CELESTIAL SPHERE/Celestial Equator
If we imagine extending the surface of Earth indefinitely on the heavens on all sides, with the observer remaining at the centre, we get what is known as the Celestial sphere, the cosmic sphere or Bramhand. As observer can observe only half of this celestial sphere as an inverted bowl over his head. Similarly, if we extend the Earth’s axis indefinitely on both the side to meet the celestial sphere, we get the celestial north and celestial south poles.
Similarly if we extend the Earth’s equator indefinitely on the heavens, we will get an imaginary great circle, which will divide the celestial sphere into two equal halves. This is known as the celestial equator. It divides the celestial sphere into Northern celestial hemisphere and Southern celestial hemisphere.
The entire celestial sphere together with all the planets appears to rise in the east and set in the west. This is due to the earth’s own rotation on its axis from west to east.
2. ECLIPTIC/Kranti Vrit
As explained earlier, we follow the geocentric observations with the Earth at the center of the universe and all other heavenly bodies in revolution around it. Sun also likewise appears to revolve around the Earth, rising in the East and setting in the West. The apparent path of the Sun (which is actually the path of Earth around the sun) is known as ecliptic. This ecliptic is tilted at an angle of 23 degrees 26 minutes approximately to the celestial equator due to the slant of the Earth’s axis.
The actual distance of the Earth from the Sun doesn’t cause the seasons. In January, Earth is at its closest point to the Sun although it is actually winter in the Northern Hemisphere.
3. THE FIRST POINT OF ARIES OR THE VERNAL EQUINOX
When the Sun crosses from the Southern celestial Hemisphere to the Northern Hemisphere or reaches the point of intersection of celestial equator ecliptic, it is known as the first point of Aries in Sayana system of Astrology. It is also called the Vernal or Spring equinox. It occurs around 21st march each year. Day & Night are of equal period or 12hrs. each on this date.
4. THE FIRST POINT OF LIBRA OR THE AUTUMNAL EQUINOX
When the Sun crosses from the Northern celestial Hemisphere to the Southern Hemisphere and reaches the point of intersection of celestial equator and ecliptic, it is known as the first point of Libra in Sayana system. It is also called the Autumnal equinox and occurs around 23rd September each year. On this day also day & night are equal.
5. SUMMER SOLSTICE AND WINTER SOLSTICE(Kark Sankranti & Makar Sankranti)
When Sun starts its Northern Course that is on 13th January every year it is called the Winter solstice. Sun enters Capricorn on this date. In Hindu Astrology. It is called Makar Sakranti or entry of Sun in Capricorn. Similarly entry of Sun in Cancer is called the Summer Solstice.
In Hindu astrology after applying Ayanamsa correction, the Sun moves to Uttarayana around 14th January each year in the present era. This coincides with Makar Sankranti or the solar ingress into the Capricorn Sign. Similarly around 21st June Sun turns Dakshinayana in Sayana system which means that it turns Southwards in its sojourn [HALT].
6. Sidereal Period
Sidereal Period is the true time of complete revolution of a planet around the Sun with reference to distant stars.
7. Sidereal Year
The Sidereal year is the mean period of revolution of Earth in its orbit with respect to a fixed star. The length of the sidereal year is 365.256 in mean solar days.
8. Tropical Year
Tropical year is the time of the passage of the Sun from one vernal equinox to the next vernal equinox. The vernal equinoctial point recedes by 50.2388475”. Therefore the sun has to travel this much distance less than 360 degrees to arrive at the new vernal equinoctial point therefore the tropical year is less than sidereal year by the time taken by the Sun to traverse 50.2388475” which is about 20 min. 26 sec.