Posts Tagged ‘magnitude’


Albedo of an astronomical body

A measure of how reflective a body is. Albedo is expressed as a percentage, the higher the percentage, the higher the albedo and therefore the more reflective the object is.

On the face of it, albedo is a straightforward thing, however, the albedo of a planet varies from place to place. Dark surfaces absorb more light than light surfaces. Rough surfaces scatter light in all directions and therfore reflect less back to the observer. Thus when talking about the albedo of an astronomical body, one generally means the average albedo.

Cloudy planets like the gas giants and Venus have high albedos because clods are good reflectors whilst the rocky planets have lower albedos.

The light reaching your eye from an object is reduced thr further it has to travel so the albedo at the edge of a planet is less than at the centre, assuming the same surface composition. Accurate measurements of albedo need to take this into account and so there are two types of albedo, spherical and geometrical. The latter assumes the planet, asteroid, moon or whatever is a uniform sphere whilst the latter compares the reflecting power of the object with that of a flat white disc of the same diameter and distance as the object.

Absolute Magnitude

The absolute magnitude of a star is the brightness that a star would appear if it was at a distance of 10 parsecs from the Earth. It is a very convenient way of comparing brightness of different stars as it is a standardised measure.

The Sun, as our nearest star, is also the visually brightest, however, when compared to other stars by using the absolute magnitude scale, it is fairly faint – it has an absolute magnitude of 4.8, faintly visible to the naked eye buch a lot dimmer than we see the stars of the Plugh constellation.

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