## measurements and coordinate systems on the celestial sphere.

When you observe the stars from the Earth they appear to be on a sphere that surrounds the Earth. This is called the celestial sphere. The apparent distances on the celestial sphere are measured in a different way to most other conventional forms of measurement like metres, kilometres, miles or even the measurement in light years.

So when we measure distances between objects on the celestial sphere we measure them in degrees. So that 360° covers the whole celestial sphere. Since we can only see one half of the sky at once (horizon to horizon) the sky we see can be divided up into 180°. It's worth noting here that 1° can be divided up further into 60 seconds.

You can try measuring these distances for yourself by using your hand:

Hold your hand at arm’s length from you and up to the sky.

- The width of your first finger is approximately 1°

- with all your fingers held together the distance between your thumb and little finger is approximately 10°

- with your fingers spread out the distance between the tip of your thumb and little finger is approximately 22°

So when we measure distances between objects on the celestial sphere we measure them in degrees. So that 360° covers the whole celestial sphere. Since we can only see one half of the sky at once (horizon to horizon) the sky we see can be divided up into 180°. It's worth noting here that 1° can be divided up further into 60 seconds.

You can try measuring these distances for yourself by using your hand:

Hold your hand at arm’s length from you and up to the sky.

- The width of your first finger is approximately 1°

- with all your fingers held together the distance between your thumb and little finger is approximately 10°

- with your fingers spread out the distance between the tip of your thumb and little finger is approximately 22°

This system of degrees as a measurement of location for an object on the celestial sphere is used to form a coordinate system of the night sky.

There are two main types of coordinate systems, the first and simplest coordinate system is the alt-az which is more commonly known as the horizontal coordinate system. The second and slightly more complex coordinate system is the equatorial coordinate system.

The horizontal coordinate system is based upon the observer’s horizon and thus is not practical to use in cataloguing or recording positions. For this reason it is not widely used and I will not describe this coordinate system.

The equatorial coordinate system on the other hand is based upon the north and south poles and the celestial equator and thus it's coordinate system is much more practical since the observers position on the Earth's surface does not matter. This means that this coordinate can be used to catalogue astronomical objects.

In the equatorial coordinate system there are two values similar to longitude and latitude when we measure coordinates on the Earth. The celestial equivalent to latitude is the

**declination**and the celestial equivalent to longitude is the

**right ascension**

The declination is the angular distance (measured in degrees, minutes and seconds) from the celestial equator along the Meridien [1] to the astronomical object. The declination north of the equator is 0° at the equator moving up to 90° at the North Pole. The declination south of the equator is 0° at the equator moving down to -90° at the South Pole.

Right ascension is measured in hours, minutes and seconds. So that one hour is 1/24th of a circle. Right ascension is measured to the east of the vertical equinox. The vertical equinox is where, at the March equinox, the sun intersects the celestial equator from south to north. This is one of two locations where the elliptic intercepts the celestial equator.

[1] Meridien definition; circle on the celestial sphere which passes through the two celestial poles and the vertical equinox.