Photometry is often used to study the
brightness variations of asteroids over a period of time, obtaining curves where
these variations of brightness are represented: the lightcurves.
But what is exactly a light curve? First of all, it is important to know that asteroids have irregular shapes and usually rotate with some period P, 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 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.