Determining Brightness using the Magnitude System


In astronomy the brightness of any star is measured using the magnitude scale. This method was devised originally by the Greeks, who classified the stars as first magnitude (brightest) to sixth magnitude (dimmest). This rough method was altered in the 1800's so that magnitude (mag.) 1 stars were defined as being exactly 100 times bright than mag. 6 stars. Thus magnitude could be expressed as varying logarithmically with the stars brightness. With the advent of accurate modern photometry the scale was extended in both directions.

Thus at one extreme the Sun is magnitude (mag.) -27 and some of the faintest observed stars around mag. +24. The full moon is mag. -12.5, Sirius the brightest star in the night sky mag. -1.5, whilst the faintest stars visible to the naked eye under good conditions are around mag. +6.

The advantage of this method is of course that the stars are readily at hand for comparison with a satellite (given knowledge of stellar magnitudes).

Further discussion of the magnitude value for brightness can be found on the Bright Satellites & Resources page.

Determining a Satellite's Magnitude

It is useful to know some stellar magnitudes in order to estimate the brightness of a satellite during a pass. This also helps in judging the current viewing conditions; it is no good looking for a mag. +5 satellite if atmospheric conditions limit you to seeing only down to mag. +3.

A quick guide to conditions and satellite brightness may be gleamed from examining a suitable constellation. In the Northern hemisphere Ursa Minor (the 'Little Bear') is ideal. Circumpolar and thus often visible, it contains stars covering magnitudes +2 down to +6. Brighter apparitions can be gauged by spotting some of the more brilliant stars (Sirius, Altair, Vega, Deneb, etc).

[Northern sky magnitude guide]This diagram indicates the magnitudes of the component stars of Ursa Minor; an excellent guide to seeing conditions.

The closest equivalent to Ursa Minor in Southern skies is Crux. Similarly circumpolar it contains stars ranging from mag. +0.8 to +6.5.

[Southern sky magnitude guide]This diagram indicates the magnitudes of the component stars of Crux as a guide to seeing conditions.

The Binocular Aided Eye

Naturally use of binoculars or a telescope improves the viewing; much fainter objects can be seen at the expense of field of view. The larger the aperture of the instrument, the fainter the satellite that can be seen. As a rough guide a decent 50mm pair of binoculars will take you from the naked eye limit of around mag. +6, to about mag. +10 (in suitable skies). A six or eight inch reflector telescope will stretch this further to around mag. +14.

Here again, acquaint yourself with a suitable starfield (naturally you will need a good star atlas this time) in order to determine the seeing conditions. By noting down the path of the satellite through your field of view, and detailing stars of similar brightness, you can refer back to the atlas once more in order to figure out the satellite's magnitude after the pass.

Of course following a satellite in a large telescope needs a motor driven mount, with accurate co-ordinates for the pass. Even when using valid, up-to-date USSPACECOM elements, the tracking error can amount to up to one degree. This is even without considering the maneuvering that the shuttle will perform regularly. Still, images can be obtained.

Links to the: VSO Home Page, Observing Guide, and Satellite Predictions