What is Sidereal Time?
I have read about different times called sidereal time, etc. I am a bit confused about all of it. Could you please enlighten me on it? Mean time is the sort of time we’re used to, where a day is 24 hours, the time it takes for the Sun to complete one trip around the sky and return to its original position. Sidereal time is measured according to the positions of the stars in the sky. A sideral day is the time it takes for a particular star to travel around and reach same position in the sky. A sidereal day is slightly shorter than a mean day, lasting 23 hours, 56 minutes, and 4.1 seconds. A sideral day is divided into 24 sideral hours, which are each divided into 60 sidereal minutes, and so on. The reason that sidereal days are shorter is that while the Earth rotates on its axis, it is also moving around the Sun. Both motions are counter-clockwise as viewed from the north pole. You may find it helpful to draw a diagram. The sun can be represented with a point. Draw the Earth. Let it be
Well, the earth spins on its own axis once every 24 hours. It also goes around the sun once every 365 days. The effect of the two motions is that whilst the sun appears to go around our sky once every 24 hours, the stars go round in only 23 hours 56 minutes. Sidereal time is where the length of each second is adjusted so that the stars go round in 24 hours according to the sidereal clock, i.e. gaining time at a rate of 1 in 365. (Actually, 1 in 365.25, because the earth takes 365.25 days to go round the sun, not 365, hence leap years every 4 years). In 1987 there was an article in the journal of the British Astronomical Association about a modification made by John Watson to an ordinary alarm clock (see below). The strategy was to count 1421 fiftieths of seconds, then add 4 all at once. This causes the clock to gain at the required rate of 1 in 365.25. The way I made the clock, there is a small red wire inside which can be connected diffently to switch to ordinary time (this is shown i
Sidereal time refers to the length of days and other measurements of time with respect to stars other than our sun. Whereas a solar day lasts 24 hours, a sidereal day is about four minutes shorter. In everyday life, solar time is always used, but astronomers often find it useful to use sidereal time when expressing the apparent movement of stars in the sky, as observed from earth. The word “sidereal” comes from the Latin word “sider,” meaning “star.” Because the earth orbits around the sun as it rotates, the earth must rotate slightly more than one full rotation on its axis to complete one solar day. In one day, the earth moves through a small portion of its orbit around the sun, slightly changing the apparent angle at which the sun faces the earth. This small addition of about one degree to the angle of the sun means that the earth must rotate for an additional four minutes to “catch up” to where the sun was shining precisely one rotation ago. However, because stars other than our sun
—————————————————————– [ root | Back to sidereal Time Calculations ] Imagine the equator and poles of the Earth projected onto the Celestial Sphere. Each instant, stars move from East to West across the sky. Most are on small circles parallel to the Celestial Equator. For instance, Mintaka in Orion’s belt happens to be almost exactly on the equator, and if you follow the Hunter’s progress tonight (I’m writing this in January) this star will trace out the Celestial Equator. The Earth’s axis is tilted at about 23.5 degrees to the plane of the Earth’s orbit around the Sun – and the path of the Sun in the sky is called the Ecliptic. The Equator and Ecliptic are ‘great circles’ – the planes that contain them pass through the centre of the Celestial sphere. The two circles intersect at two points on the Celestial Sphere, and the Sun passes through one of the intersection points around March 21st each year. At this vernal equinox, the Sun has a decli
Sidereal time is a measure of the position of the Earth in its rotation around its axis, or time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical to, the motion of stars. They differ by the precession of the vernal equinox in right ascension relative to the stars. Earth’s sidereal day also differs from its rotation period relative to the background stars by the amount of precession in right ascension during one day (8.4 ms).[1] Its J2000 mean value is 23h56m4.
