SJAA Ephemeris June 2006 | SJAA Home | Contents | Previous | Next

The Shallow Sky

Broken Comets and Drifting Dunes

Akkana Peck

This is a picture of Schwassmann-Wachmann 3 made in early May using a 10" SCT and a CCD camera. It is actually two 10-second exposures: one is taken at 2:39 a.m. on May 7, 2006 and the second was made 15 minutes later at 2:54. The two images were aligned in Photoshop so that the triangular asterism (imagine a nearly isoceles triangle with its apex in the upper right) lined up. The different exposures show how far the comet moved in 15 minutes. The first exposure shows the comet outside of the triangle and then later it is inside the triangle. According to, the comet at this time was .09 AU from Earth and around 7th magnitude. It was traveling through the Lyra constellation and, as you can see, it was moving pretty fast as it neared perihelion. Photo by Paul Kohlmiller.


Jupiter is a month past opposition, which means it’s perfectly placed for observing, rising before nightfall and reaching its maximum altitude well before midnight. Unfortunately, that maximum altitude is a hair under 40 degrees, so it never gets up into really steady air. Still, 40 degrees is enough that you’ll be able to see quite a bit of detail if you look. See the May 2005 Ephemeris for a chart of some of Jupiter’s prominent features and a guide to observing them.

Neptune rises a bit before midnight, and Uranus a bit later; both are well placed for observing if you’re staying up late.

Pluto reaches opposition on June 16, blazing at a magnitude 13.9, and is visible all night. Count this as the beginning of “Pluto season”: grab yourself a good star chart (the chart in the RASC Observer’s Handbook has proven to be very accurate for Pluto, while not all planetarium programs are) and your biggest scope, and head on out to your favorite dark-sky site!

How much aperture do you need for Pluto? Opinions vary. I recommend at least a 12.5”, though I’ve heard occasional reports of experienced observers nailing the dim planet with 8” telescopes or even smaller). Some people say that going bigger than 12.5” is a waste of effort for Pluto: all the extra stars you see (too faint to show up on the charts) will just confuse you. I don’t agree myself: I find Pluto easier to find when I’m not straining to see it, so more aperture is helpful. Everyone is different: your mileage may vary, void where prohibited by law.

Last month, Comet 73P/Schwassmann-Wachmann 3, which broke into five large pieces back in 1995, made a fairly close flyby. In June, 73P/Schwassmann-Wachmann 3 is still visible, but it’s moved into the morning sky, rising after 3 a.m., so you’ll have to stay up late (or get up early) to catch it. The pieces should still be bright enough to see in most amateur telescopes.

Venus sits very low in the dawn sky, showing a slightly gibbous phase.

On the other side of the sun, Mercury is visible in evening twilight early in the month, but becomes lost in the sun’s glare near month’s end. Mars hovers a bit above Mercury in the evening sky, and Saturn is nearby. On the night of the 4th Saturn makes a very close pass by the Beehive cluster, M44, in Cancer. Then on the 14th Mars joins the show: a fairly wide field should be able to show Saturn, mars and the Beehive all together in one field. (If your telescope can’t show a field that wide, binoculars should give a lovely view of this trio.)

Take a look at low power, then crank up the magnification a bit and zoom in on Saturn: it’s not too late to get a good view of Saturn’s rings and some of its brightest moons. In particular, Titan.

What’s so interesting about Titan? Well, the latest research from the Cassini spacecraft includes some very interesting photos of Titan, showing what looks remarkably like dune fields. In other words, all those areas on Titan that everyone thought were vast methane seas now appear to be dry desert areas, blown by wind into dunes like the ones we can see in the Mojave or Death Valley, up to 500’ high and extending for hundreds of miles. Pretty impressive considering that one mile per hour counts as a high wind on Titan. But since Titan is so small (well, it’s big for a moon, nearly twice as massive as ours, but small for a planet, only about 4/10 the mass of Mercury) it doesn’t take much to build up big dunes.

What sort of sand makes up these dunes? Nobody knows yet. They might be sand, like dunes on Earth, or they might be tiny ice crystals or some other material. Stay tuned! I’m sure Cassini will have more surprises for us.


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