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How Astronomers Describe the Position of Stars

Let us not get too complicated. Read the notes below and if you wish to learn more start looking up some of the suggested terms in the last paragraph on Google to understand this better.

The simple position on the sky using angles is called the azimuthal coordinates. Go out and look at the stars and you can easily describe the position of any star by making three imaginary lines.

1. From you to the star 2. From the star to the horizon directly under the star 3. From you to the horizon due north of you

First there is the altitude of the star or alt and it is measured in degrees. Think of the angle made by pointing one arm to the star and one arm to the horizon directly below the star. The angle between your arms is then the altitude or alt of the star in degrees. If the star is on the horizon then its altitude is 0 degrees, if it is directly above your head, the place we call the zenith, then its altitude is 90 degrees and if it is half way to the zenith it is at 45 degrees.

If you are looking at the pole star then its altitude is the same as your latitude on the surface of the Earth in degrees. So if I stand in my garden in Bradford which is about 54 degrees north in latitude on the Earth’s surface, then the pole star is 54 degrees from the horizon or 54 degrees in altitude.

So you’ve got altitude!

The next is called azimuth which is a measure in hours or degrees ("What!!", I hear you say, "have hours to do with degrees?", I will come back to that in a minute, let us stick with degrees first.) Start off with your line to the horizon due north of you and then turn around clockwise by starting to move towards the East and carry on until you get to the line to the horizon which is under your star. You will have then turned through a number of degrees which is the azimuth of your star. Thus if the star is in the east then the azimuth is 90 degrees. If it is in the south then the azimuth is 180 degrees, and if it is in the west then the azimuth is 270 degrees.

There you have azimuth! - Oh, apart from the hours!

Well astronomers are a bit lax about degrees and hours. The azimuth of the Sun changes by 15 degrees every hour and so we convert degrees into hours by dividing by 15. Of course the Sun is not moving it is the Earth that is spinning 15 degrees every hour and so a star in the West has an azimuth of 6 hours, one in the south 12 hours and one in the west 18 hours.

So now you have an immediate description of the position of a star but it is not much use because by the time you have followed all this the star will have moved. (15 degrees per hour or nearly that because during that hour the Earth will have moved a little around the Sun as well and so the Sun appears to travel further than the stars each day - about four minutes - sorry one degree --- and there you have another clue one degree per day and 360 degrees in a year --- well even the ancient Babylonians thought 360 was a much neater number than 365¼). If you really want to check if you have this all correct think how it should be worded for people living in the southern hemisphere where we exchange the south for the north and the direction clockwise doesn’t seem to work.

The simple position on the sky with respect to all the other stars - equatorial coordinates

The stars are fixed in their positions on the sky or the celestial sphere. The equatorial coordinates or Right Ascension and declination coordinates are fixed for each star and do not vary with time. Within that coordinate system various motions takes place including the Earth spinning on its axis every day and the Earth orbiting around the Sun every year.

There are two planes and one direction that help you with visualising these coordinates.

1. The direction is the north pole which is directly over your head if you stand at the north pole of the Earth. It is an extension of the axis of rotation of the Earth onto the celestial sphere. It is a point quite close to the pole star 2. One plane is the plane that the Earth’s equator makes if it is extended into the sky This is called the celestial equator 3. The other plane is defined by the path the Sun makes through the stars and is called the ecliptic

These two planes cross each other one where the Sun moves from south to north around the 21st March each year called the vernal equinox and the other when the Sun moves from north to south around the 21st September each year called the autumnal equinox. Where the planes cross produces a line which goes out into space and produces two points when we look from the Earth to the celestial sphere. For our purpose we have an imaginary point on the celestial sphere where the two planes cross for the vernal equinox and this is called the first point of Aries and it is the zero point for measuring angles in degrees (or hours) around the equator.

In this way every star position is defined by a couple of angles which can be visualised using three points on the celestial sphere. The star itself, the pole and the point where the shortest line across the sky from the pole to the star continues on and crosses the celestial equator. This point where the line through the star crosses the celestial equator defines the Right Ascension of the star which is measured moving eastwards (clockwise in the north) from the first point of Aries towards that point.

Measurements of angles are given in degrees with each degree divided into 60 minutes and each minute divided into 60 seconds. The atmosphere normally limits us seeing anything smaller than a second of arc even with large telescopes. The Bradford Robotic telescope produces good images which are centred around one pixel that is 1.4 arc seconds across in Galaxy cam and more in the other cameras.

Measurement of angles in hours similarly has each hour divided into 60 minutes and each minute into 60 seconds. Measurement of right ascension is normally in hours minutes and seconds and declination is in degrees minutes and seconds and clearly you must look whether the angles are in hours or degrees since the minutes and seconds are very different angles.

The descriptions above are simplified from the real position. The plane of the ecliptic precesses about 1 degree every 71.6 years, this means that the position of the pole describes a circle on the celestial sphere which takes 25,800 year for one loop. The equatorial coordinates change due to precession and they are given for a particular epoch. Most astronomers currently use year 2000 coordinates as the epoch. Before that they used 1950 as the epoch for the coordinates.

There are other changes in the positions of stars due to nutation, aberration, parallax, proper motions and refraction. There are also other systems of coordinates but the above is enough for a start.

By John Baruch, 10th August 2005

University of Bradford
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