The Night Sky

The Night Sky


Astronomy Coordinate Systems

It is helpful (though incorrect) to imagine all the stars to be fixed onto the inside of a vast crystalline dome - the celestial sphere.

Absolute distances between stars are of less importance than their angular separation in the sky. Adopting the Babylonian system of angular measure, the distance from horizon to zenith (directly overhead) is 90°. From North to South along the horizon is 180°.

The constant proportions of the human body give fairly accurate references that you can use to estimate angular separations in the night sky:

Held at arms length

The Azimuthal angle is the number of degrees measured clockwise from True North to the star's position projected onto the horizon (ranging from 0 to 360°).

 The Altitude of the star is its elevation in degrees from the horizon (ranging from -90° to +90°).

This co-ordinate system can only be used if the precise time is also stated, since stars move about the observer.


The Magnitude System

The most obvious thing about stars is that they are all different brightness or apparent magnitudes. We must distinguish this from the absolute magnitude of the star (the brightness if it were exactly 32 light year away). It is tempting to believe that the fainter stars are the most distant and the brighter ones are close by. This is untrue, in fact Sir Arthur Eddington stated that most of the bright stars we can see are the "Whales among the fishes". There are many nearby stars that are too faint too pick out.

In 130 B.C. Hipparcos devised a scale of apparent magnitude where the brightest stars were of the 1st magnitude (1M) and the faintest visible with the naked eye were 6th magnitude. It so happens that a 1M star is 100 times as bright as a 5M star. Since this is a logarithmic scale, the difference between one magnitude and the next is nearly 2.5 times

The limit of naked eye visibility is 6.5M.

 10 x 50 binoculars can show stars down to 9M

 Larger telescopes can detect greater magnitudes.

 The scale is extended backwards for objects brighter than 1M. For instance

• Vega in Lyra is 0M

• Sirius is mag -1.4M

• Venus attains -4.4M

• A Full Moon is -12.7M

• The Sun is -26.7M

Larger telescopes can detect greater magnitudes.


Some of the brighter stars appear to form groups in the sky, these we call constellations. Most constellations were named a very long time ago by the Greeks or Arabs. People thought they could see the shapes of animals or their gods and named the constellations after them. In most cases it is very hard to imagine how they saw the shape that the star pattern is supposed to represent but we still use the same names today.