Diurnal is something that happens daily or an event that happens once every 24 hours. It also refers to something, such as an organism, that is active during the day. Diurnal motion is the apparent daily motion of celestial bodies across the sky from east to west, including the rising and setting of the Sun, caused by Earth’s rotation. A diurnal circle is the apparent curving track of a celestial object relative to the horizon. For diurnal aberration see aberration of starlight, for diurnal libration see libration, and for diurnal parallax see geocentric parallax.