Sidereal Day
A sidereal day is approximately 23 hours, 56 minutes, 4.091 seconds. That corresponds to the time it takes the Earth to complete one rotation relative to the vernal equinox. Since the vernal equinox precesses (every 26,000 years) in a westward direction relative to the fixed stars, the sidereal day is some 0.008 seconds shorter than the Earth’s period of rotation relative to the fixed stars. The longer true sidereal period is called a stellar day.
You may wonder what the point of knowing what a sidereal day is. Well, sidereal time is a system astronomers use to keep track of the direction to point their telescopes to view a star. From a given observation point, a star found at one location in the sky will be found at the same location on another night when observed at the same sidereal time. Solar time and sidereal time use the regularity of the Earth’s rotation about its polar axis, except that solar time maintains orientation to the Sun while sidereal time maintains orientation to the stars in the night sky. The exact definition of sidereal time fixes it to the vernal equinox, so sidereal time correlates closely to the inertial rotation of the Earth, whereas common time on a typical clock correlates to a slightly faster rotation, accounting not only for the Earth’s axial rotation but also for the Earth’s annual revolution around the Sun of about one degree each day.
All the solar planets beyond Earth are similar to Earth in respect to the sidereal day. They experience many rotations per revolution around the sun, so there is only a small difference between the length of the sidereal day and that of the solar day. The case is quite different for Mercury and Venus. Mercury’s sidereal day is about two-thirds of its orbital period, so(using the prograde formula) its solar day lasts three times as long as its sidereal day. Venus rotates retrograde with a sidereal day that lasts about 243.0 Earth days, so by the retrograde formula. its solar day is about 116.8 Earth-days.
Observing a star is easy as long as you know the right ascension, declination, and are able to observe it at the same hour of the sidereal day each time you look for it.
We have written many articles about sidereal day for Universe Today. Here’s an article about how long a day is on Earth, and here’s an article about sidereal time.
If you’d like more info on Earth, check out an article about Telling Time by Stars. And here’s a link to an article about Sidereal Time.
We’ve also recorded an episode of Astronomy Cast all about Solar System Movements and Positions. Listen here, Episode 171: Solar System Movements and Positions.
References:
NASA: Reinventing Time
NASA: Venus Litograph
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