Monday, April 28, 2008

Checking In With The Gods

This weekend, late Saturday night, I was up, out and about around 3 am.

I wasn’t thinking about astronomy, but astronomy is never far from my mind. When I saw the waning moon [the next new moon will be May 5th] half-full in the southern sky with a bright star just a few degrees north of the moon, I started thinking about astronomy.

I started thinking about astronomy because I couldn’t figure out what the bright star near the moon was.

There are only four “stars” near the ecliptic that are bright enough to be visible through moonlight.

The only “real” star bright enough to see when the moon is nearby is Sirius, in Canis Majoris. But Sirius is—famously—an evening star in the spring and by 3am evening stars will have moved far to the west, or even will have set below the western horizon.

The other “stars” bright enough to be seen through moonlight are the planets Venus, Jupiter and Saturn.

But Venus is an inner planet. That means from our perspective, from the Earth, Venus will sometimes be a morning star in the east and sometimes an evening star in the west but Venus never will rise too far above the horizon. We will never see Venus in the southern sky.

So the “star” I was seeing near the moon was either Jupiter or Saturn.

I hadn’t observed either Jupiter or Saturn for a couple of months, since the last lunar eclipse back in February. Back then, Saturn was an evening star in the east and Jupiter was an evening twilight star in the west. My general, off-the-cuff feeling late Saturday night was that at 3:30am Saturn would be too far west to be the bright star near the moon and Jupiter, now a morning star, would be too far to the east.

But I was wrong about something. Either I had somehow forgotten some obvious, bright star or my off-the-cuff thinking about Jupiter or Saturn was out-of-pace with the season.

The easiest and most fun way to figure out what was going on was simply to take a look at the bright “star” through a telescope!

So that’s what I did.

When I got home, a little before 4am, I set up my telescope—quickly, I only took out one eyepiece, an 18mm for 50x—and took a look.

After sighting the bright star in my finder scope, I took a look at rough focus through my telescope. I knew instantly I hadn’t forgotten any obvious, bright star. I was looking at a blurry, ochre-colored disk with a tiny point of light just off to one side. Obviously a planet and a moon.

For that quick instant, I thought I’d been wrong about Saturn and I assumed I was looking at Saturn and its brightest moon Titan.

But even as I brought the focus tighter I realized I wasn’t looking at Saturn. Saturn has two distinctive characteristics. The planet’s ring system—even now when the rings are almost on-edge from our perspective—always makes the planet look like a slight oval. And the planet, even in a small scope, is a beautiful ochre color, but ochre with a very definite golden cast to it.

I brought the image to sharp focus and I realized I was looking at Jupiter. No oval shape, and the beautiful ochre shade of Jupiter has more of an umber tint to it. And the equatorial cloud bands on Jupiter, even through a small scope, are much easier to see than on Saturn.

So I had been wrong about thinking Jupiter would be far to the east. The seasonal shift of the Earth around the Sun was bringing Jupiter farther west faster than I’d anticipated. By the end of summer Jupiter will be an evening star again.

But there was still that business of the single moon visible to the west of the planet.

Ever since Galileo first pointed his telescope toward Jupiter, everyone has known that Jupiter has four bright satellites. The four moons are relatively easy to see, even through modern binoculars. But I was seeing only one. What was up?

Well, the dynamics of Jupiter’s moons are well understood. Some amateurs study Jupiter’s moons every night. Both monthly astronomy magazines publish a chart that depicts the positions of Jupiter’s moons for every day of the month. I checked out an April chart. It turns out that Saturday night there was a reasonably rare (once a month or so) occurrence—two of Jupiter’s moons, Callisto and Europa, were either transiting the planet’s face or too close to its disk to be resolved by my small scope. A third moon, Io, was passing behind Jupiter, either eclipsed by the planet or too close to its disk to be resolved by my small scope. And that left Ganymede, Jupiter’s largest moon—in fact, the largest moon in the solar system—just west of the planet all by itself.

Although such configurations aren’t wildly rare, I’ve been observing Jupiter for almost thirty years and Saturday night was the first time I’d seen the planet with only one moon visible.

So I hadn’t been thinking about astronomy at all, but there I was just a little later looking at something I had never seen before.

Pretty cool night!

The moons of Jupiter aren’t very picturesque through a telescope, even a large scope. But it’s interesting thinking about the dynamics of the four moons. And, with a medium sized scope, it’s possible to take photographs of the shadows of the moons falling on Jupiter’s clouds when the moons transit the planet’s face. Again, it’s not very picturesque, but it’s pretty cool anyway.

What is very picturesque is the amazing, subtle colors you see when you look at Jupiter and Saturn through medium or high power, even with a small telescope. There’s something almost magical about the shimmering, simple ochre colors and the ever-so-subtle differences between the two gas giants.

It would be an interesting photographic project to try and capture the amazing hues that appear visually. It would be interesting, too, to try and capture the subtle colors and contrasts in paint. I’m not sure photography can duplicate the sensitivity of a human eye. And I’m pretty darn sure I don’t have the skill to do it with paint.

But I’m going to be giving both projects more thought. They’d both be fun and worth doing.

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