Solar System: Our solar system is centered around the Sun. Nineplanets viz. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto along with a belt of asteroids revolve in elliptical orbit around the Sun. In Vedic astrology, Uranus, Neptune, Pluto fined no place. On the other hand, the classics recognize the Moon and two shadowy points known as Rahu & Ketu. The planets Mercury and Venus orbit in the space between the Sun and the earth and are known as a inner planet or inferior planets. Mars, Jupiter and Saturn, whose orbits are on outer side of the earth, are known as outer planets or superior planets. Rahu & Ketu always move in reverse direction, i.e., they have retrograde motion.

Zodiac: The Sun’s annual path is known as ecliptic. The belt of heavens extending 8 degrees on either side of ecliptic is known as the zodiac. In this belt the Moon and the planets appears to move. The Zodiac is divided into 12 equal parts of 30 degree each which are called the signs or rashi of the zodiac. These have been named as Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces.

Star or Nakshatras: The entire zodiac has been divided into 27 equal parts of 13 degrees and 20 minutes each known as nakshatra or constellation. Every constellation has been assigned a planetary ruler. Ketu is the ruler of first constellation starting from 0 degree Aries and then the ruler ship of Venus, Sun, Moon, Mars, Rahu, Jupiter, Saturn and Mercury follows in the sequence till the end of sing cancer. The Same Sequencestarts from 0 degree Leo and Sagittarius and continues till end of Scorpio and Pisces respectively.

Time: All are aware of time measure known as local mean time, standard time for any country or zone and Greenwich Mean Time. There is another system/measure of time known as sidereal time that is derived from earth’s rotation with respect to fixed stars. The duration of sidereal day is 23 hour 56 minutes. Whenever the time is reckoned with reference to sidereal day, it is called sidereal time. The sidereal time is zero hour when the beginning of Aries crosses he observer’s meridian.
In astrology, the first house is reckoned from legna point. The lagna or the ascendant is the rising sign in the eastern horizon at a particular epoch. It is the point of intersection of ecliptic at given time with the horizon of place. The lagna is dependent on the rotation of the earth on its axis and so it is dependent on the sidereal time of a place at a given moment. The longitude of lagna point thus calculated forms the middle point (cusp) of the first house.

The Houses: There are 12 houses. The sign where lagna point falls is known as the first house or lagna. The next sign is second house, next the third and so on. In Vedic system the sign and the house are deemed synonymous. However, in western astrology the cusp of the house is commencing point and the first house is constituted by space covering about 30 degree beyond the cusp; but in Vedic astrology, the cusp is the mid point of house and extends to 15 degrees on either side. Zodiac is constantly moving and all the 12 signs get an opportunity to become lagna in 24 hours comprising a day.