The Study of Jyotisha · Chapter 2 of 54 · Part One · Beginnings

#The sky and the zodiac

#Why we begin with the sky

A horoscope is a picture of the sky. Before you can read that picture you need to know what it shows: where the Sun, the Moon and the planets are, measured against what, and from where. This chapter builds that knowledge from things you can see with your own eyes on any clear night. Nothing here needs mathematics beyond adding and subtracting degrees.

By the end of the chapter you will know:

• what the ecliptic is and why every planet stays near it;
• what the zodiac is, and the difference between a sign and a constellation;
• how a position such as "Moon at 15°20′ Leo" is written and what it means;
• what precession is;
• why Indian astrology uses a zodiac fixed to the stars, while Western astrology uses one fixed to the seasons, and how the two are converted with a number called the ayanamsa.

#The sky as a sphere

Stand outside on a clear night and the sky looks like a great dome resting on the horizon, with the stars fixed on its inside. Watch for an hour and the whole dome turns. Stars rise in the east, climb, cross the south (if you live in India or anywhere north of the equator) and set in the west. One star, near the north, hardly moves at all: it marks the pole of the turning.

Astronomers in every culture have described the sky this way, as a sphere around the observer, because it is the simplest way to say where things are. We still do. This imaginary globe is called the celestial sphere. The Sanskrit astronomers called the same idea the gola, "the sphere", and whole chapters of their books are devoted to it.

The sphere seems to turn, but it is the Earth that turns. Aryabhata said so in 499 CE, with a lovely image: just as a man in a boat moving forward sees the trees on the bank moving backward, so a person on the Earth sees the fixed stars moving westward (Aryabhatiya, Gola 9). Most later Indian astronomers kept the older view of a turning sky, but for our purposes it does not matter: the appearance is the same either way, and astrology works with the appearance — with the sky as seen from the place of birth.

#A few words you will need

• Horizon: the circle where the sky seems to meet the ground.
• Zenith: the point directly overhead.
• Meridian: the imaginary line running from the north point of the horizon, up through the zenith, and down to the south point. A star is highest in the sky when it crosses the meridian.
• Celestial poles: the two points the sky seems to turn around — one above the Earth's North Pole, one above the South Pole.
• Celestial equator: the great circle on the sky directly above the Earth's equator, halfway between the two poles.

These five ideas are enough to understand the daily motion of the sky. Now we add the one idea that matters most for astrology.

#The Sun's road: the ecliptic

If the stars were visible in daytime, you would see that the Sun is not fixed among them. Each day it slips a little — about one degree — eastward against the background of stars. After a year it has gone all the way round and is back among the same stars. Its yearly path around the sky is a great circle called the ecliptic. (The name comes from the fact that eclipses happen only when the Moon is on or near this circle.)

The ecliptic is tilted to the celestial equator by about 23½°. This tilt is the reason for the seasons. When the Sun is on the northern half of the ecliptic, it stands high at noon in India and the days are long; when it is on the southern half, it is lower and the days are shorter.

Four points on the ecliptic have special names:

The Indian astronomers called the Sun's northward half-year the uttarayana ("northern course", from the December solstice to the June solstice) and its southward half-year the dakshinayana ("southern course"). You will meet these words again in the calendar and in muhurta.

#Why the planets stay near the ecliptic

The Earth and all the planets go round the Sun in nearly the same flat plane, like runners on a flat track. From the Earth, therefore, every planet appears to travel along a narrow band of sky centred on the ecliptic. The Moon's orbit is tilted about 5° to the ecliptic, so the Moon wanders up to about 5° north or south of it. The planets stray a few degrees as well (Venus and Mars a little more at times). But none of them ever leaves the band.

This band is the zodiac. Every body that matters in a horoscope — the Sun, the Moon, Mars, Mercury, Jupiter, Venus and Saturn, and the two lunar nodes called Rahu and Ketu — is always somewhere in the zodiac. That is why a horoscope can be drawn as a single circle.

#The zodiac and its twelve signs

The zodiac is a circle, so it has 360 degrees. Astronomers in India, as in Babylon and Greece, divided it into twelve equal parts of 30° each. Each part is a sign, called rashi in Sanskrit. Varahamihira lists six words for "sign" — rashi, kshetra, griha, riksha, bha and bhavana — and says they all mean the same thing (Brihat Jataka I.4).

Learn this table by heart, with the numbers. You will use it every day. Chapter 7 describes the nature of each sign.

#Signs are not constellations

This is the most common confusion in all of astrology, so it is worth stating plainly. A constellation is a group of stars that people have pictured as a shape — a ram, a bull, a pair of twins. Constellations are of different sizes: some are large and some small, and they overlap the ecliptic by different amounts. A sign is an exact 30° piece of the zodiac. The signs take their names from the constellations that lay roughly in those places, but a sign is a measurement, not a group of stars. When an astrologer says "the Moon is in Leo", the meaning is "the Moon is between 120° and 150° of zodiacal longitude", not "the Moon is next to the stars of the Lion".

Indian astronomy also divides the same circle into twenty-seven nakshatras, each 13°20′. They are older in India than the twelve signs and are so important that Chapter 4 is given entirely to them. Varahamihira ties the two divisions together at the very start of his book: the zodiac begins at the first point of Aries, which is also the first point of the nakshatra Ashvini, and each sign holds exactly nine quarters of nakshatras (Brihat Jataka I.4). Since 9 × 3°20′ = 30°, the two systems fit perfectly.

#How a position is written

A position along the zodiac is called longitude (more exactly, ecliptic longitude). It is measured in degrees from 0° to 360°, starting at the first point of Aries and going eastward — the direction in which the Sun moves through the year.

Each degree (°) has 60 minutes (′), and each minute has 60 seconds (″). The Indian astronomers used the same sixties: a sign (rashi) of 30 bhagas (degrees), each of 60 kalas (minutes), each of 60 vikalas (seconds).

There are two common ways to write the same position:

1. As a single number of degrees, for example 135°20′.
2. As a sign and a degree within the sign, for example Leo 15°20′, or 15°20′ Leo.

To convert from the second to the first, add the starting degree of the sign from the table above. To convert from the first to the second, subtract the largest starting degree that fits.

Example 1. The Moon is at 15°20′ Leo. Leo starts at 120°. So the Moon's longitude is 120° + 15°20′ = 135°20′.

Example 2. Saturn is at 337°45′. The largest starting degree not more than 337°45′ is 330° (Pisces). 337°45′ − 330° = 7°45′. So Saturn is at 7°45′ Pisces.

Example 3. Jupiter is at 89°59′. That is 29°59′ of Gemini — one minute short of Cancer. A planet near the very end or the very beginning of a sign is said to be at a sign boundary (sandhi). The classical texts treat such planets with care, as you will see in later chapters.

A note on counting. Sanskrit texts often describe degrees by their ordinal number — "in the tenth degree" — which means from 9° to 10°. When Varahamihira says the Sun is highly exalted in "the tenth degree" of Aries (Brihat Jataka I.13), the modern way of writing that is 9°–10° Aries, and many astrologers simply write "10° Aries". Keep the difference in mind when you compare books.

#The two zodiacs

Now we come to the question that divides Indian and Western astrology.

Where exactly is the first point of Aries? There are two possible answers, and both are reasonable.

1. Fix it to the seasons. Put 0° Aries at the March equinox, the point where the Sun crosses the celestial equator going north. This is the tropical zodiac, used in Western astrology. In it, the Sun always enters Aries on about 21 March, every year, forever.
2. Fix it to the stars. Put 0° Aries at a particular place among the stars, and keep it there. This is the sidereal zodiac, used in Indian astrology. Its signs stay with the same stars forever.

Sanskrit names these two sayana, "with the movement (of the equinox)" — the tropical zodiac — and nirayana, "without the movement" — the sidereal zodiac. Classical Indian horoscopy uses the nirayana zodiac, beginning, as Varahamihira says, with the star group Ashvini (Brihat Jataka I.4).

If the equinox stayed in one place among the stars, there would be no difference between the two zodiacs. But it does not stay. It moves.

#Precession: the slow turn of the equinox

The Earth spins like a top, and like a top its axis slowly wobbles. One wobble takes about 25,800 years. As the axis swings round, the celestial equator swings with it, and so the two points where the equator crosses the ecliptic — the equinoxes — slide slowly westward along the ecliptic, against the background of stars. This sliding is called the precession of the equinoxes.

The rate is small: about 50.3 seconds of arc each year. That is one degree in about 72 years, or a whole sign (30°) in about 2,150 years. Over a single lifetime you will not notice it. Over the history of astrology it adds up to a great deal.

The Indian astronomers knew of this movement and called it ayana-chalana, "the motion of the solstices". The Surya Siddhanta describes it, in the translation of Ebenezer Burgess, in verses that are famous for being hard to read. It says the "circle of the asterisms" moves back and forth, giving a correction it calls the degrees "of the precession (ayana)", and it explains how to check the correction by observing the Sun at the solstices and equinoxes with a shadow-stick (Surya Siddhanta III.9–12). Burgess explains that the theory meant by these verses is an oscillation (a back-and-forth swing, which he calls a libration) of 27° either side of a fixed point, at a rate of 54 seconds a year. Modern astronomy describes precession as a steady circling, not a swing, and its rate is a little slower, but the Surya Siddhanta's authors had clearly measured the motion and built it into their method.

#The ayanamsa: the gap between the zodiacs

Because the equinox slides westward, the tropical zodiac slides westward against the sidereal one. The gap between their starting points keeps growing. This gap is called the ayanamsa, "the portion (amsha) of the motion (ayana)".

To turn a tropical longitude into a sidereal one, subtract the ayanamsa:

sidereal longitude = tropical longitude − ayanamsa

(If the answer is less than 0°, add 360°.)

#Where the two zodiacs were the same

Since the gap grows by about a degree every 72 years, there must have been a time when it was zero — when the equinox stood exactly at the sidereal first point of Aries. Which year that was depends on exactly which star position you take as the sidereal starting point. Under the convention most used in India today, the two zodiacs coincided in the late third century CE, around the year 285. This is close to the period when Indian horoscopy took its classical form, which is one reason the tradition named its starting point at the end of the star group Revati and the beginning of Ashvini, where the equinox then was.

#Lahiri, the Indian standard

Because the sidereal starting point is not marked by any bright star exactly on it, astrologers have had to choose how to define it. Different choices give slightly different ayanamsas. In 1955 the Calendar Reform Committee of the Government of India, chaired by the astrophysicist Meghnad Saha and with N. C. Lahiri as a leading member, adopted a definition that places the bright star Chitra (Spica, in the constellation Virgo) at exactly 180°, the first point of Libra. This is called the Lahiri or Chitrapaksha ayanamsa. It is the official standard of the Indian national calendar and is used by most Indian astrologers and almanacs today. Its value was 23°15′00.658″ at the March equinox of 1956, and it grows with precession. In 2026 it is about 24°13′.

The calculators on this website use Lahiri and print that choice next to every result.

#Other ayanamsas

You will meet other ayanamsas in books and software:

• Raman (used by B. V. Raman), about 1½° smaller than Lahiri.
• Krishnamurti (used in the KP system), a few minutes of arc smaller than Lahiri.
• Fagan–Bradley (used by Western sidereal astrologers), a little under 1° larger than Lahiri.
• Yukteshwar, and several others.

The differences are small — usually under two degrees — but they matter. A planet at 0°30′ Leo by one ayanamsa can be at 29°40′ Cancer by another, and then it is in a different sign. This is exactly the kind of "convention" Chapter 1 warned about. When you compare two charts or two books, always check which ayanamsa each uses.

#A worked example

On 20 March 2026 the Sun reaches the March equinox: its tropical longitude is 0° Aries, that is 0°. The Lahiri ayanamsa on that date is about 24°13′.

sidereal longitude = 0° − 24°13′ = −24°13′ → add 360° → 335°47′

335°47′ is 5°47′ Pisces. So at the March equinox the Sun is still in sidereal Pisces. It will need about another 24 days, moving roughly a degree a day, to reach 0° of sidereal Aries. That is why the Sun's entry into sidereal Aries — called Mesha Sankranti, celebrated as the solar new year in many regions of India (Vishu, Puthandu, Pohela Boishakh, Baisakhi) — falls on about 14 April and not on 21 March.

The same arithmetic explains why many people find that their "Sun sign" in Indian astrology is one sign earlier than in Western astrology. Someone born on 1 April has the Sun in tropical Aries but, today, in sidereal Pisces.

#Latitude: north and south of the ecliptic

Longitude tells you how far along the zodiac a body is. Latitude (vikshepa or shara in Sanskrit) tells you how far north or south of the ecliptic it is. The Sun's latitude is always zero, by definition. The Moon's latitude goes up to about 5°. For most of horoscopy only longitude matters: a planet's sign, house and aspects are all judged by longitude. Latitude matters for eclipses (Chapter 3), for the visibility of planets, and for the rare question of whether two planets in the same degree actually pass close to each other in the sky.

#The daily turning and the rising sign

The zodiac is fixed among the stars, and the stars turn round the sky once a day. So in one day all twelve signs rise in turn over the eastern horizon, each taking on average about two hours.

The sign that is rising on the eastern horizon at a given moment is the lagna, or ascendant. It is the single most important point in a horoscope, because it changes so quickly: about one degree every four minutes on average. Two people born on the same day in the same city, a few hours apart, will have different lagnas, and so their charts will look quite different. Signs do not all take the same time to rise — some rise quickly and some slowly, and the difference depends on the latitude of the place. Varahamihira expected an astrologer to know these "times of the rising of the signs" (Brihat Samhita II), and Chapter 6 shows how they are used to cast a chart.

#Summary

• The sky appears as a sphere turning once a day; really, the Earth turns (Aryabhatiya, Gola 9).
• The Sun's yearly path among the stars is the ecliptic. All the planets stay within a narrow band around it, called the zodiac.
• The zodiac is divided into 12 signs of 30° each, starting from Aries. A sign is an exact measurement, not a constellation. Each sign holds nine quarters of the 27 nakshatras (Brihat Jataka I.4).
• Positions are written in degrees, minutes and seconds of longitude, either from 0° to 360° or as a degree within a sign.
• The equinox slides westward among the stars about 50.3″ a year (precession). Indian astronomy knew of this motion (Surya Siddhanta III.9–12).
• Western astrology uses the tropical zodiac (tied to the seasons); Indian astrology uses the sidereal zodiac (tied to the stars). The gap between them is the ayanamsa. The Indian standard, Lahiri, puts Chitra (Spica) at 180° and was about 24°13′ in 2026.
• All twelve signs rise in a day. The sign rising at birth is the lagna.

#Exercises

1. Write these positions as a single number of degrees: 12°00′ Taurus; 29°30′ Scorpio; 3°15′ Pisces.
2. Write these positions as sign and degree: 95°10′; 181°00′; 359°59′.
3. A planet's tropical longitude is 10°00′ Aries and the ayanamsa is 24°13′. What is its sidereal longitude, and in which sidereal sign is it?
4. Explain in two or three sentences, to a friend who knows no astronomy, why the Sun enters sidereal Aries about 24 days after the March equinox.
5. Two astrologers give the same planet as 0°40′ Leo and 29°50′ Cancer. What is the most likely reason?

Answers: 1. 42°00′; 239°30′; 333°15′. 2. 5°10′ Cancer; 1°00′ Libra; 29°59′ Pisces. 3. 10°00′ − 24°13′ = −14°13′ → 345°47′ = 15°47′ Pisces. 5. They are using different ayanamsas.