Sidereal Time

Sidereal Time

Jason Harris

Sidereal Time literally means “star time”. The time we are used to using in our everyday lives is Solar Time. The fundamantal unit of Solar Time is a Day: the time it takes the Sun to travel 360 degrees around the sky, due to the rotation of the Earth. Smaller units of Solar Time are just divisions of a Day:

However, there is a problem with Solar Time. The Earth doesn't actually spin around 360 degrees in one Solar Day. The Earth is in orbit around the Sun, and over the course of one day, it moves about one Degree along its orbit (360 degrees/365.25 Days for a full orbit = about one Degree per Day). So, in 24 hours, the direction toward the Sun changes by about a Degree. Therefore, the Earth only has to spin 359 degrees to make the Sun look like it has traveled the full 360 degrees across the Sky.

In astronomy, we are concerned with how long it takes the Earth to spin with respect to the "fixed" stars, not the Sun. So, we would like a timescale that removes the complication of Earth's orbit around the Sun, and just focuses on how long it takes the Earth to spin 360 degrees with respect to the stars. This rotational period is called a Sidereal Day. On average, it is 4 minutes longer than a Solar Day, because of the extra 1 degree the Earth must rotate to make a full 360-degree spin. Rather than defining a Sidereal Day to be 24 hours, 4 minutes, we define Sidereal Hours, Minutes and Seconds that are the same fraction of a Day as their Solar counterparts. Therefore, one Sidereal Second = 1.00278 Solar Seconds.

The Sidereal Time is useful for determining where the stars are at any given time. Sidereal Time divides one full spin of the Earth into 24 Sidereal Hours; similarly, the map of the sky is divided into 24 Hours of Right Ascension. This is no coincidence; Local Sidereal Time (LST) indicates the Right Ascension on the sky that is currently crossing the Local Meridian. So, if a star has a Right Ascension of 05h 32m 24s, it will be on your meridian at LST=05:32:24. More generally, the difference between an object's RA and the Local Sidereal Time tells you how far from the Meridian the object is. For example, the same object at LST=06:32:24 (one Sidereal Hour later), will be one Hour of Right Ascension west of your meridian, which is 15 degrees. This angular distance from the meridian is called the object's Hour Angle.


The Local Sidereal Time is displayed by KStars in the Info Panel, with the label ‘ST:’. Note that the changing sidereal seconds are not synchronized with the changing Local Time and Universal Time seconds. In fact, if you watch the clocks for a while, you will notice that the Sidereal seconds really are slightly longer than the LT and UT seconds.

Point to the Zenith (press Z or select Zenith from the Location menu). The Zenith is the point on the sky where you are looking ‘straight up’ from the ground, and it is a point on your Local Meridian. Note the Right Ascension of the Zenith: it is exactly the same as your Local Sidereal Time.

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