Asteroids have irregular shapes and usually rotate with some period P (called the length of the asteroid's day). When an irregularly shaped object rotates, it will reflect different amounts of light as time goes on, so the brightness of the point of light observable will change with time, depending on the observable area. Time series measurements of the asteroid's brightness variations produce what are usually called light-curves.


The motion of asteroid Geographos combined with its light curves variations.

In other words, as the asteroid rotates it is sometimes possible to see a lot of its surface while other times, only a little portion of its surface can be seen. So the solid curve at right, which is the plot of total brightness vs. time, goes up and down. Graphs of this sort are called light curves. The time it takes for a lightcurve to start repeating is the length of the asteroid's day, called its rotation period. The lightcurve amplitude (how much the curve goes up and down) gives us some information about the asteroid - in other words, it tells us something about how stretched out into a cigar shape the asteroid is. The lightcurve of a sphere would not go up and down at all, so any lightcurve variations immediately tell us that the asteroid is non-spherical and/or possibly has surface features.

Adapted from Spaceguard Science pages (


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