For the Students of Hindu Vedic Astrology by Dr. Shanker Adawal

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What is 'Sidereal Time' and why it is different from clock time.

Sidereal time is ‘Significant’, we must consider during calculation. This is like Ascension, being measured along the celestial equator, but using hours, minutes and seconds instead of degrees, minutes and seconds.

The two terms are in fact convertible, either being employed according to convenience, right ascension being open expressed in hours and minutes although strangely enough. Well, ‘Sidereal time’ is never expressed in degrees. It is easy to convert one into the other like this….

1 Day = 360 degrees
1 House = 15 degrees
1 Minute = 15’
1 Second = 15”

In every observatory there is an ‘Astronomical Clock’, an accurate adjusted chronometer, regulated in a way like 24 hours – no more and no less, a one complete revolution of the earth.

Suppose, the moment of the Vernal Equinox, when the Sun has reached g0 °0’0” (an official commencement of the spring quarter) happen on March 21st each year. Let us suppose that at Greenwich Observatory, the Sun is found to be culminating, exactly on the meridian (m.c.), perpendicular to the east-west horizontal line. These are all different ways of saying the same thing. In other words, it is exactly the noon.

The sidereal clock will show 0h. 0m. 0s. (Astronomical time, always counts from noon to noon or civil time, from midnight to midnight) and this will then be the Sidereal Time at Noon on that day.

Now let us suppose that a complete day elapses, and the astronomer observes the Sun exactly culminating again, what will the clock show?

-       24h. 0m. 0s., or 0h. 0m. 0s.?

No, because in 24 hours, during which the earth has made one complete rotation, the Sun has moved forward 1° or very nearly and consequently the earth must turn 361° or thereby, instead of only 360°, before the Sun can be exactly on the meridian. Therefore, the clock will show about 0h. 4m. 0s. This will be the ‘Sidereal Time’ at noon on that day. Similarly, next day at noon, the clock will show about 0h. 8m. 0s., so on every day, gaining about four minutes every 24 hours.

Hence on any particular day, according to the time of the year the ‘Sidereal Time’ at Noon may be anything from 0h. 0m. 0s. to 23 hours 56 minutes 0 seconds. It will be evident that the sidereal clock must gain one whole day in the year.

Dr. Shanker Adawal,

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