Lesson: the sky model

Take these one at a time. Each step is one idea. Play with the pictures before you move on.

Picture the sky as a ball around you

Go outside on a clear night and look up. The stars look as if they are painted on the inside of a huge ball, and you are standing at the centre.

Astrology uses exactly that picture. The Earth sits in the middle. The sky is a ball around it.

Space is not really built like that. But this is how the sky looks from here, and astrology only cares about how the sky looks from here.

The Sun walks the same road every year

Watch the Sun for a whole year and you will see it creep slowly past the stars behind it. It always follows the same road. After one year it is back where it started.

That road has a name: the .

Press play and watch one year go by.

Two numbers to keep:

The Sun moves about 1 degree a day.
The Sun spends about one month in each sign.

The zodiac is the strip of sky along that road

The Moon and the planets do not wander all over the sky. They all stay close to the Sun’s road.

So there is one narrow strip of sky where everything happens. That strip is the zodiac. Think of it as a belt around the ball.

One number tells you where a planet is

The belt is a circle. A circle has 360 degrees.

So we can say where a planet is with a single number from 0 to 360. Start at the beginning of the belt and measure round to the planet.

That number is called the planet’s .

Example: “The Sun is at 124.2°” means the Sun is 124.2 degrees round the belt from the starting point.

Cut the belt into twelve equal signs

360 degrees is a lot to picture. So the belt is cut into 12 equal pieces. Each piece is 30 degrees wide. Each piece is a sign.

Number	Sign	Starts at	Ends at
1	Aries	0°	30°
2	Taurus	30°	60°
3	Gemini	60°	90°
4	Cancer	90°	120°
5	Leo	120°	150°
6	Virgo	150°	180°
7	Libra	180°	210°
8	Scorpio	210°	240°
9	Sagittarius	240°	270°
10	Capricorn	270°	300°
11	Aquarius	300°	330°
12	Pisces	330°	360°

Drag the gold dot around the wheel. Watch the three numbers change.

Turn a longitude into a sign and a degree

Nobody says “the Sun is at 124.2°”. They say “the Sun is at Leo 4.2°”. Here is how to get from one to the other.

The recipe

Divide the longitude by 30.
The whole number tells you how many full signs have been passed.
Add 1 to get the sign number.
What is left over is the degrees inside that sign.

Example: 124.2°

124.2 ÷ 30 = 4, with 4.2 left over.
So 4 full signs have been passed: Aries, Taurus, Gemini, Cancer.
4 + 1 = 5. The planet is in sign number 5, which is Leo.
The left over is 4.2. So it is 4.2 degrees into Leo.

Answer: Leo 4.2°

To go the other way, do the reverse. Leo is sign 5, so 4 signs come before it. 4 × 30 = 120. Add the 4.2 and you are back at 124.2°.

But where does the belt start?

Here is the catch. A circle has no beginning. Somebody has to choose where 0° goes.

People made two different choices. That is why there are two zodiacs.

Zodiac	Where 0° is	Who uses it
Tropical	Where the Sun is on the March equinox, the day in March when day and night are the same length. It is tied to the seasons.	Western astrology
Sidereal	A fixed place among the stars.	Vedic astrology

Sidereal just means “by the stars”.

The two starting points are slowly drifting apart

If both starting points stayed put, this would not matter. But one of them moves.

The Earth spins like a top. And like a top, it wobbles. The wobble is very slow: one wobble takes about 26 000 years.

Because of that wobble, the March equinox point slides backwards against the stars. It moves about 1 degree every 72 years.

Around the year 285, the two starting points were in the same place. They have been drifting apart ever since.

Press play to watch the gap grow. Then try the three chart buttons.

The gap has a name: the ayanamsa

The gap between the two zodiacs is called the . Say it “eye uh NAHM sha”.

It grows slowly, so it depends on the year.

Year	Ayanamsa
285	0.0°
1900	22.5°
1950	23.2°
1964	23.4°
1987	23.7°
1991	23.7°
2000	23.9°
2016	24.1°
2025	24.2°

One honest detail. Astrologers do not all agree on exactly where the star fixed starting point is, so there are a few versions of the ayanamsa. India’s official one is called Lahiri. It is the one almost every Indian astrologer uses, and the only one this course uses.

Converting from Western to Vedic

Most books, apps and websites give positions in the tropical zodiac. To get the Vedic position, you take away the gap.

That is the whole trick. A Vedic chart is a Western chart with every planet moved back by about 24 degrees.

This also explains something you may have noticed. A sign is 30 degrees wide and the gap is about 24. So for most people, their Vedic Sun sign is the sign before their Western one.

What you just learnt

The zodiac is the belt of sky the Sun, Moon and planets travel along.
A planet’s position is one number, its longitude, from 0° to 360°.
Twelve signs, 30° each. Divide by 30 to find the sign.
Two zodiacs: tropical (seasons, Western) and sidereal (stars, Vedic).
The gap between them is the ayanamsa, about 24° today.
Sidereal = tropical − ayanamsa.

NextA worked example