Your Sign... or Mine?

All right. In yesterday’s blog, I promised to talk about astrology, so let’s start with the most common form of astrological knowledge: your “sign”. Literally everybody knows what their sign is, right? Mine happens to be Cancer; yours is most likely one of the others, perhaps Leo, Virgo, or Libra; maybe Scorpio, Sagittarius, or Capricorn; possibly Aquarius, Pisces, Aries, Taurus, or Gemini. It is also common knowledge that our signs are dictated by the date of our birth, with each of the 12 signs roughly aligned with one of the 12 months of the calendar year.

Astrological signs are so deeply embedded in popular culture that horoscopes run everyday in major newspapers, and knowledge of one’s sign is often considered an important form of personal information (“Hi! “I’m a Cancer. What’s your sign”)? But what are astrological signs, really? Where do they come from? Why are they called “Sun” signs”? To answer these answers, I’m going to ask you to look at some familiar “landmarks”, and some familiar locations, from a new and different perspective.

If you happen to look up at the stars tomorrow night, one of the first things that will strike your eye is the fact that the stars seem to form recognizable patterns, called constellations. Each astrological sign is named after a particular constellation. People born this month are Aries, because their astrological sign corresponds to the constellation of the same name. So, when you look at the sky in April, are you able to see Aries? The answer is a most emphatic “No”! Right now, Aries is as invisible as astronomical objects ever get!

The diagram below tells the story. It depicts where the Earth is in its orbit at roughly this time of year (the illustration says “The Earth in May” instead of "The Earth in April" because a slight discrepancy has emerged over the centuries between the real position of the sun against the stars, as shown in this figure, and the designation of where the sun is in astrological terms.  I'm deliberately glossing over this discrepancy for purposes of this blog). If you draw a straight line between the Earth, and Sun, and the sky beyond, the arrow points directly at Aries. That’s why you can’t see it. Right now, the Sun and Aries occupy the same location in the sky. They rise and set together, so Aries is not visible at night.

Once the sun sets, however, other astrological (or more precisely, zodiacal) constellations ARE visible. Near the Western horizon are the constellations Taurus and Gemini. Cancer is small and not very prominent, but the big spring winner of the zodiacal constellation sweepstakes, Leo, is very prominent; you can identify Leo by the stars that are arranged in the shape of a mirror-image question mark. The other zodiacal constellations visible early tonight, Virgo and Libra, are further to the east, and harder to spot because they are lower in the sky. But, if you wait until later on, and give the Earth a chance to rotate further east, the more familiar summer constellations of Scorpio and Sagittarius will rotate into view.

But wait, you may be saying. I’ve just named the astrological signs for birthdays that come later on this summer and fall. If astrological signs are defined by which constellation the sun is aligned with that month, and the sun is in Aries right now, it must have to be shift position to align with these other constellations over the course of the next few months. That is, the sun must be moving constantly along an imaginary line that connects these constellations.

Well, it is certainly the case that the sun appears to move along such a line. The figure above tells the real story, though. It is really the Earth that is moving along its orbital track. Every day, we shift position to the right along that imaginary line by some 1.6 million miles. This movement forces the line extending from the Earth to the Sun and beyond to pivot leftward. As you can see, the effect is to “push” the Sun through the constellations that happen to be lined up with the Sun’s position in the sky: that’s what defines a zodiacal constellation. By next month, May, the Earth will have moved far enough to co-locate the Sun with the stars that make up the constellation Taurus, the Bull, which is why May babies have Taurus as their Sun sign.

Simple, right? In the figure below, we’ve fast-forwarded exactly half the year, to early November.  In that amount of time, the Earth has shifted an enormous distance, enough to push the Sun all the way to the position in the sky that’s occupied by Libra. Then, the Sun “enters” Scorpio, the sign of you mid-November babies (again, the astrological dates for Libras and Scorpios are somewhat earlier than the dates would be if the astrological dates were aligned with the actual present-day position of the Sun in relation to the zodiacal constellations).

That’s pretty much the story, Whabbloggers, at least for astrology. In the third blog of this series, I’ll return to the issue of how the apparent movement of the Sun through the zodiacal constellations changes the stars we see in the night sky. Would you care to steal my thunder by speculating on that in the comments section?