The astronomy word of the week is “ecliptic”

Posted in Astronomy Word of the Week

Imagine you are 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 center column of the carousel. The column goes out of focus as you begin to notice what’s on the other side. The ticket booth goes by, then a food vendor. A family posing for a picture, then some benches with people resting there feet. Around and around, the scene drifting behind the central column of the carousel changes until you have come all the way around and are looking at the ticket booth again. And the scenery begins to repeat itself.

Now replace the horse with the Earth, the center column with the Sun, and the background panorama of an amusement park with distant stars. As the Earth flys around the Sun at 67,000 mph, the “scenery” behind the Sun changes. The stars themselves appear to drift into then out of our field of view. Standing in deep space with no reference to guide us, it seems like the Sun itself is drifting from west to east across the starry background.

The path of the Sun – shown by the dashed line – across the winter the sky. The dates mark the location of the Sun on those days.

This is a sight no Earth-bound humans can ever see. We can’t see what’s “behind” the Sun because, of course, we can only see the Sun during the day. And while the stars are still there, the daytime sky is much too bright for us to see them. But if we could see the stars at noon, we would notice a motion of the Sun of which few of us are aware. That the Sun moves from east to west every day is apparent to anyone with eyes to see. But lacking the visual reference of daytime stars, we never notice that the Sun is ever so slowly drifting across the stars from day to day in an easterly direction.

Of course, it’s not the Sun that’s moving, it’s us. The annual motion of the Sun, like its daily trek across the blue sky, is an illusion. Nevertheless that the apparent position of the Sun changes with respect to the stars is a very real phenomenon. That motion traces out a path on the sky along which the Sun endlessly loops. Astronomers call this line in the sky the “ecliptic”.

The ecliptic is very much like the celestial equator from last week’s article. It’s another line on the sky, plastered on to the celestial sphere and completely encircling the Earth. But because it charts the path of the Sun, it has always held greater significance to humans then the completely imaginary celestial equator. This line can be seen, in a sense. As the Sun drifts from West to East, it passes through, or “in front of” if you prefer, a number of constellations. In early March, the Sun is flanked by the stars in the constellation Aquarius. Then it moves to Pisces. Then Aries, Taurus, Gemini, Cancer, Leo, and so on. If those constellation names sound familiar, it may 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 – those which are intersected by the ecliptic. Though Western astrologers have only ever recognized twelve signs, there are actually thirteen constellations that lie along the ecliptic. The thirteenth which didn’t make the astrologer’s cut is Ophiucus, the water-bearer serpent-bearer, who sits between the summer constellations of Scorpius and Sagittarius.

The ecliptic gets its name because it is along that line that eclipses occur. A lunar eclipse happens when the Moon passes through the shadow of the Earth – it is then directly opposite to the Sun on the sky. During a solar eclipse the Moon passes directly between the Earth and the Sun momentarily blocking out it’s light and warmth. Though the Moon circles the Earth roughly once a month, eclipses don’t happen nearly that frequently because the Moon’s orbit is tilted ever so slighty relative to that of our planet. Our satellite actually spends most of it’s time either above or below the plane of the 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. Hence why eclipses happen only a couple of times a year at most!

The ecliptic is also very useful to astronomers for another less obvious reason. Last week we talked about the word “declination” and how it allows astronomers to measure how far north or south a star is in the sky. But what about the east-west direction? For that, astronomers use a system similar to longitude on the earth. Imaginary lines that run north to south in the sky, from one pole to another, mark off what astronomers call “right ascension“. This is the east-west equivalent of declination. But declination has a natural “zero point” – a point from which to start measuring. That would be the celestial equator. But there’s nothing like that for measuring right ascension – no natural line dividing the sky into east and west hemispheres.

On Earth, we’ve picked a rather arbitrary line of longitude and called it zero – the Prime Meridian which runs through the Royal Observatory in Greenwich, England. The reasons for choosing that particular spot for zero longitude are predominately political, having to do with the British Empire’s supremacy at the time longitude lines were made official. Fortunately, the ecliptic gives us a way of picking a line of zero right ascension that’s less mired in politics.

You see, there’s another way to think about the ecliptic. Like the celestial equator is a projection of the Earth’s equator onto the night sky, the ecliptic is also a projection – of the Earth’s orbit. It marks out on the sky the plane in which we orbit the Sun. Because the Earth’s axis is tilted by about 23 degrees relative to our orbit, the lines of the ecliptic and the celestial equator intersect one another, like two great circles drawn on a beach ball. The two points where these celestial guideposts meet are called the equinoxes. And they represent very special points in the sky.

The Sun spends half of the year south of the celestial equator, and half north of it. Where the Sun crosses the equator, moving from south to north, is a point along the ecliptic called the vernal equinox. Six months later, as the Sun descends back into the southern half the sky, where it cross the equator again is referred to as the autumnal equinox. If these points in the sky sound eerily familar to days on the calendar, then you’re ahead of the class. For these are the names given to the first days of spring and autumn in the northern hemisphere, respectively. On March 21st, or thereabouts, the Sun is sitting directly on the celestial equator, heading in a northerly direction.

The vernal equinox marks the zero point of the entire celestial coordinate system. It sometimes is referred to by the slightly archaic name “the first point of Aries“. This may seem odd to modern observers who try to locate the vernal equinox on a star map. You’ll find that it doesn’t sit in the constellation Aries at all, but rather in Pisces. This is yet another effect of the precession of the Earth’s axis – the wobble of the Earth that causes the North and South Poles to point at different stars over a 26,000 year cycle. Since the Earth wobbles, and therefore the projection of the equator on to the sky drifts, the location of the equinoxes also changes (this is why in some places you’ll see the Earth’s wobble referred to as the “precession of the equinoxes”). Roughly 2000 years ago, the vernal equinox sat in Aries, but the Earth’s axis has drifted enough in that time to relocate it to Pisces. In another 2000 years or so, the equinox will be passing through Aquarius, then Capricornus, and so on around the Zodiac.

The yellow circle marks the current location of the vernal equinox or where the Sun sits in the sky on the first day of spring.  The red line is the ecliptic, the blue one the celestial equator.

You can actually see the ecliptic on any clear night, if you’re lucky. And this time of year is a good time to see it. Because the orbits of all the planets in our Solar System lie roughly in the same plane, the ecliptic is also a rough guide to where you’ll see the planets in the sky. Right now, Mercury, Venus, Jupiter, the Moon, and Mars are stretched out 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. And there you’ll see the ecliptic – the Sun’s path, the plane of our planet’s orbit, the line of eclipses, the starting point for mapping the entire celestial sphere – 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.

Updated on 3/14: One of the listeners on the 365 Days of Astronomy podcast pointed out I accidentally referred to Ophiucus originally as the water-bearer.  Ophiucus is the serpent-bearer.  The water-bearer is Aquarius.  The text above has been corrected!

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