Signs of the Zodiac

    Even though the planets move on the celestial sphere, they do not wander all over it but are confined to a narrow strip, dividing it in half. Stars along that strip are traditionally divided into the 12 constellations of the zodiac. The name, related to "zoo," comes because most of these constellations are named for animals--Leo the lion, Aries the ram, Scorpio the scorpion, Cancer the crab, Pisces the fish, Capricorn the goat and Taurus the bull.

    At any time, the Sun is also somewhere on the celestial sphere, and as the Earth turns, it rises and sets the same way as stars do.

    Like the planets, the Sun, too, moves around the zodiac, making one complete circuit each year. Every month it covers a different constellation of the zodiac, which is the real reason why those constellations are 12 in number. Of course, during that month, this particular constellation is not seen, because the sky near the Sun is too bright for its stars to be seen without a telescope (except, very briefly, during a total eclipse of the Sun).

    One can however figure out where the sun is on the zodiac (as ancient astronomers have done) by noting which is the last constellation of the zodiac to rise ahead of the Sun, or the first to set after it. Obviously, the Sun is somewhere in between. In this manner each month-long period of the year was given its "sign of the zodiac."

The Ecliptic

    The path of the Sun across the celestial sphere is very close to that of the planets and the moon. After clocks became available, it was a relatively straightforward job for astronomers to relate the path of the Sun in the daytime to the one of stars at night, and to draw it on their star charts. Because of its relation to eclipses, that path is known as the ecliptic.

The orbit of the Earth around the Sun. This is a perspective view, the shape of the actual orbit is very close to a circle.

    The significance of the ecliptic is evident if we examine the Earth's orbit around the Sun. That orbit lies in a plane, flat like a tabletop, called the plane of the ecliptic (or sometimes just "the ecliptic"). In one year, as the Earth completes a full circuit around the Sun (drawing above), the Earth-Sun line and its continuation past Earth sweep the entire plane. The far end of that line then traces the ecliptic on the celestial sphere; if you have a star chart handy (it is often included in an atlas), you will find the ecliptic traced there, too.

The Planets and the Moon

    Planets seen in the sky are always near the ecliptic, which means that their orbits are never too far from the plane of the ecliptic. In other words, the solar system is rather flat, with all its major parts moving in nearly the same plane.

What about the connection between "ecliptic" and eclipses?

    The moon's orbit cuts the ecliptic at a shallow angle, around 5 degrees, which means that on the celestial sphere the Moon, too, follows a path through the zodiac. Half the time the Moon is north of the ecliptic, half the time south of it. If the shadow of the moon hits the Earth, the Sun is eclipsed in the shadow area; if on the other hand the shadow of the Earth covers the moon, the moon goes dark and we have an eclipse of the moon.

Either of these can only happen when the Sun, Earth and Moon are on the same straight line. Since the Sun and Earth are in the plane of the ecliptic, the line is automatically in that plane too; if the moon is also on the same line, it must be in the plane of the ecliptic as well.

    It takes close to a month for the Moon to go around the Earth ("month" comes from "Moon") and during that time its orbit crosses the ecliptic twice, as it goes from one side to the other. At the time of crossing, the Sun may be anywhere along the ecliptic; usually it is not on the Earth-Moon line, and therefore an eclipse usually does not take place. Occasionally, however, it is on that line or close to it. If it then happens to occupy exactly the same spot on the celestial sphere, we get an eclipse of the Sun, because the moon is then between us and the Sun. On the other hand, if it occupies the spot exactly opposite from that of the Moon, the Earth's shadow falls on the Moon and we have an eclipse of the Moon.

(Added August 2005)

A new distant planet far from the Ecliptic