Sun is the king of all planets. All planets revolve around it. Astrology considers Sun a planet, but in terms of geography, it is a star. Revolution of Earth around the Sun changes day, night and Ayan (Uttarayan and Dakshinayan).

The secret of the sidereal time is hidden in the process of circumambulation of Sun by Earth. Earth rotates on its axis for 24 hours. Earth rotates 360 degrees on its axis in 24 hours revolving around the Sun. This completes one whole day called the solar day. Ideally, it is said that 24 hours is the rotation time, though this duration is 23 hours, 56 minutes and 4.09 seconds. The remaining 3 minutes 56 seconds are considered as the sidereal day, which is the remaining part of the solar day. In short, a sidereal day can be defined as the time taken to rotate from a one-time gap to another time gap from the middle point. The remaining time between this is called the sidereal time.

Importance of Sidereal Time in Astrology

The earth is rotating on its axis once a day, but for an observer, the celestial sphere is rotating. Thus sidereal time is local time reckoned according to the apparent rotation of the celestial sphere. In other words, whenever the time is reckoned with reference to the sidereal day, it is called sidereal time. The sidereal time is zero when the first point of Aries in Sayana or vernal equinox crosses the observer's meridian. Sidereal time can, therefore, be defined geometrically as an hour angle of the vernal equinox.

One Sidereal day has 24 sidereal hours. According to the method of calculation of time, this is considered the sidereal time. The sidereal time is very important in astrology. This is because the situation of the zodiac during this period in any country or place every day will be the same in the next cycle. That means the position of signs in one sidereal time will be the same in the next 24 hours. The position of the Ascendant, tenth and the fourth house will also be the same. This is the reason why astrologers need accurate sidereal time while they prepare birth-chart. It is only through this that they will get the right results.

Ascendant or Lagna is that sign, Rashi or the point of the ecliptic which is rising in the eastern horizon corresponding to the first house of horoscope. It, therefore, has the greatest importance without which we can’t proceed with the casting of a horoscope.

In calculating the birth chart, the first step is to calculate the sidereal time and then to go for the ascendant. It is not enough to know the rising sign, rather one must know the exact degrees of the sign.

The Sidereal Day is Longer Than the Solar Day: Why?

The sidereal day is longer than the Solar day because Earth completes one cycle in one sidereal day. However, during this period, Sun rotates in the zodiac 10 times. So Earth has to rotate more to reach the position of Sun. Earth takes 4 minutes more for this. That means Sun rotates for 360.9860.