Latitude and Longitude

The geographic coordinate system allows us to locate a position on the Earths surface. Latitude identifies how far north or south of the equator our location is, whereas longitude identifies its east-west position.

Lines of Latitude are imaginary horizontal lines drawn on a map, which are marked with an angular measurement (in degrees) that ranges from 0° at the equator (low latitude) to 90° at the poles.

Lines of Longitude are imaginary vertical lines, also marked in degrees, which identify how far west or east a location is from the prime meridian - the prime meridian being the line of zero degrees of longitude that runs through the Royal Observatory Greenwich in London. Because the Earth is a sphere, there are 360 degrees of longitude, but for historical reasons, these range from 180° East to 180° West.

Thus the position of any city in the world can be described by a latitude and longitude. The coordinates for the centre of Liverpool, for example, are given as Latitude 53.3° North and Longitude 2.8° West.

Altitude and Azimuth

The horizontal coordinate system is based around an observers (or telescopes) 360° field of view, and the position of a target is given with respect to the observers local horizon. However, because the Earth rotates, the coordinates of a star or planet will be constantly changing. Hence, we can only give an object's position for a particular moment in time.

In other words, the system is fixed to the Earth and not the stars, with coordinates determined as follows:

• The altitude (Alt) is the angle between the object and the closest point on the observer's local horizon (green curve in image). It can take any value between 0° and 90°.
• The azimuth (Az) is the angle of the object around the horizon, running from the north point towards the east (red curve in image). It can take any value between 0° and 360°.

In addition, there are two parts of the system that have special names, as follows:

• The zenith is the point right above the observer's head - i.e. 90° above all points on the horizon.
• The observer's meridian is the curved line running from north to south through the zenith.

Horizontal coordinates are useful for determining the rise and set times of an object in the sky. When an object's altitude is 0°, it is on the horizon. Due to the Earth's rotation, objects rise in the east, pass through the meridian and then set in the west. However, stars close to north celestial pole (the imaginary axis around which the Earth rotates) do not set below the horizon, they just appear to circle around the pole.

The position of the north celestial pole changes depending on where you are in the world. If you were standing on the Earth's geographic north pole, it would be right above your head. If you were standing at the equator, it would appear on the horizon.

Right Ascension and Declination

The equatorial coordinate system allows observers (or telescopes) to locate celestial objects using coordinates that are fixed in relation to the stars. The system relies on two main principles:

1. an imaginary circle called the celestial equator. This is the projection of the Earth's own equator onto the celestial sphere - the celestial sphere being our 2-D view of the cosmos that encompasses the Earth.
2. a single point fixed in space, known as the vernal equinox, which is the apparent location of the Sun at the spring equinox (around 21st March). More specifically, it is the point in the constellation of Aries, where the ecliptic (a line tracing out the path of the Sun) crosses the celestial equator.

The equatorial coordinates are then determined as follows:

• The Declination (Dec) measures the angle of an object above or below the celestial equator. It can take any value between -90° and 90°.
• The Right Ascension (RA) measures the angle of the arc that runs from the vernal equinox, along the celestial equator, to the closest point to the object. Unlike other systems, the RA is measured in hours instead of degrees, and can take any value between 0 and 24 hours. Note that there are (360/24) = 15° in one hour of right ascension.

As an example, the coordinates of the bright star Rigel under this system are as follows:

RA: 05 14 32 : or : [ 05 hours 14 minutes 32 seconds ]
Dec: -08 12 06 : or : [ -08° (i.e. south of the equator) 12' (arcminutes) 06" (arcseconds) ]

Over long periods, various effects change the Earth's orbit and thus the apparent location of the stars. This, combined with movement of the stars themselves, means that the coordinates of objects, as seen from Earth, will change. When considering observations separated by long intervals, it is necessary to specify an epoch (i.e. J2000.0) when giving the coordinates of planets, stars, galaxies, etc. In other words, give the position as measured on a particular date, from which astronomers can calculate where the object should have moved to.