Pluto And Beyond #2: The Ecliptic

Using my little telescope that I’ve had almost my whole life, I’ve observed Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune. I know I’ll never see Pluto through my telescope. But at some point in the next few years I hope to use my telescope as a guide scope mounted on a larger aperture telescope fitted with a good digital imaging system to photograph an ecliptic star field over a series of nights that will show Pluto as a tiny point of light moving against the backdrop of fixed stars. (Of course, modern digital imaging systems can function as auto-guiders, but I want to give my old faithful telescope a role in tracking down all nine planets. [I have no passion for this business of is or isn’t Pluto a planet. To me it will always be the ninth planet.])

The ecliptic is a key idea in planetary astronomy and kind of interesting, too. Today I’m just going to ramble on about it for a bit. Observing Uranus last Summer helped me get a full picture of the ecliptic visually for the first time.

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I suspect every astronomer, professional and amateur, has his or her favorite and least favorite ways of explaining the ecliptic to people new to astronomy. It’s not really that complicated of an idea but some of the typical explanations make it seem complicated. There’s a general usage and a specific usage. I think if you look at the general usage first it helps understand the specific meaning.

Wikipedia opens with a typical explanation: “The ecliptic is the apparent path that the Sun traces out in the sky. As it appears to move in the sky in relation to the stars, the apparent path aligns with the planets throughout the course of the year.”

The trouble is, nobody can ever see the Sun and the stars at the same time, so this explanation tries to create a picture that is literally impossible to ever make real.

The simplest way to communicate the idea of the ecliptic in general is to go outside on a clear night, point at whatever planets are visible, and trace a half-circle across the sky from horizon to horizon that intersects the visible planets and say that area of sky, that arc, that area of sky that includes all the planets, is the ecliptic in the general sense.

Another way to get the general sense of the ecliptic is to compare the Sun to Saturn. Most people have seen pictures of Saturn with its beautiful ring system extending out into space lined up with the planet’s equator. If you picture our Sun as a sphere in space, the Sun has no rings, of course, but the planets in the solar system extend out roughly from the Sun’s equator on a plane, just like rings would. That flat plane extended out from the Sun’s equator is the ecliptic in the general sense, the plane on which the planets travel when they orbit around the Sun.

The specific usage of the word is a result of the Earth traveling around the Sun on the same plane with the other planets. When the Earth travels around the Sun, if we were magically able to see the Sun and the stars at the same time, as the Earth shifted in space around the Sun the stars behind the Sun would change, too. The Sun appears to move, day by day, month by month, through the constellations. In fact, that’s where we get our Zodiac from. The Zodiac is the set of twelve constellations the Sun appears to move through in the course of a year. The specific path the Sun takes through those constellations is called the ecliptic.

This is an interesting time of year because it’s possible to get a good ‘view’ of the imaginary path—In Taurus, the ecliptic passes right between Aldebaran and the Pleiades, then just south of Pollux and Castor in Gemini.


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One interesting thing about the ecliptic is that the human race seems to have known about it forever. Some historians believe a Zodiac system similar to ours was in use more than 5,000 years ago.

It’s fun speculating on how the Zodiac may have gotten started. Did early star gazers notice that certain stars lined up with certain landmarks when the stars were rising or setting, and then later notice that the Sun, rising or setting, lined up with those same landmarks? Or did early star gazers notice Mars, Jupiter and Saturn always seemed to move, to wander, within a certain area of the heavens and did they then pay special attention to that area and start grouping those stars into the Zodiac?

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The ecliptic is important to planet hunters because if all the planets orbit on the same plane, if they’re all visible in the same area of the sky, then how hard can it be to track down new planets?

The accepted story of William Herschel discovering Uranus makes it sound not too hard at all. One night, so the story goes, William Herschel was out with his telescope just ‘looking around Taurus’ and he saw a star he didn’t recognize, a star that didn’t appear quite like a star but appeared to have a disk. He thought it might be a comet, but he and other astronomers quickly realized it was a new planet. It can be that easy!

However, little details make things very difficult.

All the planets orbit roughly on the same plane, but most planets orbit a little north or a little south of the exact plane of the Earth’s orbit. Most planets differ by about one and a half degrees. Pluto, however, is more than fifteen degrees off the exact plane of the Earth’s orbit.

Those little differences add up.

For instance, when I observed Uranus from my back yard, the planet was so low in the sky, so near to my local horizon, that I could use a telephone pole in the alley to key on the planet’s location when I shifted from binoculars to my telescope. On the other hand, Mars is so high in the sky right now that I need to use a right angle adaptor on my telescope to observe Mars.

Normally planet hunters consider the ecliptic at its most general to be the thirty degree band of sky that stretches from fifteen degrees north to fifteen degrees south of the actual ecliptic. And that’s a lot of sky. It took Tombaugh about a year to work around the entire Zodiac.

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Over the last eighty years or so, ever since Clyde Tombaugh discovered Pluto by systematically searching the ecliptic, many astronomers have repeated Tombaugh’s work. It would appear there are no additional large planets waiting to be discovered beyond Pluto.

Or are there?

This is where the ecliptic becomes interesting indirectly.

There are ways a traditional photographic search of the ecliptic might miss a planet. And there are theories about the outer solar system which suggest large planets beyond Pluto may orbit entirely off the ecliptic.

We are living, in fact, in the Golden Age of outer solar system studies. I’m going to ramble on about that tomorrow.





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Tomorrow: The Golden Age of Outer Solar System Studies