The constellations of the zodiac have a "reputation" as being special; they appear in daily in every newspaper (albeit, "mispelled" compared to their official names) in the horoscope section. We will not be talking about horoscopes, but we are going to learn a little about these constellations and what make them "special."
The Ecliptic Plane
We've already learned a little about celestial coordinates (right ascension and declination) and now we have to learn a little more. This time, it's about the Earth's orbit. The Earth orbits the Sun in an ellipse as do all of the planets. The Earth's orbit is, however, very nearly a circle and if drawn to scale on this page, would be indistinguishable from a circle with the Sun at the center. As the earth revolves around the Sun, it always stays in the same plane; it doesn't wiggle up and down. This plane is called the ecliptic.
In many drawings of the solar system, all the planets are shown in the same plane. Its a bit hard to draw the 3-D nature of the solar system on a 2-D sheet of paper, but it is important to realize that he planets do not all orbit the Sun in the same plane as the Earth does. Each has it's own plane. These are all very close to one another, and all are focused on the Sun. This is why, for example, that although a drawing of Pluto's orbit shows it crossing the orbit of Neptune, they don't crash into one another. Even if they were in the same place on the 2-D sheet of paper, the orbits are tilted such that Pluto passes safely above or below Neptune.
The above image shows the orbits of the inner planets on April 8, 2008. If you look closely, you can see the tilted nature of the orbits. Earth's orbit is marked in red to make it easy to find. The other planet's orbits are in blue. Parts of Venus' orbit appear to lie outside the orbit of Earth from this angle. And Mercury's orbit appears to come all the way out to Earth's. This is partially an effect of perspective, but it is also due to the relative tilt of the different orbits. If they all lay in the same plane, you would not see them appearing to cross.
But the other thing you will notice is that they really aren't tilted very much to one another. While the ecliptic is defined as the plane of Earth's orbit, all planets orbit the sun in planes very close to the ecliptic.
The above YouTube video shows a flyby through the solar system which might help visualize the 3-D structure of the orbits. At about 2 minutes into the video, we stop and take a look as we cross the ecliptic the second time so that we can see the orbit of the Earth (again in red) edge on. The view is somewhat messy with orbits from all planets shown in blue. After crossing the ecliptic, the speed picks up to "warp 8" then jumps to about warp 25(!) to exit the solar system.
The only real point in all of the above is to see that the planets all orbit in their own planes and that those planes are actually quite close to one another.
Very Nice but What about those Constellations?
Imagine we could put a dimmer switch on the Sun so we could turn down the brightness until the stars were visible in the daytime. Don't turn it off, just make it dim. If we could do this, then the Sun would appear as a (very) bright star in one of the constellations. But which one would be continuously changing as we revolve around the sun. Think of it like sitting on the merry-go-round looking through the middle and out the other side. The distant scenary is not moving, but it is changing as we move in a circle.
The constellations that the Sun appears to be moving through are the constellations of the zodiac. Wow, that was a lot of build-up to such a simple idea.
But what makes them special? Nothing really, but our ancestors, not really knowing what those moving lights in the sky were, attributed the motion to ancient gods moving through the heavens. Just like the Sun moves through the constellations of the zodiac, so do the planets simply because the plane of their orbits is so close to the ecliptic. So, they concluded, it might be important which constellation the Sun was in when you were born and where those "gods" were wondering around. Fortune tellers loved this and its popularity has never completely gone away. There is, of course, no science behind it.
The Constellations of the Zodiac
Traditionally, there are twelve of these. For astronomers, the "first" one is Pisces. It is first because astronomers measure right ascension (the coordinate which is like longitude) starting from the position the Sun occupies on the spring equinox. But since we travel around the whole sky in a big circle, it doesn't really matter where you start. We'll go over them very quickly from east to west.