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The Shallow Sky

A Daytime Occultation of Jupiter

Akkana Peck

Mars is still the Planet To Watch in early August. It's shrunk to 17 arcseconds (from its maximum of 20.8 back in mid-June). That's as big as the disk of Saturn, still big enough to see detail, but look while you can, because it will continue to shrink rapidly as the month progresses, to only 14" at month's end. Look early in the evening, while it's still (relatively) high in the southern sky, and you might see some of the detail that's been visible this opposition: both polar caps at once (unusual for Mars - usually it's tilted so that we can only see one or the other), Sinus Meridiani and Sabaeus (which have been unusually prominent this year, and should be best placed around the middle of August), dark Syrtis Major and nearby light Hellas (well placed during the third week of the month), and, on the other side of the planet, Margaritifer, Acidalia, and Niliacus Lacus (visible to us in early to mid August).

Watch for changes in the south polar region. Mars' southern hemisphere is coming out of winter, so the south polar cap should be shrinking. Sometimes that means, paradoxically, that the south pole area may get brighter, as ice turns to bright sunlight-reflecting haze.

Photographers and observers with wide-angle binoculars might want to try to get a view of Mars near the Lagoon Nebula (M8) near the end of the month, though they're not close enough to appear in the same telescopic field.

As Mars leaves us, diminishing in size as it dips lower in the evening sky during the course of the month, there's compensation: Saturn is back, rising near midnight and making a nice pattern near Aldebaran and the Hyades. The ringed planet is quite far north this year, and we should be able to look forward to a good pass. Its rings are tilted even more generously than they were last year: can you see the outer ("A") ring all the way around?

Jupiter trails Saturn by two and a half hours, and is visible to morning observers. It makes a close approach (1.2 degrees) with Venus (which hovers low in the morning twilight for most of the month) on August 6. But Jupiter's real show this month comes on August 15 during the daytime, when it passes behind a thin crescent moon. Jupiter will disappear behind the moon's bright limb at about 1:20pm, and reappear at about 2:30.

The thin crescent moon will be somewhat hard to find with the sun this high in the sky: look about 45 degrees west of the sun, then use binoculars or a telescope to spot nearby Jupiter. Will the Galilean moons be visible? Probably not, but I'll be looking. The disappearance and reappearance will take a minute or more, due to Jupiter's size; this is a great chance to share views of an occultation with coworkers or the public, without having to risk missing the "big event" yourself.

As always, when using a telescope or binoculars during the day, be careful not to aim anywhere near the sun; best is to try to set up somewhere in the shade, if possible, which not only makes it impossible to sweep across the sun by accident, but also reduces glare and greatly improves the image.

Meanwhile, the other gas giants of our solar system, Uranus and Neptune, are very well placed for observing, in Capricornus. Uranus reaches opposition on August 15th, and at magnitude 5.7 is a naked-eye object from a dark site, and an easy target for binoculars or a small telescope. Add some magnification to show the planet's distinctive green disk. Neptune just passed opposition at the end of July, and at magnitude 7.8, will require binoculars or a telescope to locate. It's a bit more challenging to locate than Uranus, because its color (somewhat bluer than Uranus) and size (only 2.3") don't give it away unless fairly high powers are used. It's located just west of the magnitude-5 star upsilon Capricorni.

Pluto, too, is well placed for observing, about three degrees due south of 20 Ophiuchi.

Mercury moves into the evening sky in mid-August, but it remains very low throughout the month, and will be at best a very difficult target.

Mail to: Akkana Peck
Copyright © 2001 San Jose Astronomical Association
Last updated: July 19, 2007

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