Sidereal time is different from the clock time of 24 hours in a day. It is based on solar timings – calculated from one noon to another or one Sunrise to another. Actually, to rotate on its axis around the Sun the Earth takes 23 hours, 56 minutes and 4 seconds – this is sidereal time which is 4 minutes short of 24 hours of solar day.
The sidereal time:
Sidereal time is calculated based on Earth’s position fixed in respect to the stars on the celestial sphere. Our Earth rotates 360 degrees around its axis, and it also moves a distance of around 1 degree on the orbit round the Sun. So, the Earth should rotate another .986 degrees to complete the 24 hour duration of solar day. This also means that Earth should rotate for additional 4 minutes before it completes one rotation to meet the Sun at the same point of time. So Earth rotates 360 degrees in a sidereal day and 360.98 in a solar day.
The sidereal zodiac:
Vedic astrology is based on sidereal zodiac or the zodiac based on stars and nakshatras. Ayanamsa is the value table that indicates the precision of Earth’s tipping motion on its axis compared to the stars that keep changing in the celestial sphere. It also indicates difference between sidereal ecliptical longitude of a star and tropical ecliptical longitude around the Sun.
Due to differences in orbit of the Sun and the path of the stars, a tropical year differs from a sidereal year. Position of planets in the Solar system is indicated by ecliptic coordinates and as celestial spheres of planets keep changing, the calculations keep changing too. But in astrology Earth’s celestial path is quite fixed in reference to the position of stars. So, when a star is observed from any point on Earth – it is important to note when the same star will be visible again from the same meridian.
Ephemeris and Panchang:
A log book of ephemeris gives positions and times of planetary movements of the Solar system, while Panchang (the Hindu Almanac) tells us about duration of stars and rising signs or ascendants of a particular place. Deciding the Ascendant in the birth chart of a native is an important part in astrology.
Role of sidereal time for calculating ascendant:
Vedic astrological predictions are made from calculations are based on sidereal stars and calculation of ascendant in a birth-chart . An Ascendant is a sign seen rising on the Eastern horizon from a specific location on Earth. Ascendant sign on the horizon keeps changing every 2 hours because every sign occupies an expanse of 30 degrees. So even 10 people born on Earth at the same time will have different ascendants based on the the places where they were born.
Suppose, Sirus is born at 11.00 am at Chennai in April, his ascendant will be calculated based on the Sun rise at 6.00 am. He was born with the Sun in Capricorn, so his rising sign will be Pisces. Based on some additional calculations, it is found that difference between the time of his birth and Sunrise is 5 hrs. If each zodiac sign including Capricorn takes 2 hours to cross horizon of the Earth at the particular place then Pisces sign was on the eastern horizon at time of his birth. So, the ascendant sign of Sirus is Pisces.