The concept of sidereal time
The earth rotates around its axis in 24 hours, in what may be termed a mean solar day. In other words, the mean solar day is a function of earth's rotation in relation to the Sun. Considered with reference to any fixed star in the zodiac, the earth completes its one rotation in approximately 23 hours and 56 minutes (23 hours, 56 minutes, 4.09 seconds, to be precise). One rotation of the earth in relation to a fixed star is called a sidereal day. Said in another manner, a sidereal day is the time interval between two successive transits of a fixed star over the meridian of a place. A sidereal day is 3 minutes and 56 seconds (or roughly 4 minutes) shorter than the mean solar day.

A sidereal day consists of 24 sidereal hours. Time reckoned according to this method is called the sidereal time .Since the sidereal time considers the angular rotation of the earth in relation to the fixed stars of the zodiac, the earth will attain the same position with reference to the zodiac every day at the same sidereal time. In other words, for any location, for the same sidereal time, the disposition of the signs of the zodiac (including the rising sign, the setting sign, the tenth house, the 4th house, etc.) will be the same. This is the reason why it is essential to obtain the correct sidereal time for the purposes of erecting an astrological chart for any given moment of mean solar time as provided by the watch.

Why is the mean solar day longer than the sidereal day? By the time the earth rotates once, with reference to a fixed star, i.e., in one sidereal day, the Sun has moved by approximately 1 thereby consuming approximately 4 additional minutes each day.
Precession of equinoxes
The earth revolves around the Sun once in 365 days 5 hours 48 minutes and 46 seconds. Considered from the earth, the Sun appears to complete one round of the ecliptic during this period. This is called a tropical year .In the span of a tropical year, the earth regains its original angular position with the Sun. It is also called the year of seasons since on this Earth-Sun cycle depends the occurrence, and timing, of seasons. If we consider the revolution of the Sun around the earth from one vernal equinox (around 21st March, when the day and night all over the globe are equal) to the next vernal equinox, it takes one tropical year to do so.

However, if at the end of a tropical year from one vernal equinox to the next, we consider the position of the earth with reference to a fixed star of the zodiac, the earth appears to lie some 50.26 seconds of celestial longitude to the west of its original position. In order for the earth to attain the same position with respect to a fixed star after one revolution, it takes a time span of 365 days 6 hours 9 minutes and some 9.5 seconds. This duration of time is called a sidereal year .The sidereal year is just over 20 minutes longer than the tropical year; this time difference is equivalent to 50.26 seconds of celestial longitude.
Each year, the Vernal equinox will fall short by 50.26 seconds along the zodiac reckoned along the fixed stars. This continuous receding of the Vernal equinox along the zodiac is called as the precession of equinoxes.
Causes of precession
The earth rotates around its axis like a spinning top. In doing so, its north pole (and, therefore, the celestial pole), describes a dircle of some 47 degrees around the pole of the ecliptic.

The wobble of earth's axis in a clockwise direction causes the precession of equinoxes.
The axis now points towards polaris;in about 13,000 years from now
it will have to be moved to a point within a few degrees from vega.

This in other words, means that the plane of the equator intersects the plane of the ecliptic at a constantly shifting point. This point, the first point of Aries or the vernal equinox, goes on receding westward at a rate of approximately 50.26 seconds of arc each year. This is called the precession of the equinoxes. The result of this precession is a slow increase in the right ascensions of almost all fixed stars in the zodiac. This precession takes some 25,800 (or approximately 26,000) years to complete one circle. As will be seen, an appreciation of this precession is of paramount importance in the understanding of the basic concepts of Vedic astrology.
Fixed and movable zodiacs
The fixed or the sidereal zodiac considers the nakshatras as its basis. Its first degree begins as the first degree of Mesha (Aries) from a particular point in the Revati group of stars. There is another zodiac, however, which is reckoned from the Vernal equinoctial point; here the first point of Aries begins from the Vernal equinox. This is called the movable or the tropical zodiac. As has been seen, the movable zodiac continues to recede westward along the stars, which characterise the fixed zodiac?
Ayanamsha; the sayana and the niryana system
It has been seen that because of the precession of equinoxes at a rate of 50.26 seconds per year, the distance between the Vernal equinox (the 1st point of the movable zodiac) and the 1st point of Mesha (Aries) on the fixed zodiac has been progressively increasing. This distance at any given epoch is called as the Ayanamsha .The ayanamsha thus indicates the difference between the fixed zodiac and the movable zodiac. The system that considers the fixed zodiac is called the Niryana  (without ayana!) system, while the one that considers the movable zodiac is called the Sayana  (with ayana!) system. The Niryana values of planetary longitudes can be obtained by subtracting the ayanamsha for a given time from the Sayana longitudes.

The Niryana and the Sayana zodiacs coincided in the year 285 AD when the ayanamsha was zero. At the rate of precession of equinoxes stated above, the ayanamsha on the 1st of January, 1995 is 2347'26". The equinoctial precession completes one round in aproximately 26,000 years, as mentioned earlier, so that the fixed and movable zodiacs coincide regularly after this time span. The ayanamsha reckoned on the basis of considering the year 285 AD as the year when the Sayana and the Niryana zodiacs coincided is called the Chitrapaksha ayanamsha.
Ancient method of time reckoning
  The Vedic seers had an elaborate method of reckoning time. They combined genius with religion so that it appealed to the intellectual and the devoted alike. There were several methods of reckoning time. One standard method was as follows:

1 Asu (or Prana) = 4 (sidereal) seconds
6 Asus = 1 sidereal Pala (or Vighati or Vinadi or 24 seconds)
60 Palas = 1 Ghati (24 minutes)
60 Ghatis = 1 day (24 hours)
30 days = 1 month
12 months = 1 year
43, 20, 000 years = 1 Yuga
72 Yugas = 1 Manu
14 Manus = 1 Kalpa (or 1008 Yugas)
2 Kalpas = A day and night of brahma
30 day-nights of Brahma = 1 month of Brahma
12 months of Brahma = 1 year of Brahma
100 years of Brahma = Life of Brahm (or 1 Mahakalpa
In addition to the above, the following methods of reckoning of time were used for astrological purposes :
1. Sidereal day = Time interval between one star-rise to the next
2. Civil day = Time interval between obe sunrise to the next
3. Lunar month = One new moon to the next
4. Solar maonth = Interval between entry of Sun from one sign to the other.
5. Solar year = Period of one solar revolution
6. Jupiterian (Barhaspatya) year = Period of Jupiter's motion through a sign.


Circular divisions
Measures of angles have been similarly described by Vedic astronomers.                  

60 Pratatparas =    1 Tatpara
60 Tatparas    =        1 Vilipta (or Vikala or second)
60 Vilipta   = 1 Lipta (or Kala or minute)
60 Lipta   1 Lava (or bhaga or amsha or degree)
30 Lavas =       1 Rasi (Sign)
12 Rashis  = 1 celestial circle or a revolution
Planets and the zodiac
The planets revolve around the Sun at different velocities in elliptical orbits. They also appear to revolve around the earth in elliptical orbits.

Says Parashara:

"Although the grahas proceed towards the east, they appear as if they are
moving in the westward direction, under the influence of the 'force of flow'."

The following three factors are of importance:

I. The rotation of the earth from west to east direction : Even as this causes the day as well as the night, it also makes the planets appear to be moving from east to west across the earth.

II. Daily revolution of the zodiac from east to west  The rotation of the earth makes the whole zodiac also appear as if it is making one daily revolution around the earth. In one day-night duration, all the signs of the zodiac (and all the nakshatras) successively rise in the east and set in the west.

III. The movement of the planets from west to east  : Although the daily rotation of the earth makes the Sun and other planets appear to be moving from east to west, in effect they move from west to east along the zodiac. Thus a planet in Mesha will actually proceed to Vrisha, and then to Mithuna, and so on.

Order of the planets: Aryabhata  describes the order of planets thus:

"Beneath the asterisms lie (the planets) Saturn, Jupiter, Mars, the Sun, Venus,
Mercury and the Moon (one below the other); beneath them all lies the earth....."

Planets as lords of days and Horas
: Aryabhata  continues:

"The (above mentioned) seven planets beginning with Saturn, which are arranged in the order of increasing fourth in order of increasing velocity are the lords of the successive days, which are reckoned from sunrise."

There are 24 Horas in a day. Each Hora is being (approximately!) equivalent to an hour. The first Hora on a day, starting from sunrise, belongs to the lord of the day itself. Subsequent Horas follow in the order as given above, i.e., Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon, Saturn, etc., until the end of day at next sunrise.

From Saturn, the fourth in order is the Sun; therefore, the Sun is the lord of the day following the day of Saturn. That is, Sunday follows Saturday. From the Sun, the fourth in order is the Moon. Therefore, Sunday is followed by the day of the Moon, i.e., Monday.

Inner and Outer planets:
The planets Mercury and Venus have their orbits between the Sun and the earth. They are called inner or inferior planets.  These planets cannot go far away from the Sun. Mercury can only move a maximum of 27 degrees from the Sun and Venus a maximum of 47 degrees from the Sun.

The planets Mars, Jupiter and Saturn, whose orbits lie outside the orbit of the earth, are called outer or superior planets .

Phenomenon of apparent retrogression in an outer planet as viewed from the earth

Retrogression and direct motion: Planets move along the zodiac from west to east, around the Sun. However, when seen from the earth, sometimes their motion appears to be occurring in a reverse direction against the background of the stars. This apparent motion in the reverse direction is called as retrogression of planets and has special significance in predictive astrology. Rahu and Ketu, which are not true planets, however, always move in retrograde direction.

Combustion of planets: Planets when too close to the Sun become invisible and are labelled as combust. A combust planet loses its strength and tends to behave adversely according to predictive astrology. Aryabhata  has the following to say about combustion:

"When the Moon has no latitude (i.e., when it is at zero degree of latitude) it is visible when
situated at a distance of 12 degrees from the Sun. Venus is visible when 9 degrees distant
from the sun. The other planets taken in the order of decreasing sizes (viz., Jupiter, Mercury,
Saturn and Mars) are visible when they are 9 degrees increased by twos
(i.e., when they are 11, 13, 15 and 17 degrees) distant from the Sun."

The degrees as mentioned above are generally taken as the limits within which the respective planets are said to be combust.

Planets as gods
Planets represent concentrations of energy. They influence the terrestrial phenomena by their disposition in the heavens. The sage Parashara, the father of Vedic astrology as understood and practised today, considers the planets as the representatives of gods. According to him:

(i) The Sun represents lord Rama.

(ii) The Moon represents lord Krishna.

(iii) Mars stands for lord Narsimha, the half human-half lion form of the lord.

(iv) Mercury represents lord Buddha.

(v) Jupiter represents lord Vamana, who attained the form of a dwarf to rid the world of the rule of demons. (vi) Venus represents lord Parshurama.

(vii) Saturn represents Kurma, the Tortoise incarnation of the lord.

(viii) Rahu represents Sookar, the Boar incarnation of the lord.

(ix) Ketu represents Meena, the Fish incarnation.

Even as the lord, according to Gita incarnates to safeguard the interests of the righteous and to punish the evil-doers, so also the planets undertake their benevolent and punitive actions. In other words, they behave as benefics and malefics in a horoscopic chart. All planets, true to their godly nature, produce both good and bad results. The actual results produced by them manifest according to the inherent nature of these planets.