The way astronomers measure the brightness of stars and other objects in the sky is not very logical and can cause confusion. You would expect to give a big number to something that is bright, a little number to something that is small or insignificant. But on the magnitude scale, the LOWEST numbers refer to the BRIGHTEST objects. This is not sensible, but we are stuck with it!

How did such a silly system evolve? Even today we describe people as coming first in a race when we mean they ran the fastest. The person with the highest speed has the lowest position! It was the same with noble people in Greece around 120 BC. The greatest people in the land were ranked first, they led processions and had priority in all things. People of lesser importance had second, third and higher rank. So when the Greek astronomer Hipparchus ranked stars he put them on a magnitude scale from "first" for the brightest stars to "sixth" for those just detectable in a dark sky by the unaided eye.

Around 1850 astronomers wanted to be able to give a definite magnitude to each star. N. R. Pogson proposed that a 6th magnitude star should be 1/100 of the brightness of a star of the 1st magnitude. To keep the steps between magnitudes equal, this meant that stars of the 2nd magnitude are about 2/5 as bright as a star of the 1st magnitude.

Now the question arose, which star has the first magnitude? One of the stars near the north pole of the sky was chosen, along with a group of about 100 others covering a wide range of brightness, to be used as standards for the astronomical magnitude scale. They were chosen so they can be seen for comparison purposes at any time of the year from northern telescopes.

Unfortunately this introduced yet another complication! To include very bright objects, the scale had to be extended to zero, and then to negative numbers! So a star of magitude 0 is 2.5 times as bright as a star of magitude 1, and a star of magnitude -1 is 2.5 times as bright again.

The Sun's magnitude is -26.7, the full Moon is about -12, Venus at its brightest is -4.7 and the bright star Sirius is -1.5. The faintest stars visible through the largest telescopes are of (approximately) apparent magnitude 20.

Absolute magnitude
The apparent magnitude discussed so far, that is the brightness as seen from the Earth, depends on both the intrinsic luminosity and the distance of objects. Absolute magnitude on the other hand is a measure of intrinsic luminosity, regardless of how far away objects are. This is measured by imagining that all objects are brought to the same distance from the Earth and then measuring their magnitude.

This standard distance is 10 parsecs (32.6 light-years). The Sun's absolute magnitude is 4.8.

Visual magnitude
The magnitude of an object depends upon which type of radiation you use to look at it. Visual magnitude is the magnitude measured when using the light to which the human eye responds. Photographic magnitude is the magnitude measured by a standard photographic emulsion, which responds chiefly to the blue and violet part of the spectrum (although different photographic materials have very different colour responses). Bolometric magnitudes use all radiation, both visible and outside the visible range.

The term "apparent magnitude" is usually taken to mean apparent visual magnitude, the relative brightness as seen by the human eye.