This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

The Zodiac

by Dr Jamie Love Creative Commons Licence 1997 - 2011

The Zodiac is a series of constellations positioned along the same plane as the Earth's orbit. As the Earth moves in its orbit, the Sun appears to be "projected" against a background of the Zodiac constellations.
[Note - some books don't capitalize "zodiac" but I like to because it reminds me that it's full of constellations. I'm tempted to write it as "ZODIAC" because I like to capitalize all constellations but I fear you might feel that I've gone too far! ]

This path of the Sun among the stars is called the ecliptic and it is actually a projection of the Earth's orbital plane along the celestial sphere.

You will recall that the Earth's orbital plane was created as the Earth formed from the accretion disk billions of years ago. Because all the planets formed from the same accretion disk they have orbital planes that are close, but not exactly, like that of the Earth's. Each planet's orbit is slightly "inclined" with respect the ecliptic.

The Earth's orbital plane is defined as having an inclination of zero degrees (0o) and each planet's orbital inclination is a comparison of that planet's orbital plane to the orbital plane of the Earth. Mercury's inclination is at 7o from the ecliptic and that causes it to be more variable in its position in the sky than any other visible planet. Venus' orbital plane is at an inclination of 3.4o so it would be closer to the ecliptic. Mars' orbital plane is at an inclination is of 1.8o with respect to the Earth's orbital plane so the path of Mars' orbit is close to the path of the Sun. Jupiter's orbital plane is closer to the ecliptic, a mere 1.3o, so Jupiter is even closer to the ecliptic (and, thus, the Sun's path). Saturn, on the other hand, has an orbit whose plane is at a steeper angle (2.5o) to that of the Earth's orbital plane. Uranus' orbital plane is only 0.8o, the smallest inclination of any other planet so it tracks very close to the path of the Sun. Neptune has an inclination of 1.8o. Pluto's orbital plane is inclined a whooping 17.15o from the ecliptic so Pluto's orbit can carry it pretty far from the path of the Sun.

Yes, (we covered this during the lesson on orbits) but what does this have to do with the Zodiac?!

The Zodiac is a band of constellations through which the Sun appears to move as the Earth orbits the Sun. The planets appear to move among the Zodiac too.

Take a good look at this diagram and you will see that Mars, Venus and the Sun will appear in different parts of the zodiac - as seen from the Earth.

Question - Would you really be able to see the Sun against that starfield on the right?
Answer - No. The Sun would so outshine the stars that all you would see is the bright Sun. From Earth you would just see the bright Sun against a blue sky. From space (say, in orbit around the Earth) you would see a bright Sun against a black sky. (The stars would be so dim compared to the Sun that your eyes would not be able to distinguish them.) Of course, you wouldn't be stupid enough to stare at the Sun anyway! So this is a "trick question" (and a useful one too ).

The ecliptic is the center of the Zodiac band and that band extends 9 degrees on either side of it. The apparent paths of the Sun, the Moon (whose orbit is inclined 5.2o from the ecliptic) and all the planets (except Pluto) are always within this 18o band that passes around the celestial sphere. Here's the entire Zodiac with the ecliptic (the Sun's apparent path) in orange.

The ecliptic line is actually a circle so in this image the ecliptic leaving VIRGO enters LEO.
The Zodiac is also a circle - it has no beginning or end (but many folks start with AIRES for historical reasons).

You should recognize most of these constellations. You'll learn the rest tonight! You should remember that the ecliptic passes through all twelve of the Zodiac constellations and it passes close to several familiar stars - very close to Regulus (in LEO), Aldebaran (in TAURUS), Antares (in SCORPIUS), very close to Zubenelgenubi (in LIBRA) and Spica (in VIRGO). All astronomers are aware of the ecliptic and can trace it out in the sky by following familiar stars and constellations.

The Zodiac is divided into twelve zones, each 30 degrees long, and assigned a Zodiac constellation. Sort of.
In a sense there are two Zodiacs. The mathematically correct Zodiac is divided up evenly so each section is exactly 30o wide and the Sun spends exactly equal amounts of time in each one as it moves along the ecliptic through its annual cycle. However, the constellations that make up the Zodiac are in a variety of sizes and their borders are not along this 30o division. So, the Sun will spend more time in some Zodiac constellations than in others.
Generally speaking, as the Sun moves through the sky it enters a different Zodiac constellation each month (approximately).

This allowed the Ancients to use the Sun's progress through the Zodiac as a calendar, enabling them to estimate the seasons and assign the best times for planting and harvesting crops. These early day astronomers were held in very high regard by their kings and countrymen because the ability to predict crops and seasonal weather patterns was very important. To those unfamiliar with the Sun's movement through the Zodiac these predictions seemed magical. The Ancient astronomers used the ignorance of others to maintain power. They called themselves "astrologers" and promoted a superstitious belief that the position of the Sun, Moon and planets in the Zodiac could predict not only seasons and crops but also a person's personality or destiny!

Today, some people are so ignorant of the laws of nature and so desperate to know their future that they turn to astrology. Modern day astrologers make ridiculous claims about gravity or magical energy from planets in order to prop up their silly "science" but there is nothing factual about astrology. Astrologers have learned how to make money and sometimes fame by manipulating the ignorance of others. Some astrologers even fool themselves! And all astrologers are masters at making up excuses for failed predictions or recalling only their successes.
You should not confuse the science of astronomy or the work of astronomers with the nonsense of astrology or the deceit of astrologers.

OK. Why is it called "the Zodiac"?

"Zodiac" is an Ancient word meaning "animal circle". At one time there were only eleven Zodiac constellations and they were all either animals or people (and people are a kind of animal). So that name was suitable. Later astronomers chopped the claws off of SCORPIUS to make LIBRA. That brought SCORPIUS a little bit closer in size to that of other constellations and gave us as many Zodiac signs as there are months of the year. (Astrologers had to rewrite some of their nonsense to fit the change but it helped them give each month a "Zodiac sign".)

You should understand that astronomers divide the sky into many different sections depending upon what they are trying to accomplish. They divide the Zodiac into twelve equal 30o sections BUT the constellations do not fit well into each section. SCORPIUS is still huge and the Sun spends a lot longer in it than in LIBRA (for example). Zodiac constellations are not evenly spaced along the ecliptic nor are they the same length, but astronomers like to keep with traditions and still use the names of the Zodiac for each 30o section of the belt. In fact, OPHIUCHUS (the Serpent-bearer) crosses into the Zodiac zone so we should have 13 signs of the Zodiac! Astronomers aren't too concerned about that inconsistency because they know the monthly sections are just an easy way to remember the patterns and maintain a little bit of tradition.

What about those Zodiac constellations you mentioned earlier?

Oh, good point! This month all of the Zodiac constellations you have yet to learn are well positioned so let's make them part of our sky lesson for this month.

Start with the constellations you already know.
Use the Great Summer Triangle as a pointer to our next constellation.
Imagine a line from Vega to Altair and continue that line further towards the horizon until you come to a dim group of stars that form a twisted triangle.
That's CAPRICORNUS the Seagoat, a mythical animal, half-fish and half-goat.

The Great Summer Triangle makes for a wonderful pointer. Notice that a line from Deneb through Altair leads to SAGITTARIUS. (A line from Vega through Deneb points to Mirach in ANDROMEDA, but you cannot see it here because the image doesn't extend far enough to the left.)

If you look towards the southwest evening sky this month you will see that SCORPIUS is setting and the familiar teapot shape of SAGITTARIUS is following him.
Following SAGITTARIUS is our new constellation, CAPRICORNUS.

Following CAPRICORNUS is the next member of the Zodiac, AQUARIUS. This is a very dim constellation (it has no stars brighter than the 3rd magnitude) and easy to miss so find it by first finding CAPRICORNUS and then moving slightly east (left).

I think it looks like a "stick figure" of a person reaching towards CAPRICORNUS.

The two brightest stars form the base of a triangle with M2. This is a globular cluster in our galaxy's halo. (So it's full of Population II stars. Right?) M2 is about 50,000 light-years away and has a magnitude of 6.5 so you will need binoculars and a clear moonless night to see it. Some folks say they can see it unaided! It's worth a try. Notice how it is away from the glow of the Milky Way. (Here I am using the name "Milky Way" for the glow of our galaxy's disk.) Use your imagination to picture how M2 would be positioned far from the galactic arms (disk) in a very distant part of our galaxy's halo. This gives you some practice thinking about the shape of our galaxy.

Once you are satisfied that you have found AQUARIUS, follow the Zodiac eastward to PISCES the Fishes. This is a large but rather dim constellation.

The best way to find it is to look just to the east of AQUARIUS and find a group of five to eight stars (the number depends upon how "good" the sky is) forming a loop or circle. That is the start of PISCES. Sweep eastward from the loop following the brighter stars in a long arc that parallels the south end of the Square of Pegasus.

This arc ends at the brightest star in PISCES, alpha-PISCES. This star is not particularly bright (magnitude 3.8) and it is actually a binary pair about 100 light-years away. Once you reach this point in PISCES you will see a few stars heading back north. The entire constellation is supposed to be two fish but I think it looks more like a cowboy's lariat.
Note and warning - don't be fooled by an occasionally bright star near the bend in PISCES. It isn't part of PISCES and we'll talk about that star in December. Until then try to get a "feel" for the bend at the "bottom" of PISCES. The "bend" is more like a "U" than a "V" because of the very dim neighboring star that adds some bend to the bottom.

Notice that in the image above you should still be able to find AQUARIUS and CAPRICORNUS as well as most of SAGITTARIUS. (PEGASUS, Altair and DELPHINUS too.) Give them a try. If you can't see them, page up and have a look at them again.

Immediately behind PISCES is the last of your Zodiac constellations, ARIES the Ram. (A ram is a male sheep.)
This is a very boring and inconspicuous constellation with only the bright stars Hamal (magnitude 2.0) and Sheratan (magnitude 2.6) marking the main section of a dull arch. Hamal is slightly reddish and is type-K2.

According to legend Aries was a flying ram with a golden fleece.

Now you know all the constellations of the Zodiac.
These form a complete band around the celestial sphere and act as convenient "landmarks" in the night sky.
The ecliptic passes through the center of the Zodiac and every planet, except Pluto, can be found within this band of constellations. Actually even Pluto is usually found in the Zodiac.

So, I understand why it is important to be able to identify the constellations of the Zodiac.

Good.

Of course, the positions of the planets will change as time goes by because the planets are in orbit around the Sun. That's why you should keep up to date with planet positions by consulting an updated sky map (like the one I provide each month). However, once you get the hang of it, things start to "make sense".
You begin to expect that the two inner planets, Mercury and Venus, will always be in the same Zodiac constellation as the Sun or maybe one constellation to either side of it. Of the two inner planets, only Venus is bright enough to really stand out and by following its motion over the course of several weeks you will grasp how it waltzes around the Sun visiting the Zodiac neighbors.
Jupiter takes 11.9 years to complete one orbit of the Sun so each year it will move eastward by one Zodiac constellation (roughly). However, you should know that some constellations are bigger than others and, because it's an outer planet, Jupiter can retrograde (slightly). So don't take Jupiter's "one constellation per year" migration as a solid rule. It's only a trend.
Mars and Saturn can be followed and predicted to behave in a logical manner too as they move through the constellations of the Zodiac.

If you love planets (who doesn't? ) then you'll love the Zodiac. Once you become familiar with the stars of the Zodiac your attention is immediately draw to any bright light that is "out of place". That makes planet watching easier. I've learned to recognize Venus, Mars, Jupiter and Saturn by their color (although I have to admit that Jupiter and Saturn are pretty similar) and behavior (Venus is always somewhere near the Sun). Sometimes, without even consulting an updated sky map, I can identify those planets because I know roughly where they should be in the Zodiac and their color. My point is, if you see a bright "star" in the Zodiac but you know it doesn't fit any constellation (because you know the Zodiac well enough to know at least the brightest stars) then, it must be a planet!

Or, it's flying saucer! But seriously folks - Venus is commonly mistaken as an ET craft because it is so bright and it appears to be "hovering" behind (in the evenings) or ahead (in the mornings) of the Sun. Venus will even play "games" with your mind because it, like the Moon, can appear to be following you around! Spooky but easily explained as a common illusion.

You should learn all twelve of these important constellations and the order in which they present themselves because some are best found by knowing which obvious constellations they follow. If you really like to memorize information you might want to memorize when the Sun will be in each constellation. Remember, the Sun passes through a different Zodiac constellation each month so when the Sun is in a particular Zodiac constellation that constellation cannot be seen because the Sun blocks it out (except during a total eclipse of the Sun).

Here I've placed a month below each Zodiac constellation to give you a rough idea when the Sun will be in it. Remember, these are just rough estimates because neither the months nor the Zodiac constellations are equal in length. Notice that the "calendar" runs backwards like the "clock" you learned about in your first lesson.

The Zodiac also acts as our point of reference when determining right ascension (RA).

Huh?

To learn what I'm talking about you can continue on to your next lesson where you will learn more about the Zodiac and we will revisit some old information about celestial coordinates in a new light.




This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.