It states that the imaginary line joining a planet and the Sun sweeps out equal areas of space during equal time intervals as the planet orbits (see diagram 2). Basically planets don’t move with a constant speed. The areas of the two cones in the diagram are the same but notice that the distance the planet has to travel at perihelion is longer than at aphelion. So for the planet to sweep out these areas in the same time interval the planet has to travel faster along its orbit at perihelion to make sure it sweeps out the area in the same time.
The Earth is at aphelion in the summer in the northern hemisphere and at perihelion in the winter in the northern hemisphere. The reverse is true for the southern hemisphere. This has an indirect effect on the summers and winters in the different hemispheres. Because Earth's orbital speed is minimum at aphelion and maximum at perihelion, the planet takes longer to orbit from June solstice to September equinox than it does from December solstice to March equinox. Therefore the length of the summer is slightly longer in the northern hemisphere (93 days) compared to summer in the southern hemisphere (89 days). See aphelion.
For more information about the northern and southern hemispheres have a look at the celestial sphere.
Diagram 1.
Diagram 2.
A planet's elongation is the angular separation between the Sun and the planet, with Earth as the reference point. Greatest elongation refers to the position of either of the inferior (or inner) planets - Mercury or Venus - when they are at their greatest angle of separation from the Sun. At this position in their orbit the planet appears farthest from the Sun as viewed from Earth, so it is in the best position for viewing.
When the planet is at maximum EASTERN elongation the planet is seen in the evening before sunset. When the planet is at maximum WESTERN elongation, the planet is seen in the morning before sunrise
The angle of maximum elongation (east or west) for Mercury is between 18° and 28°, while that for Venus is between 45° and 47°. These values vary because the planetary orbits are elliptical rather than perfectly circular. Another factor contributing to this inconsistency is orbital inclination, in which each planet's orbital plane is slightly tilted relative to a reference plane, like the ECLIPTIC.
The word equinox comes from the Latin aequinoctium: aequus (equal) and nox (night). At the time of the equinox, Earth’s two hemispheres are receiving the Sun’s rays equally and the length of the day and the length of the night are almost (but not quite!) equal all over the planet. This is due to the position of the Earth in its orbit around the Sun and the way it tilts on its axis. It is defined as the time when the Sun crosses the celestial equator (see image) and when the centre of the visible Sun is directly above the equator. At the equinoxes the Sun’s rays are perpendicular to the tilt of the Earth’s axis. At any other time of the year either the southern or northern hemisphere of Earth, tilts a little towards the Sun. Summertime is tilted towards the Sun and wintertime is tilted away from the Sun (see image). The lengths of day and night are not exactly 12 hours each at the moment of the equinox because of the way sunset and sunrise are defined and because of atmospheric refraction.
There are 2 equinoxes during the year: the spring or Vernal (from the Latin meaning of or pertaining to spring) equinox and the Autumnal equinox. The spring equinox in the northern hemisphere is in March and the autumnal equinox is in September. This is the reverse if you live in the southern hemisphere. During the March equinox the Sun crosses the celestial equator from south to north and is also known as northward equinox. During the September equinox the reverse is true and is also known as the southward equinox. Although there doesn’t seem to be anything official about it, the March equinox signifies the beginning of astronomical spring in the northern hemisphere and astronomical autumn in the southern hemisphere. The September equinox signifies the start of astronomical autumn in the northern hemisphere and astronomical spring in the southern hemisphere.
A globular cluster is a nearly symetrical, spherical collection of hundreds of thousands of very old stars held tightly together by gravity. The stars are probably some of the first stars that formed in our Milky Way galaxy. The highest concentration of stars is in the central region making it look like a glowing almost 3 dimensional ball through a telescope.
The name is derived from the Latin globulus meaning a small sphere and there are currently about 158 known globular clusters in the Milky Way. In larger galaxies there may be many more. In fact every galaxy of sufficient mass has an associated group of globular clusters.
Several globular clusters with extremely massive cores may even have a black hole lurking at their centre but there is no scientific evidence to prove this yet.
Jupiter's most famous feature is its Great Red Spot (GRS). The spot was first seen by the English astronomer Robert Hooke in 1664. This means that it has been raging for at least 349 years. In fact nobody knows how long it has been active. It is still huge but seems to be shrinking.
It was named the Great Red Spot around 1878 when it turned a vivid brick red, but more recently it has faded to a much less conspicuous pale tan. This huge, long-lived storm is spinning like a hurricane. However, unlike hurricanes on Earth, the GRS rotates in an anticlockwise direction in Jupiter's southern hemisphere, showing that it is a high-pressure system. In fact the storm gets weaker and stronger over time and the higher the pressure the redder is the spot.
Recent observations show that the storm seems to be coming apart, with streamers “peeling off” the main spot as often as every week. Some reports have called this process “unraveling” although that isn’t really the best description. Could the Great Red Spot actually be self-destructing? Is it nearing its end?
Jupiter’s Great Red Spot is perhaps the most iconic feature of any planet in our Solar System. It’s instantly recognizable, and the massive cyclone has been swirling for so long that we’ve taken for granted that it’ll always be there. Recent observations have shown that, unfortunately, that’s not the case. The storm is dying — the latest data from the Juno spacecraft suggests it might actually be gone within our lifetimes — and a new research paper by scientists at NASA suggests that it’s actually changed in both shape and color as it enters its twilight years.
Juno’s latest images have revealed some surprising changes to the storm, which is now smaller in diameter than has ever before been observed (some 16,000 km long compared to around 48,000 km when it was first viewed). Its swirling winds are reaching higher into the planet’s atmosphere than before, stretching the storm taller as they swirl upward. At the same time, its iconic crimson hue is becoming more orange, likely as a result of the highest gasses being exposed to ultraviolet radiation.
The image courtesy of NASA/JPL/University of Arizona shows a true-color image of Jupiter.
A total lunar eclipse takes place when the Moon enters the shadow cast by Earth (see diagram). This means that they only occur when it is a FULL Moon and when the Sun, Earth and Moon all line up together. The astronomical term for this is syzygy which is from the Greek word for being paired together.
When the Moon is behind Earth like this it doesn’t get any direct light from the Sun. We only see the Moon because the light from the Sun reflects off its surface; no sunlight, no reflection! BUT….
We do actually see the Moon as a glowing red or orange ball during a total eclipse. This is because Earth's atmosphere bends sunlight and indirectly lights up the Moon's surface.
When sunlight passes through the Earth's atmosphere, it gets refracted (bent) towards the Earth's surface. Sunlight is made up of all the colours of the rainbow – the visible spectrum. Each colour has a different wavelength and those with shorter wavelengths get scattered by Earth’s atmosphere (the blue end of the spectrum). The red end of the spectrum has longer wavelengths and this light is able to pass through the atmosphere and therefore falls on the surface of the Moon making it look orange or red or even reddish-orange! We often call this a ‘Blood Moon’.
So what are the different shadows?
The umbra is the dark central core of the shadow. Imagine a light source and an object casting a shadow. If you are standing within the umbra, you will not be able to see any part of the light source as the object blocks all direct light rays. For a total lunar eclipse to occur, the Moon has to enter the Earth’s umbral shadow.
The penumbra is the outer part of the shadow. It is only a half-shadow because the Sun’s light is only partly covered by the Earth.
Does this mean we have different kinds of eclipse?
Yes! We can also get partial lunar eclipses when only a bit of the Moon is in the umbral shadow. When this happens it looks like someone has taken a bite out of the Moon. The size of the bite depends on how much of the Moon is in the umbral shadow. We can also get penumbral eclipses when the Moon is just in the penumbral part of the shadow. However, if this happens some sunlight still reaches the surface of the Moon making it difficult to distinguish them from a normal full Moon.
This raises another question. We get a full Moon every 29.5 days so why don’t we get lunar eclipses every time it is full Moon? The answer is, because the Sun, Earth and Moon don’t always directly line up with each other. The diagram shows you that the Moon’s orbital path around Earth is inclined by an angle of 5° to Earth's orbital plane around the Sun. Earth’s orbital plane around the Sun is called the ecliptic. The points where the two orbital planes meet are called lunar nodes. Lunar eclipses occur when a Full Moon happens near a lunar node. Total lunar eclipses count for about 29% of all lunar eclipses and occur about once every 2.5 years.
A total lunar eclipse usually happens within a few hours. Totality can range anywhere from a few seconds to about 100 minutes. The July 26, 1953 total lunar eclipse had one of the longest periods of totality in the 20th century—100 minutes and 43 seconds.
There are 7 stages of a total lunar eclipse:
Penumbral eclipse begins: This begins when the penumbral part of Earth's shadow starts moving over the Moon. This phase is not easily seen with the unaided eye.
Partial eclipse begins: Earth's umbra starts covering the Moon, making the eclipse more visible.
Total eclipse begins: Earth's umbra completely covers the Moon and the Moon is red, brown, or yellow in colour.
Maximum eclipse: This is the middle of the total eclipse.
Total eclipse ends: At this stage, the Moon is moving away from the Earth’s umbra.
Partial eclipse ends: The moon is no longer in Earth’s umbral shadow.
Penumbral eclipse ends: At this point, the eclipse ends and the Moon is no longer in any part of the earth’s shadow.
Almost everyone on the night side of Earth can see a total eclipse of the Moon. This is very different from a solar eclipse because the shadow of the Moon is so much smaller than the Earth itself and therefore only a small part of Earth experiences a solar eclipse. In a lunar eclipse the Earth is bigger than the Moon so its shadow covers the whole of the Moon.
Magnitude is a measure of how bright a celestial object looks. Those objects that can be seen with the naked eye are ranked in 6 magnitudes from first to sixth magnitude. First magnitude is the brightest and 6th magnitude the faintest, which always seems a little odd! Anyway a sixth magnitude object is exactly 100 times less bright than a first magnitude object. This means that the difference between a first and second magnitude object is approximately 2.51 times. To get the difference between a first and second magnitude object all you do is multiply 2.51 x 2.51 = 6.3.This means that a third magnitude object is about 6.3 times less bright than a first magnitude object.
To make things a little more complicated, an object 2.51 times brighter than magnitude 1 becomes magnitude 0. An object 6.3 times brighter than magnitude 1 becomes magnitude -1.
Sirius is the brightest STAR in the sky and has a magnitude of -1.44. The full Moon has a magnitude of -12.7 and the Sun has a magnitude of -26.7.
The meridian in astronomy is an imaginary line on the celestial sphere that passes through an observer’s zenith (the imaginary point on the celestial sphere directly above the observer) and the north and south points of their horizon. If you look at the whole sphere in the image you will see that the observer’s meridian passes through the north celestial pole, the observer’s zenith, the south celestial pole and the observer’s nadir (the point directly opposite the observer’s zenith). The observer’s upper meridian passes through the zenith of their location and the lower meridian passes through the nadir. It is perpendicular to the horizon.
Because the Earth is rotating west to east, celestial objects such as planets and stars appear to move from east to west. At some point they cross or transit the observer’s upper meridian. At the point of contact with the upper meridian the object is at its highest point in the sky and is referred to as culmination.
Transit in this instance should not be confused with the passing of one body in front of another.
Charles Messier was an 18th Century French astronomer. In particular he hunted for comets. He is noted for his catalogue of nebulae and star clusters called the Catalogue des Nébuleuses et des Amas d'Étoiles ("Catalogue of Nebulae and Star Clusters") first published in 1774.
Today the catalogue is known as the 'Catalogue of galaxies, nebulae and star clusters.' The reason why he did not mention galaxies is because originally all the celestial objects with a nebulous appearance were assumed to be part of the Milky Way Galaxy and not galaxies in their own right. He drew up the catalogue with his friend and assistant Pierre Méchain to distinguish the permanent visually diffuse objects from the transient ones such as comets. This avoided wasting time sorting out comets from the permanent “fuzzy objects” and helped to prevent fellow comet hunters making mistakes.
The first version of Messier's catalogue contained 45 objects. In addition to his own discoveries, this version included objects previously observed by other astronomers, with only 17 of the 45 objects being those observed by Messier himself. By 1780 the catalogue had increased to 80 objects. The final version of the catalogue containing 103 objects was published in 1781. However, due to what was thought for a long time to be the incorrect addition of Messier 102, the total number remained 102. Today there are 110 Messier objects catalogued with astronomers other than Messier, using side notes in Messier's texts, adding to those already catalogued.
Meteor showers are often called shooting stars but they are nothing to do with stars at all. They are caused by streams of dust and rock called meteoroids. These meteoroids are tiny pieces of debris left behind (usually) by comets as they orbit the Sun. A comet is like a great big dirty snow ball or a snowy dirt ball! When it approaches the Sun the ice begins to turn from a solid straight into a gas. This is called sublimation. The ‘dirt’ in the comet then gets swept along behind the comet in a meteoroid stream also known as a dust trail. As Earth passes through this dust trail every year at the same time, meteoroids will enter the Earth's atmosphere and "burn up" leaving a bright visible streak called a meteor. If meteors occur only seconds or minutes apart then it is known as a meteor shower and it takes its name from the constellation from which the shower appears to originate (the radiant point). For example the Perseids appears to originate in the constellation Perseus and the Taurids appears to originate from the constellation Taurus.
Most meteoroids that cause meteors are the size of a grain of sand. Their bright streaks become visible between 40 and 75 miles (65-120 km) above the Earth's atmosphere and they disintegrate at altitudes of around 30-60 miles (50-95 km). At speeds of from 11 -72 km/s (25,000mph to 160,000mph) they are visible for between fractions of a second to seconds.
Meteoroids vary in size from a dust particle to a small boulder. The number of meteoroids appears inversely proportional to their size for instance there are more meteoroids the size of a dust particle than the size of a grain of sand and there are more meteoroids the size of a grain of sand than the size of a pebble etc. Millions of meteors "burn up" in the Earth's atmosphere every day but most of them are so small that they are invisible. Also many enter the atmosphere during the day.
So why do we see these streaks of light? As the meteoroid leaves the vacuum of space and enters the Earth's atmosphere at high speed there is a rapid compression (squashing) of the air in front of it, which becomes superheated so much so that it ionises. This is what causes it to glow and because it is travelling so fast it leaves a glowing trail behind it. It can also shed glowing material too adding to the bright trail behind the meteor. The larger and faster the meteor, the longer the trail and the easier it is to spot. The meteor itself disintegrates at such high temperatures. However, if the meteor is large enough and survives its traumatic entry through the atmosphere, it will fall to Earth. This is now called a meteorite. Scientists think that up to 50 metric tons of meteors fall on the Earth as meteorites each day, but most are no bigger than a pebble. Of course some fall through the atmosphere during the day and we will never see them.
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The hidden object is either smaller or appears smaller to the observer than the object passing in front of it. In the example shown here there is an occultation of Jupiter by the Moon.
Star clusters form from the same molecular cloud. An open cluster is one that can contain from a few dozen to a few thousand stars all of similar ages. They are much more loosly bound by gravity than a globular cluster and also generally contain younger, hot stars. Because they are more loosely bound they can become disrupted by passing objects such as other clusters or clouds of gas and stars can be lost from the cluster.
Open clusters are commonly found in the disk of a galaxy but only in spiral or irregular galaxies where active star formation is still ongoing. They generally survive for a few hundred million years, with the most massive ones surviving for a few billion years.
The Pleiades or the Seven Sisters in the constellation Taurus is an example of an open cluster. It contains a group of newly formed B-type stars. It is 115 million years old and is easily seen with the unaided eye. when you start to see Pleiades rising over the eastern horizon before midnight then you know the end of summer is nigh and Autumn is on its way. The brightest stars have a surface temperature of 30,000 Kelvin, much hotter than the surface of our Sun which is about 6,000 Kelvin.
Other examples of Open Clusters include the Double Cluster in Perseus and the Coma Star Cluster in Coma Berenices (also known as Melotte 111). More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist.
In this example opposition is the time when a celestial body is on the opposite side of the Earth to the Sun. At this point in the orbits of the 2 bodies the outer planet (shown in orange) is the closest it will be to the Earth. However because of the elliptical nature of planetary orbits the distance between the Earth and the outer planet will be different at each opposition.
The diagram is simplified to show opposition and conjunction. The elliptical orbits are very exagerated.
The Orion Nebula is also known as M42. The M refers to Charles Messier an 18th century French astronomer and comet hunter. He compiled a list of deep sky fuzzy looking objects so they would not be mistaken for comets. Not all Messier objects were actually discovered by Charles Messier himself.
Nebula (plural nebulae) is Latin for mist and they are vast areas of cloud and dust between the stars. The Orion Nebula is so huge it is visible to the naked eye even though it is 1,344 light years away. It appears to cover 1 degree of the sky, an area twice the size of the full Moon, it is actually 24 light years across. A light year is roughly equivalent to 9.5 million million Km! It is a region where new stars are formed. These new stars at the centre of the dust cloud light up the surrounding gas making it visible through a telescope.
The Orion Nebula is in the constellation of Orion, which is a very prominent constellation in the winter sky. The nebula is located in the "sword" of Orion which hangs below the 3 stars that depict his belt (see diagram on right)
One of the new stars at the centre of the Orion Nebula is Theta-1 Orionis. It is easy to understand why it is called the Trapezium because, through the telescope, you should see 4 prominent stars in the shape of a trapezium (see Diagram below).
This false colour mosaic was made by combining several exposures from the Hubble Space Telescope Image credit: NASA Picture of the day
Perihelion is the nearest point that a body gets to the Sun in its orbit. The word comes from peri which is Greek for around and Helios which is the Greek god and personification of the Sun.
The image shows the exaggerated elliptical orbit of the Earth around the Sun. All planetary orbits around the Sun are elliptical. Ellipses are oval shaped – like a stretched out circle and the Sun is not at the centre of the ellipse, as it would be if the orbit were circular. This is why the planet is not always the same distance from the Sun. Instead, the Sun is at one of two points called "foci" (which is the plural form of "focus") that are offset from the centre. This means that each planet moves closer towards and further away from the Sun during the course of each orbit. Not all planets have the same shaped oval orbit; some are more elliptical than others. The Earth’s orbit is almost circular but not quite! See Elliptical Orbit for more information.
At the moment the Earth reaches perihelion in early January approximately 14 days after the winter solstice. This means that in the northern hemisphere the closest approach to the Sun is in winter while the closest approach to the Sun corresponds to summer in the southern hemisphere. At perihelion the centre of the Earth is about 0.98 astronomical units or 147,098,070 km (91,402,500 miles) from the centre of the Sun. The date of perihelion changes over time due to precession and other orbital factors which follow cyclical patterns. Because perihelion falls in the winter months in the northern hemisphere and the summer months in the southern hemisphere, the winter is shorter in the northern than in the southern hemisphere. See Elliptical Orbit for more information.
As the Moon orbits the Earth it shows different phases to observers on Earth. The reason for this is explained pictorially in the diagram.
The whole of the Moon is only ever half illuminated - the half that faces towards the Sun (inner part of the diagram). However, looking from Earth, we only see a portion of this illuminated face because as the moon orbits the Earth it changes position in space relative to the Sun and Earth. (outer part of the diagram).
New Moon is considered as the beginning of the cycle, hence the term New Moon. However, because the moon is between Earth and the Sun the Sun only illuminates the face that we do not see, the ‘Far Side of the Moon'. The face that we normally look at is in total darkness and therefore we cannot see it.
The next phase is a crescent Moon but because more and more of the illuminated face of the Moon will become visible to observers from Earth in the following days it is called a waxing crescent.
The next phase is called First Quarter. The reason why we call this phase of the moon first quarter is because it is one quarter of the way through the lunar cycle. The lunar cycle is the number of days it takes to go from one New Moon to the next and is 29.53 days. This is one lunation. First quarter is often referred to as half moon. This is because only half of the face of the moon that we see is lit by the Sun. Halfway through the lunar cycle is Full Moon when we can see the whole of the illuminated face because it is opposite the Sun with respect to the Earth.
As the illuminated face becomes less the Moon is said to be waning and when it has orbited three quarters of the way around the Earth it is said to be a Last (or Third) Quarter Moon. The next time the moon is a crescent it is a waning crescent and the edge that is illuminated is the opposite edge that is illuminated when it is a waxing crescent. Remembering this becomes easy by saying "if the Light is coming from the Left the Moon is getting Less."
Native Americans have names for each full Moon. For more information have a look at our section on Full Moons and Supermoons.
The Earth spins on its axis. This axis is an imaginary line running through the Earth. If you were to be high above the Earth, looking straight down along the axis, all the points on Earth would appear to move in circles around the axis. This why we get day and night. If you followed this axis out into space from the northern hemisphere on Earth, it would point toward a particular star in the sky. We call that star the North Star or the Pole Star.
At the moment, the star known as Polaris is the North Star. However, Polaris has not always been the North Star and will not always be the North Star. To understand that, we need to look at how the Earth spins on its axis.
The spin axis of the Earth undergoes a motion called precession. If you have ever watched a spinning top, you know that its spin axis tends to stay pointed in the same direction. However, if you give it a slight nudge, the axis will start to change its direction, and its motion traces out a cone. This changing of direction of the spin axis is called precession. So what gave the Earth the "nudge" it needed to start precessing? The Earth bulges out at its equator, and the gravitational attraction of the Moon and Sun on the bulge provided the "nudge" which made the Earth precess. It was the ancient Greek astronomer and mathematician Hipparchus who first estimated the precession of the Earth's axis around 130 B.C. The period of precession is about 26,000 years. In other words, it takes 26,000 years for the axis to trace out the cone one complete time.
So now you can see why Polaris will not always be aligned with the north spin axis of the Earth - because that axis is slowly changing the direction in which it points! Right now, the Earth's rotation axis happens to be pointing almost exactly at Polaris. But in the year 3000 B.C., the North Star was a star called Thuban (also known as Alpha Draconis), and in about 13,000 years from now the precession of the rotation axis will mean that the bright star Vega will be the North Star. Don't feel bad for Polaris, however, because in 26,000 more years it will once again be the Pole Star!
By the way, there is not currently a star in the direction of the southern hemisphere spin axis. So we do not now have a "South Star".
(Source: NASA)
The Ring Nebula (also known as M57), is a planetary nebula in the constellation of Lyra (the most notable star in Lyra is Vega). Planetary Nebulae are formed when some types of star die (stars that are between 0.8 and 8 solar masses). The remnants of our own Sun will end up becoming a planetary nebula with a white dwarf star at its centre. This will happen after it has reached the red giant phase of its life. The nebula comes from a shell of ionized gas which is expelled into the surrounding interstellar medium as it goes through its final death throes.
The rings of Saturn are made up of icy particles ranging in size from micrometres to metres. Almost entirely water ice, the particles are contaminated with some dust and other chemicals. Reflected sunlight from these particles, contribute a great deal to the brightness of Saturn as viewed from Earth and this brightness appears to change over time. This is due not only to the change in the distance from Earth to Saturn as both planets independently orbit the Sun, it is also due to the changing aspect of the rings (see diagram), which in turn depends on where Saturn is in its orbit around the Sun. The thickness of the ring system is estimated at only 10 metres deep.
The diagram shows the orbit of Saturn shown at two/three year intervals between the years 1993 and 2020 AD. The orbit of the Earth is seen close to the centre, marked at various dates by a blue-green globe (the orbits are not shown to scale). The dates in blue are the dates of Saturn's opposition to the Sun, i.e. when the planet is closest to the Earth and appears at its brightest for the year. The images in the grey circles show how the planet appears from the Earth (orientated with Celestial North at the top). The points of Saturn's perihelion (i.e. its closest point to the Sun) and aphelion (its most distant point from the Sun) are also marked. The constellation in which Saturn appears, as seen from the Earth, is shown in green. The First Point of Aries is the 'zero point' from which the longitudes of the planets are measured (diagram based on a graphic by space artist David A Hardy).
It takes 29.457 Earth years for Saturn to orbit the Sun. During this time we see the rings from different angles. Previously the South Pole of Saturn has appeared tilted towards Earth and we have been looking at the underside of the rings. When the rings start to slowly open up again we will begin to see the top side of the rings as the North Pole appears tilted in our direction. By the time Saturn has completed one orbit the ring cycle, from our point of view will start all over again.
Another ring around Saturn was discovered on 6th October 2009 using the Spitzer Space Telescope, which revealed an infrared glow thought to come from sun-warmed dust in a tenuous ring. The ring spans from 128 to 207 times the radius of Saturn - or further - and is 2.4 million kilometres deep. It is the largest planetary ring in the solar system but is quite diffuse making it very difficult to detect using visible light. The source of the ring's material seems to be Saturn's outer moon Phoebe, which orbits the planet at an average distance of 215 times the radius of Saturn. If space rock hits Phoebe the impact may generate the debris which has made the ring.
Diagram and caption courtesy of: http://www.nakedeyeplanets.com/saturn-orbit.htm
It takes 23 hours, 56 minutes and 4.1 seconds for the Earth to make one complete rotation on its axis (i.e. to spin 360 degrees). This is called a sidereal day. Sidereal means of or with respect to the distant stars (i.e. the constellations or fixed stars, not the Sun or planets).
Earth does not just rotate anticlockwise around its axis it orbits the Sun anticlockwise at the same time. In fact it moves a little less than one degree around the Sun during the time it takes for 1 full axial rotation. So, for the Sun to appear in the same place again the next day, the Earth has to rotate about an extra one degree (in fact 360.99 degrees altogether not just 360 degrees). This is called a solar day. It is also referred to as the synodic period for Earth in relation to the Sun. It takes about 4 minutes to rotate 1 degree so this is why a solar day is longer than a sidereal day by about 4 minutes.
Currently it takes 23 hours, 56 minutes and 4.1 seconds for 1 full rotation of Earth around its axis. However, the Earth is spinning a little slower every year because of tidal forces (gravitational pull) between Earth and the Moon. Every 100 years, the sidereal day gets about 1.4 milliseconds, or 1.4 thousandths of a second, longer.
A solar eclipse occurs when the Sun, Moon and Earth line up. As the Moon passes in front of the Sun it blocks out the sunlight and casts a shadow on the Earth. If you happen to be in the shadow you will see an eclipse. How can this be you may ask; the Moon is so much smaller than the Sun! It just so happens that the Moon although 400 times smaller than the Sun is 400 times closer to Earth than the Sun; it’s all a matter of perspective.
There are 2 types of shadow cast by the Moon; the inner dark umbral shadow when you see a total or annular eclipse and the lighter outer penumbral shadow when you see a partial eclipse. So what’s the difference between an annular and a total solar eclipse? The Moon’s orbit around Earth is not exactly circular; it is an oval shape or an ellipse. Sometimes when the Moon lines up with the Sun it’s at its closest point known as perigee (or near to its closest point) to Earth in its orbit. This is when we see a total eclipse and the Moon is able to cover the whole of the Sun. However, if the Moon is further away from Earth in its elliptical orbit (close to or at its furthest point which is known as apogee) it just can’t manage to cover the whole of the Sun’s disc and it leaves a ring around the outside. This is called an annular eclipse.
The Sun is very dangerous and you must never look directly at it even if you are wearing sunglasses.
There are 2 solstices each year. One in June and one in December. The June solstice is the longest day in the northern hempisphere and the shortest day in the southern hemisphere. It marks the start of astronomical summer and the start of astronomical winter respectively in the two hemispheres. The December solstice is the shortest day in the northern hemisphere and the longest day in the southern hemisphere. It marks the start of astronomical winter and the start of astronomical summer in the two hemispheres respectively.
In 2022, the June solstice occurs on Tuesday, June 21 and the December solstice is on Wednesday 21 December.
In the Northern Hemisphere, the June solstice is also known as the summer solstice. It occurs when the Sun reaches its highest and northernmost points in the sky. At this time the Earth arrives at the point in its orbit around the Sun where the north pole is at its maximum tilt of 23.5 degrees towards the Sun.
On the day of the June solstice, the Northern Hemisphere receives sunlight at the most direct angle of the year.
The word “solstice” comes from Latin solstitium—from sol (Sun) and stitium (still or stopped). Due to Earth’s tilted axis, the Sun doesn’t rise and set at the same locations on the horizon each morning and evening; its rise and set positions move northward or southward in the sky as Earth travels around the Sun throughout the year. Also, the path of the Sun in the sky becomes higher or lower throughout the year. At the June solstice the Sun reaches its northernmost point in the sky and at the December solstice it reaches its southern most point in the sky. At each of these points, the height of the Sun’s path does not change for a brief period of time. The Sun itself of course isn’t moving (unless you consider its own orbit around the Milky Way galaxy); instead, this change in position in the sky that we on Earth notice is caused by the tilt of Earth’s axis as it orbits the Sun, as well as Earth’s elliptical, rather than circular, orbit.
Does the Solstice Always Occur on the Same Day?
The timing of the solstices is not based on a specific calendar date or time; it all depends on when the Sun reaches its northernmost and southernmost points from the celestial equator. Therefore, the solstice won’t always occur on the same day. Currently the June solstice shifts between June 20, 21, and 22 and the December solstice shifts between December 20, 21, 22 and 23.
The Year’s Longest Day
The Summer Solstice is the day with the longest period of sunlight and the winter solstice is the day with the shortest period of sunlight. In the UK we can expect about 16 hours 38 minutes of sunlight in London at the summer solstice and about 7 hours 49 minutes of sunlight at the winter solstice. However, in Edinburgh we can expect about 17 hours 36 minutes of sunlight at the summer solstice and 6 hours 57 minutes of sunlight at the winter solstice. It all depends on your latitude!
The Sun appears highest in the sky at the summer solstice when its rays strike Earth at a more direct angle, causing the efficient warming we call summer. Because the Sun is highest in the sky on this day, you’ll notice that your shadow (at local, or solar, noon, not clock-time noon) is the shortest that it will be all year.
What is the difference between the start of meteorological summer and the start of astronomical summer?
Most meteorologists divide the year into four seasons based on the months and the temperature cycle, which allows them to compare and organize climate data more easily. In this system, summer begins on June 1 and ends on August 31. Therefore, the summer solstice is not considered to be the first day of summer, meteorologically speaking.
Astronomically, however, the first day of summer is said to be when the Sun reaches its highest point in the sky, which occurs on the summer solstice (June 20–22). Therefore, the summer solstice is considered to be the first day of summer, astronomically speaking.
Why are the dates of the solstices not the same every year?
The summer solstice in the Northern Hemisphere ranges in date from June 20 to 22. This occurs in part because of the difference between the Gregorian calendar system, which normally has 365 days, and the tropical year (how long it takes Earth to actually orbit the Sun once), which has about 365.242199 days. To compensate for the missing fraction of days, the Gregorian calendar adds a leap day about every 4 years, which makes the date for summer jump backward. However, the date also changes because of other influences, such as the gravitational pull from the Moon and planets, as well as the slight wobble in Earth’s rotation.
Fun Facts
When the Sun appears highest in the sky near the summer solstice, the full Moon opposite the Sun generally appears lowest in the sky!
On the winter solstice, just the opposite occurs: The Sun is at its lowest in
A transit is an astronomical event that occurs when, as seen from an observers viewpoint, one celestial body appears to move across the face of another celestial body, hiding a small part of it. Examples of this would be Venus or Mercury crossing the face of the Sun, satellites crossing the face of the planet that they are orbiting.
A shadow transit is when you see the shadow of the celestial object passing across the face of the larger celestial body. The image shows the triple shadow transit of Callisto, Io, and Europa on October 11-12, 2013. Callisto is at far left and Io is the upper of the pair at center. Notice the distorted shadow of Callisto. The shadow lies close to the edge of a Jupiter's globe, which curves sharply away from the observer.
Image by John Sussenbach (see image of the triple shadow transit of three moons of Jupiter in 2013).
If the larger celestial body hides a major part, or all of, the smaller celestial body (i.e from an observers point of view the smaller celestial body moves behind the larger body), then it is an occultation rather than a transit.
