What is Sidereal Time?
Sidereal Time is a convenient way for astronomers to keep time since it’s based on the daily and nightly motion of the stars across the sky, rather than the Sun. The time on your wristwatch or the clock is based on the Solar Day- Civil Time is one way to refer to this timekeeping method. Like the Solar Day, Sidereal Time is divided up into 24 hours, each with 60 minutes. Each of these minutes have 60 seconds. However, the Sidereal Day is a few minutes shorter than the Solar Day! That’s why Sidereal Time and Civil Time almost never match.
In Civil Time, a Solar Day starts at midnight and if you go outside right at Noon and look South, you will (more or less) see the Sun cross your local meridian. Twenty-four hours later if you go outside you’ll see the Sun cross at (more or less) the same time. It might not cross exactly at Noon from your location because there are all these things like the how far East or West you are in your time zone, Daylight Savings or Summer Time, or how far along the Earth has moved in its yearly orbit around the Sun. But, for our purposes let’s just say you are somewhere the Sun is crossing your meridian right at Noon by the Civil Time on your watch.
Let’s say instead of a brilliantly glowing orb the Sun was transparent and you could see stars around it in the middle of the day. You might be able to pick out a bright star that’s right in your line of sight, behind the Sun. If you go out exactly 24 hours later and look for the Sun, the star won’t be directly behind it now. It will be just a little to the West. In fact, all the stars will be shifted just a little to the West compared to the same time yesterday! Why is this?
At Noon, viewer sights along a line that goes through the local meridian and the Sun and sees a distant star.
Picture looking down on our Solar System at the North Pole of the Sun and the Earth. Every day, the Earth spins counter-clockwise exactly once (not really, but very very close). While it’s doing this, it’s moving counter-clockwise around the Sun on its yearly orbit. Yesterday when you went out exactly at Noon you could see the Sun and a distant star exactly lined up. Twenty four hours went by and now you’re seeing the Sun in exactly the same place in the sky, because the Earth moved along its orbit and turned just a little past the line of sight where you could see that star yesterday- it and all the other stars in the heavens shifted West like they do on their yearly cycle through the seasons.
Exactly 24 hours later...the star is so far away it might as well have stayed in place compared to where the Sun appears as the viewer again sights along the local meridian.
Forget about the Sun for a minute and just pick any star and watch it cross your local meridian at Noon or at any other time. When you go out the next day and see that star cross your meridian, that would have been exactly one Sidereal Day. The Sun won’t cross the meridian for another 4 minutes or so because we’re moving along in our orbit. It only took about 23 hours and 56 minutes (by Civil Time) for that star to swing around and cross your meridian.
Astronomers usually like to know where and when to point a telescope at a star or something so Sidereal Time is convenient for them. In fact, in the East-West direction the Celestial Sphere is divided up into 24 hours of Right Ascension, each with 60 minutes divided into 60 seconds. There is a place on the Celestial Sphere where the Sun crosses the Celestial Equator at the Vernal Equinox. This point is called the First Point of Aries and is at 0 hours, 0 minutes, and 0 seconds of Right Ascension. As you go further East along the grid system of the Celestial Sphere, Right Ascension counts up through the hours, minutes and seconds until you go all the way around the sky 23 hours, 59 minutes, and 59 seconds+ to the First Point of Aries.
Right Ascension and Sidereal Time are so conveniently tied together that if you look up a star on a chart, and wait until Sidereal Time is the same as that star’s Right Ascension, you’ll see it exactly cross your local meridian. Also, if you see some bright star cross your meridian, you can check the Sidereal Time and look up what star is at that Right Ascension. You can see why astronomers use Sidereal Time! They don’t have to worry about things like time zones, Daylight Savings or Summer Times, or whether it’s 12 midnight or 12 noon. They just have to know their Longitude and UTC.