Understanding the Ecliptic and the Zodiac

Because of the Earth's yearly revolution around the Sun, the Sun appears to move in its annual journey through the heavens with the ecliptic as its path. Technically then, the ecliptic represents the extension or projection of the plane of the Earth's orbit out towards the sky.

But since the Moon and planets also move in orbits whose planes do not differ greatly from that of the Earth's orbit, these bodies, when visible in our sky, always stay relatively close to the ecliptic line. Our solar system can be thought of as somewhat flat, with most of the planets (the exception being Pluto) moving in very nearly the same plane.

It is for this reason that most sky charts plot the position of the ecliptic; it is something of a warning to skywatchers that strange "stars" (planets) often appear near and along this path through our heavens, as well as the Moon. Usually the Moon and planets are not positioned exactly on the ecliptic (because they're not located exactly in the same orbital plane as Earth), but lie within several degrees of it and form a sort of narrow strip encompassing the entire sky which we call the Zodiac.

The ecliptic runs exactly along the middle of the Zodiac.

Twelve constellations through which the ecliptic passes form the Zodiac. The name is derived from the Greek, meaning "animal circle," also related to the word "zoo," and it comes from the fact that most of these constellations are named for animals, such as Leo, the Lion, Taurus, the Bull and Cancer, the Crab. These names, readily identifiable on sky charts, are familiar to millions of horoscope users (many of whom, ironically, would be hard pressed to find them in the actual sky).

If we could see the stars in the daytime, we would see the Sun slowly wander from one constellation of the Zodiac to the next, making one complete circle around the sky in one year.

Ancient astrologers were able to figure out where the Sun was on the Zodiac 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 had to be somewhere in between. As such, each month a specific constellation was conferred the title of "House of the Sun," and in this manner each month-long period of the year was given its "sign of the Zodiac."

Interestingly, the "sign" which has been assigned for a given month in the horoscope that you'll find in your daily newspaper is not where the Sun actually is that particular month, but where it would have been thousands of years ago! This is due to the "wobble" of the Earth's axis (known as precession) which alters the direction in the sky to which the North Pole points, also changing over time the relative positions of all the stars. Yet today's astrologers, who believe that the Sun, Moon and planets mysteriously direct our lives, continue to adhere to star positions that for all intents and purposes are out of date by thousands of years.

The ecliptic also passes through the constellation of Ophiuchus, which is not included among the so-called signs of the Zodiac. In fact, the Sun spends more time traversing through Ophiuchus than nearby Scorpius. During 2003, the Sun officially resides in Scorpius for less than a week: from Nov. 23-29. It then moves into Ophiuchus on Nov. 30 and remains within its boundaries for more than two weeks -- until Dec. 17. Yet the Serpent Holder is not considered a member of the Zodiac and so must defer to Scorpius.

In addition, because the Moon and planets are often positioned either just to the north or south of the ecliptic, it allows them to sometimes appear within the boundaries of several other non-zodiacal star patterns. For instance: from May 15 through June 5 of 2003, the planet Saturn will officially be within the boundaries of Orion, (passing across the Hunter's club). Other constellations that can be visited by the Moon and planets include Auriga, the Charioteer, Cetus, the Whale, and Sextans, the Sextant.

Because the Moon's orbit is inclined 5 degrees to the Earth's orbital plane, periodically it crosses over the ecliptic. Should this happen when the Moon is at new phase -- positioned between Earth and the Sun -- it will cross directly in front of the Sun causing a solar eclipse.

If the Moon crosses over the ecliptic when the Moon is at Full phase, while it is opposite the Earth in relation to the Sun, it will pass into the shadow of the Earth resulting in a lunar eclipse.

Usually when the New Moon is in the vicinity of the Sun it appears to pass above or below it and no eclipse occurs. Similarly, the Full Moon usually misses the Earth's shadow by sweeping above or below it. Only when all three bodies (Sun, Earth and Moon) are on a straight line occupying the plane of the ecliptic can an eclipse occur. Hence the name ecliptic (the place where eclipses occur).

 

return to top >