We have already looked at the Celestial Sphere with the Celestial Equator, celestial Poles, Ecliptic and and the equinoxes and solstices. The stars are mapped onto this imaginary sphere. That is convenient for defining their position in the sky.
This way of looking at the Earth and how we see the stars is a Geocentric view, similar to the old Geocentric cosmological model used for many centuries. We now know that the Sun is at the centre of the Solar system, but this “old-fashioned” geocentric view is still convenient for our discussion of the night sky and defining positions of celestial objects.
The co-ordinates of objects on the celestial sphere are expressed in Declination and Right-Ascension. These are analogous to longitude and latitude we use to identify positions on Earth.
Where we are in New Zealand we have a latitude
of about 43 degrees South and a longitude
of 172 degrees East of Greenwich.
Image source unknown (edited)
Latitude measures the number of degrees north or south of the Equator. Stars have a north-south position called the Declination which is similar to the idea of latitude. The declination of a star is the number degrees north or south of the Celestial Equator.
Polaris (near the Celestial North Pole) has a declination of almost 90º N. Stars over the equator have a declination of 0º.
Longitude measures the number of degrees east or west of the Greenwich meridian. Stars have an east-west position called the Right-Ascension which is similar to the idea of longitude. Right ascension is the angle eastward towards the star measured from the Vernal equinox. Right ascension is measured in the same direction as the rotation of the Earth and is therefore essentially a time measure. It is expressed in hours, minutes and seconds. One complete circle corresponds to 24 hours, equal to 360 degrees.
The right-ascension of a star is however independent of the rotation of the earth. Because it is connected to the Vernal Equinox, it is an angle that is fixed with respect to the stellar background. The value of Right Ascension and Declination for a star is practically constant, it only changes noticeably over ten ’s of thousands of years (as we discussed for constellations). These star co-ordinates Declination and Right-Ascension are listed in star catalogues.
Star charts, Planisphere
For amateur astronomers star charts are very useful to find stars and constellations and to become familiar with the night sky. Sky Charts are available in many forms and editions and are generally a good investment.
A practical tool based on a star chart is the planisphere, that allows you to display the part of the night sky that is visible for the location at any time. The planisphere consists of two disks that rotate about a common pivot. Star charts and Planispheres are available commercially, although you can find free versions on the internet to print yourself. A good location to download templates for making your own planisphere can be found here.
There are also many computer programmes available that display the night sky at any time and any location on Earth. Such planetarium software is commercially available as well as in the form of shareware or freeware.
A good place to find such software is here.