The telescopes were referred to as 'The Equatorial Group' because originally all the telescopes were mounted on equatorial mounts (see diagram).
Imagine a long stick passing through the Earth right through the north and the south poles. This is the Earth’s axis but it’s not straight up and down, it is tilted by 23.5 degrees. As Earth rotates on its axis from west to east, this tilt causes stars and indeed the Sun and the Moon to apparently move in an arc across the sky from east to west. The highest point the object reaches is in the south.
By tilting one rotational axis of the telescope parallel to the equator, an equatorial mount can follow the same arc across the sky. This tilted axis or polar axis is known as the Right Ascension (RA). This axis is now pointed towards the same point in the sky that, for us here in the northern hemisphere, the north polar axis points to; this is called the Celestial Pole. We are lucky in the northern hemisphere because we have a fairly bright star called Polaris (the North Star or the Pole Star) that is very close to the Celestial Pole. Polaris is in the constellation Ursa Minor (The Little Bear).
The major advantage of an equatorial mount is that it can be electrically driven at exactly the same speed as Earth’s rotation but in the opposite direction. This means that it can remain focused on a celestial object even as ‘travels’ on its arc across the sky (diurnal motion). Also an equatorial mount allows any instrument attached to it to stay fixed on any celestial object with diurnal motion. This in turn means that sharp images can be taken even over a long period of time (long exposure photography).
