How the Ecliptic and the Zodiac Work

How the Ecliptic and the Zodiac Work
From the Earth's perspective in late May and into June, the Sun lies along a line of sight to the constellation Taurus. Earlier in the year, the Sun would have appeared in Aries. As the Earth proceeds in its orbit around the Sun (counterclockwise in this picture), the line of sight will point to Gemini, off the left side of the image.

Of theimaginary coordinate lines that astronomers and navigators use in mapping thesky, perhaps the most important one is the ecliptic, the apparent path the sunappears to take through the sky as a result of the Earth's revolution aroundit.

Because ofthe Earth's yearly revolution around the sun, the sun appears to move in itsannual journey through the heavens with the ecliptic as its path. Technicallythen, the ecliptic represents the extension or projection of the plane of the Earth'sorbit out towards the sky.

But sincethe moon and planets also move in orbits, whose planes do not differ greatlyfrom that of the Earth's orbit, these bodies, when visible inour sky, always stay relatively close to the ecliptic line. In other words,our solar system can be best defined as being somewhat flat, with the planetsmoving in very nearly the same plane.

It is for thisreason that most sky charts plot the position of the ecliptic; it is somethingof a warning to sky watchers that strange "stars" (planets) oftenappear 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), butlie within several degrees of it and form a sort of narrow strip encompassingthe entire sky which we call the Zodiac.

Theecliptic runs exactly along the middle of the Zodiac.

The"Classic Twelve"

Twelveconstellations through which the ecliptic passes form the Zodiac. The name isderived from the Greek, meaning "animal circle," and also is related tothe word "zoo," coming from the fact that most of theseconstellations are named for animals, such as Leo, the Lion; Taurus, the Bull;and Cancer, the Crab, just to name a few.

These nameswhich can be readily identified on sky charts are familiar to millions of horoscopeusers (who — ironically — would be hard pressed to find them in the actualsky!).

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

Ancient astrologers were able to figure out where the sunwas on the Zodiac by noting which was the last zodiacal constellation to riseahead of the sun, or the first to set after it. Obviously, the sun had to besomewhere in between. As such, each month a specific constellation wasconferred the title of "House of the sun," and in this manner eachmonth-long period of the year was given its "sign of the Zodiac."

Some discrepancies

Interestingly, however, the "sign" which has beenassigned for a given month in the horoscope that you'll find in your dailynewspaper is notwhere the sun actually is for that particular month, but where it wouldhave been several millennia ago!

This is due to the "wobble" of the Earth's axis(known as precession); yet today's astrologers, who believe that the sun, moon andplanets mysteriously direct our lives, continue to adhere to star positionsthat for all intents and purposes are out of date by thousands of years!

In addition, the ecliptic crosses through the constellationof Ophiuchus, the Serpent Holder. In fact, the sun spends more time traversingthrough Ophiuchus than nearby Scorpius! It officially resides in Scorpius forless than a week: from Nov. 23 through 29. It then moves into Ophiuchus onNovember 30 and remains within its boundaries for more than two weeks — untilDec. 17. And yet the Serpent Holder is not considered a member of the Zodiacand so must defer to Scorpius!

In addition, because the Moon and planets are oftenpositioned either just to the north or south of the ecliptic, it allows them tosometimes appear within the boundaries of a number of other non-zodiacal starpatterns.In fact, as pointed out by the well-known astronomical calculator,Jean Meeus, along with Ophiuchus, there are nine other constellations thatoccasionally can be visited by the Moon and planets: Auriga, the Charioteer;Cetus, the Whale; Corvus, the Crow; Crater, the Cup; Hydra, the Water Snake;Orion, the Hunter; Pegasus, the Flying Horse; Scutum, the Shield; and Sextans,the Sextant.

So in truth, there really aren't twelve zodiacalconstellations, but twenty-two!

Origin of "Ecliptic"

Although the moon's orbit is inclined 5.5 degreesto the Earth's orbital plane, periodically there will come times when itcrosses over the ecliptic.

Should this happen when the moon is at new phase, it willend up crossing in front of the sun causing a solar eclipse. If the mooncrosses over the ecliptic when the moon is at full phase, it will pass into theshadow of the Earth resulting in a lunar eclipse. Usually when the new moon isin the vicinity of the sun it appears to pass above or below it and no eclipseoccurs. Similarly, the full moon usually misses the Earth's shadow by sweepingabove or below it.

Only when all three bodies (sun, Earth and moon) are on astraight line occupying the plane of the ecliptic can an eclipse occur.

Hence the name "ecliptic": the place where eclipses occur.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and otherpublications, and he is also an on-camera meteorologist for News 12 Westchester, New York.

Have a news tip, correction or comment? Let us know at community@space.com.