To locate a point on Earth you need two coordinates: latitude and longitude. Unlike places on Earth, objects in the night sky appear to move over time. This is because the Earth is rotating on its axis and orbiting the Sun. So celestial coordinate systems must take this movement into account.
There are two main coordinate systems used in astronomy: Altitude-Azimuth and Right Ascension and Declination.
Click on each of the following to learn more:
Latitude and Longitude
We use this geographic coordinate system to locate a position on the Earth's surface. A line on a map that connects points with equal longitude or latitude is called a meridian. Longitude and latitude are angular measurements and are measured in degrees.
Latitude tells us how far north or south a point on the Earth is. The Earth's equator has a latitude of 0°. There are 180° degrees of latitude. These range from 90° South at the South Pole to 90° North at the North Pole.
Longitude tells us how far west or east a location is from the prime meridian, where the longitude is 0°. The Greenwich Meridian, which runs through the Royal Observatory Greenwich in London, is the prime meridian. Because the Earth is a sphere, there are 360 degrees of longitude. These range from 180° East to 180° West.
We can describe the location of anywhere in the world using latitude and longitude. For example, the centre of Liverpool has a latitude of 53.3° North and a longitude of 2.8° West. We can write this as 53.3°N, 2.8°W.
Altitude and Azimuth
The Altitude-Azimuth (Alt-Az)system tells us the vertical and horizontal positions of objects in the sky. This system is fixed to the Earth and not the stars. Because the Earth rotates we can only give an object's Altitude-Azimuth position for a particular moment in time. It also depends on where on the Earth the observer is. Altitude and Azimuth are measured as angles in degrees.
- The altitude (Alt) tells you how far the object is above the horizon (green curve in image). An object on the horizon has an altitude of 0°. An object directly above you has an altitude of 90°.
- The azimuth (Az) tells you where the object is in your 360° field of view (red curve in image). We measure it starting from north and moving clockwise through east, south, west and back to north. So an object to the north has an azimuth of 0°. An object to the west has an azimuth of 270°.
Two parts of the Alt-Az system have special names:
- The zenith is the point right above the observer. This is also the point that is 90° above all points on the horizon.
- The observer's meridian is the curved line running from north to south through the zenith.
The altitude is useful for finding the times when an object is visible in the night sky. You have the best chance of viewing an object when it has an altitude of at least 30°. This is when the astronomical seeing is best.
You can also use the Alt-Az system to work out when an object will rise and set in the sky. When an object's altitude is 0°, it is either rising or setting on the horizon. Objects rise in the east, pass through the meridian and then set in the west.
The celestial poles are the imaginary points above the Earth's north and south poles. Stars near the celestial poles do not set below the horizon. They just appear to circle the pole during the night. The altitude of the celestial pole changes depending on where you are in the world. If you were standing on the Earth's geographic North Pole, the north celestial pole is right above your head. If you were standing at the equator, it is on the horizon.
Right Ascension and Declination
This system is known as the Equatorial system. It uses celestial coordinates to assign a unique position to each object in the night sky. These coordinates do not depend on the location of the observer or on the time of night of the observation.
To use this system, it’s useful to think of the Earth as at the centre of a huge sphere. This sphere is called the celestial sphere. Then we can imagine that all the objects we see in the night sky are drawn onto the celestial sphere.
The Equatorial system relies on two main ideas:
- An imaginary circle around the celestial sphere, called the celestial equator. All points on the celestial equator lie in the same plane as the Earth’s equator.
- A fixed point in space, called the vernal equinox. It is also known as the first point of Aries. This is location of the Sun on the celestial sphere at the spring equinox (around 21st March). It is also the point where the ecliptic (an imaginary line which shows the apparent path of the Sun) crosses the celestial equator.
The Equatorial system uses coordinates called right ascension and declination. These are often shortened to RA and Dec.
- The declination measures how far an object is above or below the celestial equator. It is an angle, measured in degrees and ranges from -90° to 90°.
- The right ascension measures the angle around the celestial equator. The angle starts at the vernal equinox and stops at the point on the celestial equator closest to the object. The RA angle is measured in hours instead of degrees and ranges from 0 to 24 hours. Each hour of RA contains 15 degrees.
Smaller amounts of RA and Dec can be expressed in arcminutes and arcseconds. There are 60 arcminutes in 1 hour of arc. There are 60 arcseconds in one arcminute.
Example: how do we write the RA and Dec of the bright star Rigel?
RA = 05 14 32 or 05 hours 14 minutes 32 seconds.
Dec = -08 12 06 or -08° (negative value means south of the equator) 12' (arcminutes) 06" (arcseconds).
The Earth's orbit changes over long periods of time. This means that the RA and Dec of an object varies each year. When astronomers refer to Equatorial coordinates, they state which epoch was used to locate the object. In other words, they give the position of the object on a particular date. The current epoch is called J2000.0. This means the RA and Dec of celestial objects are given relative to their positions in the year 2000.