Discovered in October 1983 this unusual asteroid 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.
Phaethon's most remarkable distinction is that it approaches the Sun closer than any other numbered asteroid. 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.
Phaethon will approach relatively close to the Earth on December 14, 2093, passing within 0.0198 AU (Astronomical Units).
The astronomical unit (AU) is a unit of length approximately equal to the distance from the Earth to the Sun. The currently accepted value of the AU is 149 597 870 691 ± 30 metres (about 150 million kilometres or 93 million miles).
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. However, half of the moon always faces towards the Sun and is therefore always lit by the Sun while the other half is in darkness (see diagram below).
Diagram 1: Courtesy of NASA (http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question3.html)
The moon orbits the Earth and therefore changes position in space relative to the Sun and Earth. At first quarter because of the relative positions of the Moon, Earth and Sun we can only see a portion of the lit face (see diagram of the phases of the moon below).
Diagram 2: Courtesy of NASA (http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question3.html)
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 refers to the position of an inner planet (Mercury or Venus) when it is at maximum angular separation from the Sun as viewed from Earth i.e. how far from the Sun Mercury or Venus appears in our sky (see diagram 1). Mercury and Venus are particularly easy to see when at greatest elongation. When the planet is at maximum EASTERN elongation, the planet is seen in the evening close to sunset. When the planet is at maximum WESTERN elongation, the planet is seen in the morning close to sunrise (see diagram 2).
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Diagram 1 The source of this diagram is Windows to the Universe |
Diagram 2 The source of this
diagram is |
An eclipse of the moon can only occur at FULL MOON and only if the Moon passes through some portion of the Earth’s shadow (see the diagram of the phases of the moon under First Quarter, diagram 2).
A TOTAL eclipse happens when the entire moon passes through Earth’s umbral shadow (see diagram below).
A partial lunar eclipse is when only part of the Moon passes through Earth’s umbral shadow. On August 16th 2008 81% of the moon will be in the northern umbral shadow at greatest eclipse. See Fred Espanak’s eclipse pages.
The timings of the major phases of the
eclipse are listed below:
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Penumbral Eclipse Begins |
18:24:46 UT 19:24:46 BST |
P1 |
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Partial Eclipse Begins |
19:36:04 UT 20:36:04 BST |
U1 |
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Greatest Eclipse |
21:10:09 UT 22:10:09 BST |
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Partial Eclipse Ends |
22:44:14 UT 23:44:14 BST |
U4 |
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Penumbral Eclipse Ends |
23:55:23 UT 24:55:23 BST |
P4 |
UT = Universal Time, BST = British
Summer Time
UT = Universal Time, BST = British Summer Time
P1 = Point when the Moon enters the Earth’s penumbral shadow
U1 = Point when the Moon enters the Earth’s Umbral shadow and partial eclipse begins
U4 = Point when the Moon finally leaves the Earth’s Umbral shadow and partial eclipse ends
P4 = Point when the Moon finally leaves the Earth’s Penumbral shadow
The partial eclipse will last for a total of 3 hours and 8 minutes. The moon will rise at 20:11 (BST) on August 16th and the eclipse will begin soon after the moon has risen making it a spectacular sight.
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.
As the Moon orbits the Earth it shows different phases to observers on Earth (see the diagram below).
Diagram 1: Courtesy of NASA (http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question3.html)
New Moon occurs when the Moon is starting its orbit around the Sun, hence the term New Moon. However, because the moon is between Earth and the Sun the Sun only illuminates the side that we do not normally look at. The side that we normally look at is in total darkness and therefore we cannot see it (see diagram below where the white part of the Moon is the portion that is lit by the Sun and therefore the only part that we can see clearly).
Diagram 2: Courtesy of NASA (http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question3.html)
Opposition is the time when a celestial body, is on the opposite side of the sky to the Sun (see diagram below).
Diagram courtesy of NASA
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 a huge area (twice the apparent size of the full Moon) 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. It is located in the “sword” of Orion which hangs below the 3 stars that depict his belt (see diagram below)
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
All of the planets in our Solar System move around the Sun in elliptical orbits. An ellipse is a shape that can be thought of as a "stretched out" circle or an oval as in the diagram below. The Sun is not at the centre of the ellipse, as it would be if the orbit were circular. 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. The point in the orbit where the planet is closest to the Sun is called "perihelion". The point in the orbit where the planet is furthest away from the Sun is called "aphelion".
