Galaxy Classification

Celestrial Coordinates

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Celestrial coordinates

The sky and everything in it is constantly moving. This makes finding things in it very problematic. This is because of several things. The Earth is spinning on it's own axis once a day, around a line drawn through the North and South poles. This line through the two poles also moves with a circular motion over about 26000 years whilst maintaining an almost constant 23.4° tilt. This movement is called precession and was discovered in the second century B.C. by Hipparchus. In effect the Earth is wobbling like a gyroscope. The celestial poles tracing a circle on the celestial

sphere, is caused mainly by gravitational effects of the Sun and the Moon on the equatorial bulge of the Earth. The Earth is also moving around the Sun (sol) about once every year. The Sun is also moving around the Milky Way galaxy every 220 million years. The Milky Way galaxy is also moving through space. In effect everything in the universe is moving in different directions at different speeds. So how can we record the position of any object in the sky and find it reliably every time and at any time?

Below are several ways of giving coordinates for objects in the sky. They all have their uses and disadvantages so they are all used depending on the situation.

Catalogues of stars and of deep-sky objects will specify the right ascensions and declinations of these objects at some particular time, or epoch. This is because of the slow movement of just about everything in the universe a discussed above.

Equatorial Coordinates

Sidereal Time.

Sidereal time is based on the motion of the Earth with relation to the stars which because of their great distance, remain fixed points in the sky. As the Earth rotates through 360° each star will pass over one of the Earths lines of longitude twice. The period between two successive transits is called a Sidereal day( That is the Earth rotates twice taking 12 sidereal hours each rotation). The sidereal day is broken into hours, minutes and seconds etc. The Solar day is slightly longer than a sidereal day. One sidereal day equals 23h 56m 4s .09053 Solar days. This is why stars seem to rise about 4 Solar minutes earlier each night.

Declination

Declination is the celestrial equivalent of latitude on Earth. It is measured, like latitude in degrees (°), minutes of arc (') and seconds of arc (") from 0° at the celestrial equator to 90° North and 90° South at the celestrial poles.

Right ascension

Right ascension is the celestrial equivalent of longitude on Earth. It is measured in hours (h), minutes (m) and seconds (s), but is sometimes measured in degrees. The zero point of right ascension is on the Greenwich meridian of longitude.

Altazimuth

Altazimuth coordinates are a pair of measurements, altitude and azimuth. These coordinates refer to the position of an object in the sky in relation to the observer. The altitude or elevation of an object is the angle above or below the plain of the observers horizon. Directly overhead the observer in the Northern hemisphere ( zenith) is 90° , directly below the observer ( nadir) is -90°) and the horizon is 0°. The azimuth is an angle measured along the horizon to a vertical line drawn from the object to the horizon. It is measured in a clockwise direction from due North (0°) through due East (90°), due South (180°), due West (270°), and back to due North (0° or 360°). These figures differ for each observer depending on his or her position and the time of the observation.

Galactic coordinates

Galactic coordinates are used to specify the position of objects in the Milky Way as observed from the Earth. The galactic equator is is a line drawn through the band of light we see in the night sky called the Milky Way.

Galactic Latitude

Galactic latitude (b) of a celestrial object is its angular distance (from 0° to 90°) North (as positive)

or South (as negative) of the galactic equator. It is measured along the great circle passing through the celestrial object and the galactic poles.

Galactic Longitude.

Galactic longitude (l) of a celestrial object is its angular distance (from 0° to 360°) measured eastwards from the galactic centre along the galactic equator to the celestrial object.
The Galactic centre lies in the direction of the constellation of Sagittarius. It was agreed in 1959 by the International Astronomical Union (IAU) and its coordinates are RA. 17h 45.6m, dec. -28° 56.3' (Epoc 2000). However more recent observation indicate the actual Galactic centre lies close to Sagittarius A West, a few arc minuets away from the galactic centre used for the Galactic coordinates system.