• See also definitions, below

Greenwich Siderial Time at 0h UT for selected dates

South Africa takes its time from 30ºE, i.e. we are 2 hours ahead of Greenwich. So, if we know the sidereal time at Greenwich, we can add 2h to get the sidereal time for our time zone's standard meridian. This is approximately correct for nearby longitudes. If you want to have a more accurate result, use your exact longitude. The table below gives approximate sidereal time for Greenwich for certain dates. Due to corrections for leap days, for example, it can only be considered accurate to about two minutes, which is adequate for most purposes. Interpolate between these dates by applying a correction of 3.9 minutes for each complete day. For greater accuracy, consult a time service (see resources), an almanac or a good astronomy program.

Definitions

The meridian for a given observing location is the imaginary "line" in the sky passing through the poles and the zenith, the point directly overhead. The meridian is a line of longitude; it goes directly North/South through the observer's position. A celestial object culminates when it is highest in the sky for any given day. This occurs when the object transits, i.e. passes the meridian.

Objects that culminate on or about midnight for any given observing session are well placed for observation, as they rise at approximately sunset and set at approximately sunrise, giving you the whole night to view them. As they culminate, you will be viewing them through the thinnest air mass (the shortest path through our atmosphere) possible for your location, thus minimising the deleterious effects of the atmosphere. 

A sidereal day is the time taken between successive culminations of any given star, and represents the time taken for the Earth to rotate once about its axis. It is approximately 3m55.91s shorter than the mean solar day, i.e. the time between successive culminations of the Sun, this being the period on which our civil time system is based. This difference is caused by the Earth's orbital movement around the Sun. The result is that the night sky appears to have moved approximately one degree for each solar day that passes, hence the apparent gradual march of the constellations across the sky from season to season.

The terrestrial coordinate system used for mapping the earth, divides the planet into lines of longitude and Latitude. Lines of longitude are "great circles", as they run completely around the circumference of the earth. They are evenly spaced a degree apart at the equator and, since they run directly north-south, converge at the poles, where they intersect. Zero longitude runs through Greenwich in England; you measure your longitude East or West with respect to Greenwich.

Latitude is measured in degrees North of South of the equator. Only the zero reference, the Equator, is a great circle. As one moves away from the equator and towards a pole, the diameter of a circle of latitude gets smaller, as they represent parallel slices through the sphere of the Earth. They too are measured in degrees from the equator, the North pole being at +90 degrees latitude, and the South pole being at -90 degrees.

Celestial coordinates are similarly arranged. Right Ascension corresponds to Longitude and Declination to Latitude. However, while declination is measured in degrees, minutes and seconds north or south from the celestial equator, right ascension is measured in hours, minutes and seconds. This is because it is convenient to take into account the rotation of the earth. The Earth rotates 360 degrees in 24 hours, i.e. 15 degrees in 1 hour and 1 degree in 4 minutes. The 0h position in the sky is also known as the first point of Aries.

The local sidereal time at any given instant is simply the right ascension that is culminating at that time. Just as solar (and thus civil) time varies with longitude, so does sidereal time. Again, the time at Greenwich is taken as a standard reference. Because of the difference between solar and sidereal days, the sidereal time at local midnight will also drift with the seasons.

By knowing your longitude, and the sidereal time at Greenwich, you can easily calculate your local sidereal time. These days, that kind of thing is relegated to computer programs, which also incorporate all sorts of little corections arising from slight inconsistencies in the Earth's motion (such as precession and nutation) as well as major adjustments arising from the peculiarities of our calendar ('leap" days or seconds, for example). A complete discussion is obviously beyond the scope of this site. For general planning of the average amateur's observing session, it is usually good enough to know the approximate sidereal time.

On this site, our tables are arranged according to when in the year the objects of interest will culminate at approximately midnight, so as to know what is readily accessible for observation at a given time of year.

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