Zodiacal Constellations

by Whabby

Yesterday, Carol wrote a beautiful blog in which she talked about a group of seven special stars, named after her aunts and uncles, that her Mom always looks for in the darkness just before dawn. Carol noted that her Mom cannot see these stars all the time, because the makeup of the pre-dawn sky changes with the seasons, but the “Group of Seven” always swings back into view eventually.

How does that work, owls? What’s going on in the sky that makes pre-dawn stars come and go with the seasons?

Let me see if I can answer this question via astrology, a topic we’re all familiar with. We all have an astrological sign. Coincidently, mine happens to be Cancer, the July sun sign. Your sign is either Cancer too, or one of the others: Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, Pisces, Aries, Taurus, or Gemini. Most of us can rattle off our astrological sign in a heartbeat; indeed, as reflected by that clichéd pick-up line, “What’s your sign”, many of us deem our sign to be an important piece of information about ourselves.

But what are astrological signs, really? To truly understand them, we’re going to have to look at some familiar “landmarks” from a new and different perspective.

Most everybody knows that their sun sign corresponds to a particular constellation in the sky. Since people born today are Cancers, when I talk about looking at familiar landmarks, you would be forgiven for thinking that I’m going to start with the constellation Cancer, the Crab.

The trouble is, you can’t see the Crab right now, because Cancer is located in the daytime sky, right smack where the sun is (that’s why the astrological constellations are know as “sun signs”). Once night falls, other astrological (or more precisely, zodiacal) constellations ARE visible. Suppose you go outside during the next couple of nights, maybe about an hour and a half after sunset. Look for the Big Dipper, and then trace out an imaginary arc that stretches from where the dipper’s handle is pointing. You should come to a bright orange star; that is Arcturus. Even further down the imaginary arc is another bright star, Spica. Spica lies in the astrological sign of Virgo, the Maiden. Spica and Virgo may actually be pretty hard to see, as they will be close to the western horizon. However, below and east of Virgo is the prominent and well defined Scorpius, the Scorpion, and beyond that, the teapot-shaped Sagittarius. They should be easier to spot.

But wait, you’re saying. The constellations I’m babbling about correspond to astrological signs for people whose birthdays are coming up this fall. If the sun is in the vicinity of Cancer right now, it must be shifting to the vicinity of these other constellations a couple of months from now. And if that is true, the sun must be moving along a line that connects these constellations.

It is certainly the case that the sun appears to move along such a line (known as the ecliptic), with two interesting consequences. To describe them, let’s return to our hypothetical outdoor observations, sometime in the next couple of days, but this time I’ll ask you to imagine going out right at sunset, when the sun is barely above the horizon. Imagine, every day, the sun has moved a little way along the line that links the astrological constellations. If so, then every successive day the sun is “sneaking up” on the next zodiacal constellation in the yearly sequence; right now the “next in line” is Leo. Leo is another constellation that is not visible at the moment; it is too close to the sun, and is swallowed up in the twilight glare just above and to the left of where the sun sets. In another week or so, the Sun will have moved out of Cancer and will be directly co-located with Leo. At that point, it will be Virgo and Libra that are slipping into the Sun’s glare, and disappearing from the night sky.

But what the movement of the sun taketh away from the early evening sky, it giveth back to the early morning sky! Right now, before sunrise, the constellations that the Sun has recently left behind, like Gemini, and Taurus, and Aries the Ram (the Spring astrological signs, in other words), are becoming visible. As the Sun gradually moves ever farther away from them, they become more and more visible in the morning sky, before the sun rises. Another way of making this point is to note that just before sunrise, were you to go out and look toward the east, a little sliver of sky would be revealed to you, that had not been visible the night before.

Day after day, the sun constantly “captures” (swallows up) a small sliver of the night sky lying to the left of itself, and continually “releases” (makes visible) a sliver of sky down and to the right of it.

Are you with me so far, owls? Hang in there… just a little further to go. Of course, we all know that the sun isn’t really moving with respect to the earth; we’ve known that since the time of Copernicus, 500 years ago. So what’s really happening? Let's return to our hypothetical observation in the next couple of nights right at sunset. Picture the Earth constantly moving in a direction sort of consistent with the eastern horizon. However, try to imagine as well that we’re not really traveling in a perfectly straight line toward the east… instead, we’re moving along an enormous curved trajectory that is so big, compared to our perspective, that it forms almost a straight line from one day to the next… a line with just a slight curve to it.

Imagining that slight curve to the trajectory, though, is crucial. It means that, in fact, we are always circling around and heading toward a point in space that is down and to the right of the sun’s location. That’s the case, every single day, every single sunset.

So what does that do to the sun? The to-the-right movement of the Earth “pushes” the sun a little bit up and to the left (to the east). The cumulative result is that the sun forever moves serenely through the astrological constellations, completing its 12-part journey in the space of a year. And that’s why, in the current night sky before midnight, you see the astrological constellations that the sun will enter in the future (Scorpius, Sagittarius), while later on at night, approaching sunrise, they are replaced with the constellations that were collocated with the sun in the near past, but have now been left behind (Taurus and Gemini).

And that’s all there is to it! Stars don’t really move; the Sun doesn’t really move; just the Earth moves. The stars named after Carol’s aunts and uncles have a fixed location with respect to the astrological constellations, so they, too, appear and disappear in the dawn sky depending on where the sun is located on that particular night.

This website, (click here) has a great animated illustration of the how the Earth's movement around the sun produces the apparent movement of the Sun through the astrological sun signs.