A limb is the apparent edge of the Sun, the Moon, a planet, or any other body having a detectable disk. The leading limb of an object as it moves due to the diurnal rotation of the Earth is known as the preceding limb, while the trailing limb is called the following limb. The lower limb of an object (usually the Sun or the Moon) is the limb that is closer to the observer's horizon. The edges of the visible face of the Moon and of Venus are known as the limb regions.