#akashvani

The Zodiac Belt - The Sun, moon, and the planets move in a certain path in the sky. Their motions are constricted to a band. This band is known as the Zodiac Belt and it is composed of 13 constellations. These 13 constellations, in order of appearance in the night sky (starting with Aries), are -

Aries

Taurus

Gemini

Cancer

Leo

Virgo

Libra

Scorpius

Ophiuchus

Sagittarius

Capricorn

Aquarius

Pisces

You may recognize the 12 bold ones as the 12 Zodiac signs! The Sun, the moon, and the planets move approximately through the center of this belt in a line called the Ecliptic, which gets it’s name from the fact that eclipses (both lunar and solar) happen on this line. 

Now, you may be wondering why Ophiuchus isn’t a Zodiac sign? The answer is related to the fact that ecliptic is a ‘line’, but not on a flat surface. Because Earth is round and the sky appears like a dome above us, ecliptic is on a sphere! And a line on a sphere isn’t a straight line at all, it is a circle! (Try drawing a straight line all around a tennis ball to find out!) 

To go around a circle, you need to turn by 360 degrees. 360 is a difficult number to divide by 13 isn’t it? But not by 12! So what astrology does, is it tries to divide the ecliptic into 12 equal parts and each of those parts corresponds to a zodiac sign. In this kinda division, poor Ophiuchus gets divided between Sagittarius and Scorpius but most of it lies in the Sagittarius section. These sections are also called ‘houses’ in astrology sometimes.

What is a horoscope? A horoscope is generated based on the position of planets, the Sun, and the moon in the Zodiac belt at the time of one’s birth. Position of the Sun during a person’s birth decides the ‘Sun Sign’ and position of the moon decides the ‘Moon Sign’. The calendar we follow (Gregorian calendar) is a solar calendar, which means its basis is the motion of Earth around the Sun. Which is why, most solar signs can be determined based on what month you were born in, without looking at a star chart. Moon signs are tricky unless you are following a lunar calendar, which is based on moon’s orbit around the Earth. Chances are, most cultures that emphasize moon signs traditionally follow a lunar calendar (like most South Asian cultures!).

What are the Chinese Zodiacs? This might need a post of it’s own and I might make one depending on the response to this post. The 12 Chinese zodiac signs are quite different from lunar or solar zodiac. They do not depend on the division of the ecliptic at all. Instead, they depend on the time JUPITER takes to go around the Sun! That, to me, is extremely cool. It takes about 12 years for Jupiter to go around the Sun. So the cycle gets divided into 12 parts where each part corresponds to an year (not a month!). However, the Chinese calendar is most definitely a lunar calendar and has nothing to do with Jupiter. We are now reaching the limits of my knowledge on this subject and I see this as a sign to stop this fairly long post. I do want to leave you with some questions about the Chinese zodiac system that I currently cannot answer and welcome people who know about this to add to this post if they can and want to. Why does the Chinese zodiac system use Jupiter instead of the Sun or the moon? The brightest planet in the sky is Venus not Jupiter. In fact, Jupiter isn’t that bright since it is so far. So what is the cultural reason behind it? Is their an astronomical reason for the ‘Zodiac of the Cat’ being the 13th disputed zodiac ‘sign’? The existence of a 13th sign, to me, sounds very similar to the Ophiuchus issue, but I haven’t found an astronomical reason for it. How did the Chinese find out that it takes 12 earth years for Jupiter to go around the Sun when most of the world at the time didn’t even know the planets were orbiting the Sun and not Earth? Please add to this post or send me an ask/message if you know!

The Earth rotates around an axis. We are introduced to this concept pretty early in school aren’t we? After all, Earth’s rotation is what causes day and night! It is a super important concept to grasp. But, that’s not all there is to Earth’s rotation. 

Precession: When you spin a top, it usually looks like this - 

Notice, the top is not vertically straight, it’s tilted. And the top is wobbling. The wobble and tilt are pretty noticeable in the gif above but even a comparably stable top like the one below, also shows some tilt and wobble -

If there was no tilt, the long pointy thing in the middle would stay put. The Earth is exactly like that wooden wobbly top. We call this wobble Precession. If you imagine a stick protruding from Earth’s north pole, very much like that wooden top, the star placed right on top of it would be the North Star or the Pole Star.

You may have heard this before that the North Star is the only star that doesn’t move in the sky. And that is true to an extent. If the top in the picture didn’t wobble, then the wooden pole would stay exactly where it is. If you were on the surface of an un-wobbling Earth, you would see everything in the sky above you circle one single point in the sky. Earth takes it’s sweet time to complete one full precession cycle - about 26,000 years for one wobble! That is much much longer than a human life isn’t it? So to us, it does definitely look like the whole sky is going around one point. But, humans have been around for thousands of years to be able to notice slight changes in the location of the sky’s immovable north pole, and many ancient civilizations have records of the effects of Earth’s precession on the North Star and other stars. 

The North Star right now is Polaris in the Ursa Minor constellation. But in the past, various stars in the Cepheus, Draco, Cygnus, and Lyra constellations, have been Earth’s pole star. If you are a regular stargazer, you know how far apart these constellations appear in the sky! If you are not well-versed in constellation positions, check these out on Stellarium to see how drastic of an effect this can be. But as I mentioned, this happens over the course of millenia so Polaris will probably remain the North Star in our lifetimes.