Astrology Primer Astronomy Basic Concepts

The Vedic System of Calculating the Ascendant

The most important point in the construction of a horoscope is the Ascendant. The ascendant is the point of cutting of the ecliptic by the eastern horizon of a place.

The earth spinning on its axis in a linear movement takes 24 hours to complete one rotation. But what exactly is the duration of a day? There are many types of days prevalent.

Sidereal day: The time taken by earth to spin one complete rotation of 360 degrees on its axis. Average duration of one sidereal day is 23 hrs, 56 min, 4.091 sec.

Savana day: The duration of time between one sunrise to another sunrise is a Savana day. For people living in northern hemisphere, from winter solstice day onwards, the sunshine hours (dinamana) increases and night hours (ratrimana) decreases. As the sunrise every day is earlier than the previous day, the duration of the savana day is less than 24 hours till the Sun reaches its maximum declination at summer solstice. After that the dinamana reduces and the ratrimana increases. Since the sunrise of every day is later than the previous day, the duration of the savana day is more than 24 hours till it reaches the winter solstice again.

Mean Solar Day: The average of all the days of a year. It’s duration is equal to 24 hours.

The Vedic system recognises a day as the duration of time from one sunrise to the next sunrise. This span, known as a Savana day, is measured in units of ghatis. One Savana day is equal to 60 ghatis and each ghati is divisible into 60 palas or vighatis.

The earth continuously spins on its axis in a west to east direction. For a person situated on the surface of the earth, different signs of the zodiac appear to rise in the eastern horizon and set in the western horizon. With the completion of one rotation of the earth, all the twelve signs of the zodiac rise and set during one sidereal day.

Rashimana (Oblique Ascension)

Rashimana is the rising periods of signs of the zodiac. As there are twelve equally divided signs of the zodiac and it takes approximately 24 hours for all the signs to rise, therefore, one sign should take about two hours to rise in the eastern horizon. But it’s not so. As the plane of the ecliptic is inclined at an angle of 23.5 degrees to the plane of the celestial equator, the rising time of different signs is not uniform. The time taken by different groups of signs at the equator is given in Table 1.

Table 1.

Time taken by different group of signs to rise at the Equator

Group Signs Rashimana in
I Aries Virgo Libra Pisces 1674 asus 1h51m36s
II Taurus Leo Scorpio Aquarius 1795 asus 1h59m40s
III Gemini Cancer Sagittarius Capricorn 1931 asus 2h08m44s

Rashimana values are calculated for Sayana signs and are measured in units of Asus. One unit of Asu is equivalent to 4 seconds of sidereal time. Rashimana values vary from one latitude to another. These values once calculated for any place do not change from year to year.

Charakhandas (Ascensional Differences)

Variations in the rising of different signs at different latitudes can be calculated with the help of Charakhandas or ascensional differences for those latitudes.

To know the Charakhandas of a particular place with the help of ‘Hindu Dial’, measure the length of the mid-day shadow, on the day of the equinox, of a shanku of 12 units length (please refer to Astrology Primer # 5, Vol.1, No.5). Put this figure at three places and multiply the first figure with 10; second with 8 and; third with 10 divided by 3. This gives the Charakhandas for I, II, and III groups of signs respectively. These Charakhanda values are in palas or vighaties. To convert these values to asus, multiply the charakhandas by six.

Signs of Long Ascension and Short Ascension

For people living in the northern hemisphere of the earth, on the day of winter solstice, when the Sun is at zero degrees Sayana Capricorn, the sunshine hours are the shortest. With the rising of the Sun, sign Capricorn rises in the eastern horizon followed by other signs in sequence. At the time of sunset, the point rising at the eastern horizon would be 180° opposite the Sun’s longitude (thus zero degrees Cancer). Therefore, during the daytime signs Capricorn to Gemini rise in the shortest duration of time, while at night the signs Cancer to Sagittarius take the longest duration of time.

Table 2.

Signs of Short and Long Ascension for Northern Hemisphere*

Signs of Short Ascension* Signs of Long Ascension*
10 – Capricorn

11 – Aquarius

12 – Pisces

1 – Aries

2 – Taurus

3 – Gemini

4 – Cancer

5 – Leo

6 – Virgo

7 – Libra

8 – Scorpio

9 – Sagittarius

* For southern hemisphere short and long ascension rashis are reversed.

When the Sun is at summer solstice (zero degrees Sayana Cancer) during the daytime signs Cancer to Sagittarius spend the longest duration of time to rise and during night signs Capricorn to Gemini take the shortest duration of time.

Sign which takes longer time in rising than the time taken by same sign at the equator, is the sign of long ascension and the sign which takes shorter time in rising is the sign of short ascension. Signs Capricorn to Gemini are short ascension signs while Cancer to Sagittarius are long ascension signs for norther latitudes. Reverse is the case for people living in the southern latitudes.

As the latitude of the observer increases, the duration of signs of long ascension become much longer while the duration of signs of short ascension become much shorter.

Calculation of rising times of different signs (Rashimana) for a particular place

After knowing the Charakhandas of a particular place, we can calculate the rashimana of different signs. Add the Charakanda values, in asus, to the rashimana values at the equator in their respective groups for signs of long ascension and subtract the Charakhandas from their respective groups for signs of short ascension.

Correlation of the earth with the Zodiac

Calculation of ascendant for any given moment is an effort to establish a relationship between the horizon of the observer on the earth with the zodiac.

The earth is spinning continuously on its axis. To an observer, being located on the surface of the earth, it appears that the earth is stationary and the sky with all the stars and heavenly bodies is drifting towards the west after rising in the east.

To establish a relationship of the earth with the zodiac, we have to refer to some identifiable point on the zodiac. The rising, setting or the meridian passage of this point is to be observed to find out the actual position of this point at any given moment of time for the place of location of the observer. Once we know the position of one point of the zodiac, we can relate the other points of the zodiac with respect to this identifiable point.

This identifiable point could be a star or a planet or the vernal equinox (zero degrees Sayana Aries point) of the zodiac. When we observe the passing of the Vernal Equinox on the meridian of a place, it is zero hours Sidereal time for that place. Sidereal time at any given moment indicates the time elapsed since the vernal equinox crossed the meridian of that place.

The Indian system makes use of the position of the Sun in the zodiac to establish a link between the earth and the zodiac. At the time of sunrise, the centre of the Sun is touching the eastern horizon. Sunrise is considered to be the beginning to the day and that day remains in force till the next sunrise. The duration of this day is considered to be equal to sixty ghatis. One ghati is roughly equal to 24 minues of time.

The longitude of the Sun is identical with the cusp of the sign rising at the time of sunrise. A track of the number of ghatis and palas passed since sunrise is kept and is called Ishtakaala.

Since the rashimana values are for Sayana signs, the longitude of the Sun is also considered in Sayana values.

Inputs to calculate the Ascendant

In order to calculate the cusp of the ascendant, we need the following:

1. The time of sunrise at the required place on the relevant day.

2. The Sayana position of the Sun at the time of sunrise at the place in question. In case the available ephe-meris provides the nirayana position of the Sun, the Sayana position may be obtained by adding to it the appropriate ayanamsha.

3. The ishtakala or the duration of time elapsed from the time of sunrise.

4. Rashimana or the duration of the rising of different signs at the particular latitude of the place.

Steps to Calculate the Ascendant

The following steps describe the method of calculation of the ascendant for a given place at a given date and time. For example, let’s calculate the ascendant rising at Gurdaspur, India (latitude 32°N02′ longitude 75°E31′) on April 1, 1997 at 12.00 hours IST.

Step 1. Calculate the Charakhandas

On the ‘Hindu Dial’, measure the length of the mid-day shadow, on the day of the equinox, of a shanku of 12 units length.

Table 3:
Length of the equinoctial shadow of a Shanku of 12 units at different latitudes
Lat. Length Lat. Length Lat. Length Lat. Length
01° 0.21 16° 3.44 31° 7.21 46° 12.42
02° 0.42 17° 3.66 32° 7.50 47° 12.87
03° 0.63 18° 3.90 33° 7.79 48° 13.33
04° 0.84 19° 4.13 34° 8.09 49° 13.80
05° 1.05 20° 4.37 35° 8.40 50° 14.30
06° 1.26 21° 4.60 36° 8.71 51° 14.82
07° 1.47 22° 4.85 37° 9.04 52° 15.35
08° 1.69 23° 5.09 38° 9.37 53° 15.92
09° 1.90 24° 5.34 39° 9.72 54° 16.52
10° 2.11 25° 5.59 40° 10.06 55° 17.13
11° 2.33 26° 5.85 41° 10.43 56° 17.79
12° 2.55 27° 6.11 42° 10.80 57° 18.46
13° 2.70 28° 6.38 43° 11.19 58° 19.20
14° 2.99 29° 6.65 44° 11.58 59° 19.97
15° 3.21 30° 6.93 45° 12.00 60° 20.78

The length of the shadow at Gurdaspur (32 degrees latitude) from the above table is 7.5 units. Now multiply this figure with 10, 8, and 10/3 respectively to get the Charakhanda values in palas or vighatis.

Group Shanku length x value Charakhanda in palas
I 7.5 x 10 = 75 palas
II 7.5 x 8 = 60 palas
III 7.5 x 10/3 = 25 palas

Multiply each with 6 to convert the values in asus.

Group Charakhanda x 6 Charakhanda in asus
I 75 palas x 6 = 450 asus
II 60 palas x 6 = 360 asus
III 25 palas x 6 = 150 asus

The derived values of 450, 360 and 150 are the charakhandas for I, II and III groups of signs respectively.

Step 2. Calculate the Rashimana

The Rashimana for different groups of signs at the equator are:

Group Signs Rashimana
I 1, 6, 7, 12 1674 asus
II 2, 5, 8, 11 1795 asus
III 3, 4, 9, 10 1931 asus

To the above rashimanas we apply the Charakhanda corrections as worked out above to obtain the rashimana for different signs at the latitude in question. Add the Charakandas to their respective groups for signs of long ascension and subtract the Charakhandas from their respective groups for signs of short ascension.

Group Signs Rashimana in
Short Ascension Asus hr-mn-sc
I 1, 12 1674 – 450 = 1224 1:21:36
II 2, 11 1795 – 360 = 1435 1:35:40
III 3, 10 1931 – 150 = 1781 1:58:44
Long Ascension
I 4, 9 1931 + 150 = 2081 2:18:44
II 5, 8 1795 + 360 = 2155 2:23:40
III 6, 7 1674 + 450 = 2124 2:21:36

Step 3. Find out the Sunrise time

From the ephemeris, calculate the sunrise time on the given date for the place of birth. For Gurdaspur the sunrise time is 6h:20m:40s (IST).

Step 4. Find out the Sayana Sun

Again from the ephemeris, calculate the position of Sayan Sun at the time of sunrise. If the available ephemeris provides the longitudes of planets in nirayana values, add the ayanamsha to the Sun’s longitude to get the Sayana value. The nirayana longitude of the sun at the time of sunrise on April 1, 1997 is 11s17°31’16”. Adding to this the ayanamsha value on the given date, i.e., 23°49’06”, we get the Sayana longitude of the Sun at the time of sunrise as 0s11°20’22”. This also indicates the longitude of the ascendant at the time of sunrise.

Step 5. Find out the Ishtakala

Ishtakala is the time elapsed since the time of sunrise to the time of birth. Traditionally the time of birth is recorded in ishtakala only. Since in our example the time of birth is in hours-minutes, etc., it can be converted to ishtakala by subtracting the time of sunrise from the time of birth.

Time of birth : 12h:00m:00s

Sunrise time : 06h:20m:40s

Ishtakala in hrs. : 05h:39m:20s

Step 6. Cusp of the Ascendant

From Step 4 above, we know the sign that the sun is in at sunrise and, therefore, the cusp of the sign rising at the time of sunrise. The duration of this sign being known (Step 2), it is possible to work out how much of this sign has yet to rise above horizon and how much time it will take to do so.

Long. of Sun (Cusp at sunrise): = 0s11°20’22”
Bal. of sign Aries yet to rise: (30°00’00” – 11°20’22”) = 18°39’38”
Time taken by 30 degrees of Aries to rise: = 1h:21m:36s (Step 2)
Time taken by 18°39’38” of Aries to rise: (1:21:36 / 30°) x 18°39’38” = 0h:50m:45s

After 50m:45s of sunrise (i.e. from 7h:11m:25s onwards), the sign Taurus will start and last for 1h:35m:40s (i.e., upto 8h:47m:05s). The next sign Gemini (with a duration of 1h:58m:44s) lasts until 10h:45m:49s. Cancer (duration of 2h:18m:44s) lasts until 13h:04m:33s which includes our time of birth (12 noon). Thus we have Cancer rising at 12 noon.

Time elapsed from the onset of Cancer lagna upto the time of birth (12:00:00 – 10:45:49) = 1h:14m:11s
Arc of Cancer rising in 2h:18m:44s = 30°
Arc of Cancer rising in 1h:14m:11s = (30° / 2:18:44) x 1:14:11 = 16°02’30”

Thus we get the cusp of ascendant at 12 noon as Cancer 16°02’30”. This is the Sayana value. Reduce the ayanamsha from this value to obtain the cusp of the ascendant in nirayana value. Thus the nirayana ascendant would be: 3s16°02’30” – 23°49’06” = 2s22°13’24” or Gemini rising at 22°13’24”.

Astrology Primer Astronomy Basic Concepts

How to Create and Use Vedic Sundial

This article explores the ingenuity of ancient Vedic scholars in extracting the vast astronomical information with the help of very simple instruments.

Here we propose to explain the method of erecting a ‘Vedic Sundial’ as described in the Narada Purana (II.50.125-131), believed to be one of the ancient classics compiled around 1700 BC, and the Surya Siddhanta, one of the most important Indian astronomical classics.

The erection of the ‘Vedic Sundial’

Select a flat smooth stone or prepare a cemented surface, and level it with water. Draw an even circle having a radius of 12 units of any measurement. In the centre of this circle fix a cylindrical shaft, called ‘Shanku’. The Shanku is divided in twelve equal parts and has a height equal to the radius of the circle.

The classics refer to the unit of measurement as angula or ‘a finger’s-breadth’. The Shanku should be two angulas in diameter, uniformly circular (cylindrical), twelve angulas in height and made of strong wood. If according to this measurement a Shanku of twelve angula is formed, it will have a height of about nine inches.

Knowing the directions

Mark the two points on the circle where the extremity of the Shanku’s shadow touches the circle, once in the forenoon and again in the afternoon. Please refer to the diagram below. Points A and B represent the forenoon and the afternoon points respectively.

Considering these two points as the centre, draw two bisecting arcs a and b, forming the shape of a ‘fish’. Draw a line passing through the ‘mouth’ and ‘tail’ of this fish. This line is the north-south direction line. The line will pass through the centre of the circle O and touch the circle at two points N and S, indicating the north and south points. This line also indicates the meridian of the place.

Again form a ‘fish-figure’ on the north-south line by drawing two bisecting arcs n and s keeping the north and south points as their centres. A line formed by joining the ‘mouth’ and ‘tail’ will be perpendicular to the north-south line and is known as the east-west line. The east-west line passes through the centre of the circle and cuts the circle at two points E and W indicating the east and west points. The east-west line, is also called the prime vertical. The point O, the point of intersection of the north-south and east-west lines, which is also the centre point of the circle where the base of the Shanku lies, is the zenith point.

Thus, after knowing the four directions, by forming similar ‘fish-figures’ (bisecting arcs) between the two points of the directions, find out the four intermediate directions and mark them appropriately.

Draw a circumscribing square, having its sides equal to the diameter of the circle and the four corners of the square touching the four cardinal direction lines NE, SE, SW, NW. The east and west sides of this square are each divided into twenty four parts to form a linear scale. Its use is to aid in ascertaining the length of any given shadow.

Find the position of the Sun

The Indian system of astronomy primarily does the calculations in the Sayana (tropical) system. To convert the planetary longitudes to Nirayana (sidereal), subtract ayanamsha for the given moment from the Sayana position.

Mid-day shadow: When the shadow of the Shanku coincides with the north-south line, the Sun is exactly on the meridian of the place. At that time it is local noon or mid-day of that place. The shadow thus formed is referred to as the mid-day shadow.

Equinoctial shadow: When the Sun is either on the vernal equinox or on the autumnal equinox, the shadow of the Shanku thus formed on the mid-day is termed as Palabha or equinoctial shadow.

Equinoctial line: Mark the extremity of the mid-day shadow on the day when the Sun is at the equinox. Draw a line parallel to the east-west line, touching this point. The line is called the equinoctial line. Refer to the diagram on the right.

North declination line: On the day of the summer solstice, when the Sun is at its maximum declination north, mark the extremity of the midday shadow and draw a line touching this point, parallel to the east-west line and mark it summer solstice line or north declination line.

South declination line: Again on the day of the winter solstice, when the Sun is at its maximum declination south, mark the extremity of the midday shadow. Draw a line parallel to the east-west line passing through this point and mark the line as the winter solstice line or south declination line.

In the above paragraphs we have referred to summer and winter solstice as applicable to persons in the earth’s northern hemisphere. For people in the southern hemisphere, for example Australia, Sun’s maximum northern declination is referred to as winter solstice and Sun’s maximum southern declination as summer solstice.

Now we have got three lines: north declination line, equinoctial line and south declination line. The equinoctial line will be in between the two maximum declination lines of the Sun. On the north-south line, divide the distance between the north declination line and the equinoctial line in three parts, and the distance between the equinoctial line and south declination line also in three parts. On the left side of these six divisions mark vertically from top to bottom 9, 8, 7, 6, 5 and 4. On the right side of these six divisions again mark from top to bottom 10, 11, 12, 1, 2 and 3. These numbers represent the rashis. If the person is located near the equator, these divisions will be uniformally spaced out. But if the person is located far north of the equator, the divisions for signs Libra to Pisces would be more spaced out compared to the divisions for Aries to Virgo. Reverse would be the case for a person located far south from the equator. For persons located at higher latitudes, north or south, it is best to mark the extremity of shadows on the days of Sayana Sankranti (entrance of Sun in Sayana rashis) either of the six rashis of Uttarayana (Capricorn to Gemini) or Dakshinayana (Cancer to Sagittarius).

Observe the extremity of the midday shadow of the Shanku on any day of the year on the scale. When the Sun is in its southward course, the shadow point on the scale indicates the position of the Sun among six rashis from Karka (Cancer) to Dhanu (Sagittarius). When the Sun is in its northward course, the midday shadow point on the scale indicates the position of the Sun among six rashis from Makara (Capricorn) to Mithuna (Gemini).

Astrology Primer Astronomy Basic Concepts

The Basic Concepts of Astronomy Relevant to Astrology

The Branches of Vedic Astrology

Jyotish is considered to be one of the Vedangas (part of Vedas) propounded by lord Brahma by the scientific study of which human beings can accomplish virtue. Jyotish shastra or the science of Vedic astrology, is a compilation of 4,00,000 verses (vide Narada Purana, II.50.2). Vedic astrology has mainly three branches – Siddhanta (the principle), Jataka or Hora (astrology for individuals) and Samhita (astrology for masses).

Siddhanta, also known as Ganita, deals with the mathematical calculations, the methodology of calculating planetary positions, knowledge about time, place, direction, lunar and solar eclipses, their rising and setting, planetary movements, conjunctions, retrogression, etc.

Jataka (Hora) deals with the techniques of interpretation of horoscopes of individuals. It describes signs, planets, their qualities, family situations/ circumstances at the time of birth, arishta (mishaps), longevity of the native, different dasha systems and their results, profession (sources of livelihood), ashtakavarga, varied types of yogas, results of planetary positions in different houses, signs, nakshatras, aspects of planets, planetary combinations, female horoscopy, circumstances at the time of death, cases of unknown birth time, etc. The term ‘Hora’ has been applied to ‘Jataka’ or natal astrology, as well as to the ‘Muhurta’ or electional astrology (i.e., selecting the appropriate moment to commence an undertaking).

Samhita is that branch of astrology which is related to masses and is a compilation of varied subjects like the results of rising and setting of planets, appearance of different types of comets, varied types of chakras, predicting about rainfalls, earthquakes, natural disasters and epidemics, results of planetary movements on kingdoms, nations, masses and commodities, etc.

The Geocentric System

It is a human tendency to refer to other things in relation to oneself. Sitting in a moving train, we see things passing by the train – trees, farms, hutments, etc. A common question arises in our mind – which is the station coming next? At the back of our mind we do know that it is not the station which is going to come, it is the train which will reach the next station. Similarly we refer to the rising and setting of the Sun. But we do know that it is not the Sun which is rising or setting, it is the spin of the earth which makes it appear so.

Because we feel stationary on the solid earth, the sky seems to spin around us in complicated ways. In our quest to understand what we see, our ancients had evolved a most innovative and powerful tool.

As nothing is stationary in the universe, whether it is a satellite or a planet or even a star, it is convenient to imagine our position in the universe – the earth – as its centre and the whole of the universe moving around us in constant motion. Thus considering the relative positions and movements of all heavenly bodies with respect to the earth is the Geo-centric system. On the other hand, when we consider the relative position of planets (including the earth) in respect of the Sun, it forms the basis of the Helio-centric system. Vedic astronomy and astrology are essentially geo-centric in their concept.

The Earth

The earth is spherical and 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 Celestial Sphere

Think of the sky as a great, hollow, crystalline sphere surrounding the earth. Imagine the stars to be attached to the inside of the sphere like thumbnails stuck in the ceiling. The sphere takes one day to rotate, carrying the Sun, the Moon, the planets and the stars from east to west. We know that the sky is not a great, hollow, crystalline sphere. The stars are scattered through space at different distances, and it isn’t the sky that rotates once a day. It is rather the earth that rotates once in a day around its axis. It is convenient as a model of the sky. This model of the sky, the Celestial sphere, is an imaginary hollow sphere of very large radius (infinity) surrounding the earth and to which the stars seem to be attached. On this imaginary sphere the celestial equator, the celestial poles, and other reference points are marked as they are done on the earth; these represent the extensions of the equator and the poles, etc., of the earth into infinity.


The earth takes one year to complete its rotation around the Sun. From the earth, it appears that the Sun moves around the earth. This apparent path of the Sun is known as ecliptic. An imaginary belt of 18 degrees width with ecliptic in its centre is known as the zodiac. Many groups of stars appear to have been studded on this imaginary belt. Vedic astrology recognizes 27 such groups of stars called nakshatras.

The zodiac encircles the earth like a circle consisting of 360 degrees. If this circle is divided into 27 equal parts, each part will be of 13 degrees and 20 minutes arc, known as a nakshatra. Each nakshatra is further divided into 4 quarters (padas or charanas), of 3 degrees and 20 minutes arc each.

Twelve divisions of the zodiac will have an arc of 30 degrees each, known as rashis (or signs).

The above figure shows rising of the Sun in the eastern horizon. The line passing through the centre of the Sun is the ecliptic, the apparent path of the Sun created by its ‘revolution’ around the earth during its annual journey. The group of stars, referred to as the nakshatras, are the fixed reference points in the zodiac used to locate the position of the Sun, the Moon and other heavenly bodies. All the planets considered in Vedic astrology for the purpose of interpretation, do not decline beyond the belt of the zodiac. They may be on the ecliptic or towards the north or sourth of the ecliptic depending on their latitude with reference to the ecliptic.

For example, the orbit of the Moon is inclined at an angle of 5 degrees to the ecliptic. The Moon does not go beyond 5 degrees on either side of the ecliptic. The orbit of the Moon cuts the ecliptic at two point. In its orbit, when the Moon is on the ecliptic while moving from south of ecliptic to north, this point is known as Rahu or the ascending node of the Moon and when the Moon is on the ecliptic while moving from north of ecliptic to south of ecliptic, this point of intersection is known as Ketu or the descending node of the Moon.

The point of sunrise with respect to the observer keeps changing during the year. If A is the point of sunrise when the Sun is at vernal equinox (around March 21 every year), the point of sunrise will appear to move northwards till it reaches the summer solstice (B) on or around June 21. from this point it will start its southernly journey (Dakshinayana) during which it reaches the autumnal equinox (again A) around September 23 and further until it reaches winter solstice (C) around December 22. At this stage it starts its northward journey (Uttarayana).

Tropical Zodiac

The most crucial point in the division of a circle is to know the starting point of the circle. The point where the ecliptic cuts the celestial equator is known as equinox. There are two such equinoxes – the vernal equinox and the autumnal equinox. When the Sun is passing from the southern hemisphere to the northern hemisphere, it cuts the equator at vernal equinox. When the division of the circle of the zodiac is with reference to vernal equinox as its starting point, the zodiac is referred to as the Sayana (or tropical) zodiac, the divisions of this zodiac into twelve equal parts are the Sayana rashis, and the positions of planets in this zodiac represent the Sayana longitudes of the planets.

The Precession of Equinoxes

If we could watch the sky for a few hundred years, we would discover that the north celestial pole is moving slowly with respect to Dhruva (Polaris) star. The celestial poles and the celestial equator, supposed to be the fixed reference marks, are moving very slowly because of the slow change in the direction of Earth’s axis of rotation. This slow top-like motion is called precession. Earth’s axis sweeps around in a cone, taking almost 26,000 years for each sweep.

Precession is caused by the gravitational pull of the Sun and the Moon. Because earth is not a perfect sphere – it has a slight bulge around its equator – Sun and Moon pull on it, trying to make it spin upright in its orbit. This forces earth’s axis to precess.

The result of this precession is that vernal equinox, the cutting point of the ecliptic and the celestial equator, drifts westward on the ecliptic by an approximate angle of 51 seconds of an arc each year. So we have a new vernal equinox every year and hence a new staring point of the Sayana zodiac. This results in the shifting of the Sayana signs.

Sidereal Zodiac

The Vedic system does not depend on this shifting zodiac and relies on a fixed point on the zodiac as its starting point. There is no clear cut demarcation of this starting point in the zodiac. Some consider this point to be 180 degrees opposite to the Chitra nakshatra. Some consider it to be slightly to the east of the Revati nakshatra, while still others opine differently.

When the division of the circle of the zodiac is with reference to the Vedic starting point, the zodiac is referred to as the Nirayana (or Sidereal) zodiac, the twelve equal parts are the Nirayana rashis, and the positions of planets in this zodiac represent the Nirayana longitudes of the planets.

The angular difference between the vernal equinox and the Vedic starting point of the zodiac is known as the Ayanamsha. When the Vedic starting point is with reference to Chitra nakshatra, the Ayanamsha is refered to as the Chitrapaksha Ayanamsha. According to this system the first point of Sayana zodiac and Nirayana zodiac coincided in the year 285 A.D. The corresponding value of this Ayanamsha on January 1997 is 23°48’56”.