Sidereal Time

Sidereal time is time kept with respect to the distant stars.


Because the Earth moves in its orbit around the Sun, the Earth must rotate more than 360 degrees in one solar day .

  • A solar day lasts from when the Sun is on the meridian at a point on Earth until it is next on the meridian. A solar day is exactly 24 hours (of solar time). Because of the Earth's revolution, a solar day is slightly longer than a sidereal day. In every day life, we use solar time.

  • The Earth must rotate an extra 0.986 degrees between solar crossings of the meridian. Therefore in 24 hours of solar time, the Earth rotates 360.986 degrees.

  • Because the stars are so distant from us, the motion of the Earth in its orbit makes an negligible difference in the direction to the stars. Hence, the Earth rotates 360 degrees in one sidereal day .

  • A sidereal day lasts from when a distant star is on the meridian at a point on Earth until it is next on the meridian. A sidereal day lasts 23 hours and 56 minutes (of solar time), about 4 minutes less than a solar day.

  • Can you figure out where this value comes from?


  • Likewise the Earth moves in its orbit around the Sun, the Moon must rotate more than 360 degrees in one synodic lunar month .

  • A synodic month is the time from new Moon to new Moon. Because of the Earth's revolution around the Sun, the Moon must orbit more than 360 degrees until it is again in direct line with the Sun.

  • Can you figure out where this value comes from?


  • [back to the topics page] [back to astro 201 home page] [back to astro FAQ home page] [back to current A201 FAQ home page]