The ecliptic is an imaginary line on the sky that marks the annual path of the sun. It is the projection of Earth’s orbit onto the celestial sphere. And it is an essential part of any stargazer’s vocabulary.
Besides define the path of the sun, the ecliptic marks the line along which eclipses occur, the moon and planets and asteroids wander, the Zodiac constellations live. The ecliptic is even the starting point for the celestial coordinate system used by astronomers to pinpoint the location of every star, nebula, and galaxy. To better understand this region of the sky, let’s start with a ride on a carousel.
Sitting on a wooden horse, your hands clasped around a cool, brass pole, you are swung around and around as the sights of an amusement park blur past your vision. As you circle around, your eyes begin to wander and become fixated on the central pillar of the carousel. The pillar goes out of focus as you begin to notice what’s on the other side. The ticket booth goes by, then a food vendor. There’s a family posing for a picture, then some benches with people resting their feet. Around and around, the scene drifting behind the center column of the carousel changes until you have come all the way around and are looking at the ticket booth again.
Now replace the horse with Earth, the column with the sun, and the background panorama of an amusement park with the distant stars. As Earth flys around the sun at 67,000 m.p.h. (108,000 kilometers m.p.h.), the “scenery” behind the sun changes.
For example, if we could see the stars during the day, we would notice that in late March and early April, the constellation Pisces (the Fish) is on the other side of the sun. As the days and weeks went by, the sun would appear to drift eastward across Pisces until moving in front of Aries, the Ram, in the second half of April. One month later, the sun would be flanked by the stars in Taurus, then Gemini, Cancer, Leo, and so on. Roughly every month or so, as Earth moes in orbit, a different constellation sits behind the sun.
If those constellation names sound familiar, it might be because you’ve seen them in your newspaper’s horoscope section. The signs of the Zodiac come from the constellations through which the sun passes. They are the constellations which lie in Earth’s orbital plane. Though Western astrologers have only ever recognized 12 signs, there are actually 13 constellations that lie along the path of the Zodiac. The 13th, which didn’t make the astrologer’s cut, is the constellation Ophiuchus. It the Serpent-Bearer constellation, partially located along the ecliptic between the summer constellations of Scorpius and Sagittarius.
The ecliptic – the line across our sky defined by the sun’s path – gets its name from the fact that eclipses can only occur along it. A lunar eclipse happens when the moon passes through Earth’s shadow, when it is directly opposite the sun on the sky. During a solar eclipse the moon passes between Earth and the sun momentarily blocking out its light and warmth. Though the moon circles Earth roughly once a month, eclipses don’t happen nearly that frequently because the moon’s orbit is slightly tilted relative to that of our planet. Our satellite actually spends most of its time either above or below the plane of Earth’s orbit and therefore is usually not nicely aligned with us and the sun. Twice a month it crosses the ecliptic – but an eclipse will only occur when that passage happens during either a full moon for a lunar eclipse or a new moon for a solar one. The need for this precise alignment is why eclipses happen only a couple of times a year at most.
Since the other seven planets orbit in approximately the same plane as Earth, the ecliptic is also a decent guide to where you’ll see the planets in the sky. To put that another way, the planets allow you to actually trace out the ecliptic on any clear night yourself. Right now (April 1, 2012), the moon, Jupiter, Venus, and Mars are stretched out from west to east across the sky shortly after sun set. Go out tonight and try to find them. They’ll be some of the most brilliant points of light in the sky. Connect the dots and you’ll see the ecliptic – the sun’s path, the plane of our planet’s orbit, the zodiac, and the line of eclipses – arcing overhead. Hopefully it demystifies the night sky, even if just a little bit, and starts you on a journey of unraveling what the nightly dance of the heavens is telling you.
Bottom line: What is the ecliptic? The ecliptic traces out the apparent annual motion of the sun across the sky. The signs of the Zodiac come from the constellations that lie along this line. You can see the ecliptic yourself by drawing a line connecting the planets and the moon.
Christopher has a Ph.D. in astronomy from the University of California, Los Angeles. After eight years of searching for exoplanets, probing distant galaxies and exploring comets, Chris realized he enjoyed talking about astronomy a lot more than actually doing it. After being awarded a 2013 AAAS Mass Media Fellowship to write for Scientific American, he left a research career at the U.S. Naval Observatory to pursue a new life writing about anything and everything within the local cosmological horizon. Since 2014, he's been working with Science News.