Perceiving the Primordial Pancake

For the last two weeks of March, all five naked eye planets - Mercury, Venus, Mars, Jupiter, and Saturn - as well as our own moon, will share the evening sky.

In addition to being a great opportunity for those of us with telescopes to view each of these planets, this also provides a fun way to teach a little celestial mechanics to your less astronomically oriented friends. To wit: the ecliptic.

Go out any night in late March.ÿ Look west, and find bright Venus, blazing at magnitude -4.3. A telescope will show it slightly crescent, becoming thinner as the month progresses. Mars is the red "star" above Venus, dim in comparison at magnitude 1. Mars is far away from us now, showing a tiny 5 arcsecond disk not much bigger than Uranus, but think of rovers Spirit and Opportunity as you look at that red dot.

If you hunt a little very early in the evening, you can find Mercury below Venus, brighter than Mars at magnitude -1 and showing its best evening apparition of the year: it sets more than an hour and a half after the sun.

Look overhead and find the bright yet steady "star" near the western foot of Gemini. That's Saturn, high in the sky and perfectly placed for telescopic observers who want to get their best view of the ring system.

Now swing your gaze over to the eastern sky, to the planet that seems almost as bright as Venus - Jupiter! Okay, at magnitude-2.5 it's not really even close to being as bright as Venus, but this near opposition (March 4th), Jupiter is about as bright as it ever gets. Probably enough to make those deep sky observers stay home! Jupiter has enough features, constantly changing in as little as a few days, to keep any telescope owner busy, and its four brightest moons are visible even in a good binocular. We're back in the season of double shadow transits now: Io and Europa on March 4 at 11:22 P.M.; Io, Ganymede and both of their shadows on the 20th at 10:38 P.M.; and a rare triple transit of Io, Europa, Ganymede and all three shadows, at midnight on the 27th-28th. Even better, that's a Saturday night, so mark your calendar!

Now that you've located all the naked eye planets, go back and start again with Mercury. Hold your arm out and point (unless you have one of those nifty green lasers). Now swing your arm through Venus and Mars, up to Saturn, then over to Jupiter, and notice ... they're all in a line!

That line is the "ecliptic", which you've probably seen as a dotted line on star charts.

The ecliptic is the plane of the earth's orbit, and, roughly, the plane in which all but one of our planets orbit. (Pluto is the exception: its orbit is inclined seventeen degrees from the ecliptic, which is one of the reasons that some people have tried to argue for its non-planethood.)

Why do all the major planets orbit in the same plane? Is it some staggering coincidence?

Actually, it's a consequence of the spinning disk of dust from which the sun and planets originally coalesced - the primordial planetary pancake. Since all the dust started out in the same plane, as bits of dust bumped into each other and gradually grew into planets, they remained in that original orbital plane. Most moons in the solar system also orbit in that plane, though a few don't.

I mentioned the zodiacal light last month, and this month is another good chance to see it.

What it is: sunlight reflecting from particles of dust and other small debris which lie along the plane of the ecliptic, making a faint column of light extending from the sun's position up along the ecliptic. In other words, some of what you're looking at might even be stardust left over from the formation of the solar system!

Usually the zodiacal light is a fairly challenging target, especially near a big city like San Jose. But it can be seen, and a perfect chance is at the Messier Marathon on March 20th. If you go out that weekend, remember, after the sky gets fully dark, to take a look in the western sky for a faint pillar of light stretching up along the ecliptic. It won't be bright - it's a lot less obvious than the Milky Way - but if the weather is favorable, you'll have an excellent chance of adding a primordial pancake plus five planets to your marathon observing list.