In the study of planets (planetology), a light curve can be used to work out the rotation period of a minor planet, moon, or comet nucleus. From the Earth many objects are so small that even the most powerful telescopes are unable to see objects clearly. Because of this astronomers measure the amount of light produced by the object over a period of time, its light curve. The time between the peaks on the graph gives the rotational period of the object. The difference between the maximum and minimum brightness, the amplitude of the light curve, can be caused by either the shape of the object, or bright and dark areas on the surface. For example, an odd shaped asteroid's light curve generally has more pronounced peaks, while a more spherical object's light curve will be flatter. When the light curve covers a long period of time, it is called a secular light curve.