Aryabhatt Vedic Lession
The geocentric astronomical framework
In order to understand the very basic astronomical concepts, as pertinent to an understanding of Vedic astrology, it is important to understand certain facts about the earth, the movements of the earth, and the apparent movement of the planets around the earth. Explanation of a few definitions is also in order.

Earth as a sphere
: The earth is spherical. It rotates from west to east around its axis. The axis of the earth is an imaginary line which, passing through its centre, connects its two poles, the north pole and the south pole. Another imaginary line running across the largest circumference of the earth, equidistant from its poles and running in an east-west direction, is called the equator. The terrestial equator is considered as the zero degree of latitude. Parallels drawn to the equator, either north or south of it, indicate the north or south latitudes, from zero degree at the equator to 90 degrees at either pole.

Imaginary lines can also be drawn on the surface of the earth connecting the north pole to the south pole. Encompassing the circumference of the earth, these correspond to the 360 degrees of longitude. They are also known as the terrestrial meridians. Ancient Vedic astrologers considered the terrestrial meridian passing through Ujjain as the zero degree longitude. At present, the meridian passing through Green witch in England is regarded as corresponding to zero degree of longitude. The longitudes are marked from zero degree to 180 degrees east or west, depending upon whether a place falls to the east or to the west of Greenwitch. The latitude and the longitude of a place are the co-ordinates, which help to locate the place accurately on the surface of the earth. The 360 degrees of terrestrial longitude represent a time span of 24 hours. One hour thus corresponds to 15 degrees, and one degree of terrestrial longitude represents 4 minutes of time.

The equator divides the earth into northern and souther hemispheres.Latitudes and longitudes help locate a place on the surface of the earth.Arrow shows the direction of the earth's rotation from west to east.
The great and the small circles : A great circle  is any circle the plane of which passes through the centre of a sphere. Equator is a great circle on the earth, equidistant from the north and south poles. Any circle the plane of which does not pass through the centre of a sphere is called a small circle.  As the equator corresponds to zero degree latitude, all parallels to it are small circles, which represent the north or south latitudes.
Imaginary extensions into space : The space around the earth extends to an infinite extent. To us, the extension of space upto the zodiac is of primary importance. Celestrial Sphere is an imaginary projection of the earth in all directions upto infinity. An extension of the plane of terrestial equator into space is called the celestial equator . Any great circle that joins the celestial north and south poles is called a meridian. The meridian of a place corresponds to the terrestrial longitude. The meridian passing through Greenwitch corresponds to zero degree of longitude, and is termed as the Principal meridian  or the Standard meridian. The angular distance between the principal meridian and the meridian of a given place (i.e., the angle subtended by the principal meridian and the meridian of a given place, at the centre of the earth) is called the longitude of a place.

The Sun crosses the meridian of a place at mid-day. The intersection of the ecliptic (i.e., the sun's apparent path around the earth) with the meridian of a place is termed as the midheaven which in other words corresponds to the cusp of the tenth house of a horoscope. The meridian of a place thus passes, around the earth, through north pole, midheaven (10th house or zenith), south pole nadir (4th house) and back to the north pole.
Declination and right ascension: Just as parallels of latitude and meridians of longitude help to locate a place on the surface of the terrentrial sphere, so do their extensions in the form of parallels of declination and meridians of right ascension help to locate heavenly bodies on the celestial sphere.Declination  of a planet is the angle subtended by it and the celestial equator at the earth. The declination of a planet, thus, corresponds exactly with the terrestrial latitude. A planet at the terrestrial equator is said to possess zero degree declination. Right ascension  of a planet is its angular distance, measured eastwards along the celestial equator, from the vernal equinox to the point where a perpendicular drawn through the said planet falls on the celestial equator.
Equator and ecliptic ; formation of seasons: The earth rotates on its own axis in twenty-four hours. Along with this rotation, it also revolves around the Sun in one year or 365.2422 days (365 days 5 hours 48 minutes 46 seconds). This span of time is called a tropical year. The path of the earth around the Sun appears to us, from the earth, as the Sun's path around the earth, and is called the ecliptic . The equator runs around the middle of earth in an east-west direction and divides the earth into a northern hemisphere and a southern hemisphere. The ecliptic, or the Sun's path, in the apparent east-west direction, does not lie along the equator but is obliquely placed to it. Half of the Sun's path thus lies to the north of the equator and a half of it to the south of the equator.

wrote over fifteen centuries ago:

"One half of the ecliptic, running from the beginning of the sign Aries to the end of the sign Virgo, lies obliquely inclined
(to the equator) northwards. The remaining half (or the ecliptic)
running from the beginning of the sign Libra to the end of the sign Pisces, lies (equally inclined to the equator) southwards."

The Sun thus happens to cross the equator twice a year, giving rise to what are termed as the two equinoxes. The vernal equinox  happens around the 21st March, when the Sun is on its northerly course. The autumnal equinox  occurs around 23rd September when the Sun is on its southerly course..

The ecliptic is inclined to the equator at an angle of 23.28'.It crosses equator at two points, the vernal equinox and the autumnal equinox.The north of the earth corresponds to the celestial north and celestial south poles.
On these two occasions, the day and night all over the globe are of equal duration. The Sun is vertically above the equator at this time. The declination of the Sun at these occasions is zero as it corresponds to the terrestrial equator which represents zero degree latitude.

After vernal equinox, the Sun progressively attains north declination unit it reaches a maximum of 2328'. This occurs around 21st June and is known as the summer solstice . The Sun is vertically above the tropic of Cancer at this time. The northern hemisphere experiences the longest day and the shortest night on this occasion. The reverse holds true for the southern hemisphere.

After the autumnal equinox, the Sun pursues a southward course and attains a maximum south declination of 2328' at the time of winter solstice . This happens around 22nd December. The Sun is vertically above the tropic of Capricorn, at this time. The northern hemisphere experiences the shortest day and the longest night on this occasion. The reverse holds true for the southern hemisphere.

The obliquity of the ecliptic to the equator thus results in the formation of seasons . When it is winter in the northern hemisphere, it is summer in the southern hemisphere. When it is summer in the northern hemisphere, it is winter in the southern hemisphere.
The horizon: It is the great circle, which represents the meeting line of the earth and the sky. It varies according to the position of the observer on the surface of the earth. For example, for an observer at the north pole of the earth, the horizon corresponds with the equator while the southern hemisphere remains out of view. For one standing at the equator, the great circle passing through the poles represents the horizon; the two poles lie on the horizon in this case. For any intermediate positions, the horizon too varies accordingly. More and more of the southern hemisphere moves out of the horizon as the observer moves northward, and more and more northern hemisphere moves out of the horizon as the observer moves southward.

The point of the celestial sphere, which is directly overhead for the observer, is called as the zenith . This is at right angles to the observer's horizon. Its opposite point is known as the nadir . The great circle that passes in a north-south direction through the zenith and the nadir, through the celestial north and south poles (i.e., the north and south poles of the equator) and through the north and south points of the horizon is called the meridian  which has been already referred to.
The rising and setting of signs
As already mentioned, the ecliptic passes through the centre of the zodiacal belt which extends some 8' to 9' on its either (north as well as south) side. The planets remain within the limits of the zodiac. The earth rotates around its axis once in twenty-four hours from west to east. As a consequence, all heavenly bodies appear to revolve around the earth from east to west once in twenty-four hours. The zodiac, with the nakshatras and rashis fixed upon it, also appears to revolve around the earth once in twenty-four hours. Thus all the signs and nakshatras on the zodiac appear to successively rise in the eastern horizon and set at the western horizon once in twenty-four hours. Six of the twelve signs appear at the eastern horizon during the day-time and the remaining six during night-time. The following points are of importance:
1. The sign that rises at the eastern horizon, at a given moment of time, is of primary importance and called the ascendant or the lagna. It is the sign where the ecliptic cuts the eastern horizon. In a horoscope this represents the first house.

The sign seventh from the ascendant is the descendant or the setting sign. That is, when a particular sign is rising in the eastern horizon, its opposite sign is setting in the western horizon. It is the sign where the ecliptic cuts the western horizon. In a horoscope this represents the seventh house

The points where the meridian cuts the ecliptic are the zenith (above the earth) and the nadir (below the earth, exactly opposite to the zenith) The Zenith (mid-heaven) represents the tenth house in a horoscope, while the nadir represents the fourth house.

Each sign takes time to rise at the horizon from zero degrees to 30 degrees. All signs are not of equal duration so that some signs take longer to completely rise above the horizon compared to the others

Signs can be divided into three groups, depending upon their rising periods (rashi maanas).

Group A       
Group B
Group C

A sign belonging to one group takes the same time to rise as another belonging to the same group at the equator. The six signs from Karka to Dhanu lengthen and the remaining six shorten as one proceeds from the equator to the north pole. On the other hand, the signs from Makara to Mithuna lengthen, while the remaining ones shorten, as one proceeds from the equator to the south pole.

For any given latitude, the rising period for different signs is fixed.

As one moves away from the equator, certain signs lengthen while the others shorten as far as their rising period is concerned. That is, certain signs remain longer on the horizon than the others.

Six signs elapse between sunrise and sunset, while the remaining six signs do so between sunset and sunrise.

This means that in winter, when the days are shorter, the six zodiacal signs that rise successively during the day have a shorter time duration, while the remaining six have a longer time duration. This gives rise to signs of short ascension and those of long ascension.

Signs of long ascension in the northern hemisphere are: Karka, Simbha, Kanya, Tula, Vrischika and Dhanu.

Sings of short ascension in the northern hemisphere are: Makara, Kumbha, Meena, Mesha, Vrisha and Mithuna. These are the signs of long ascension for the southern latitudes.

As one nears the poles, certain zodiacal signs fail to rise.