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The Shallow Sky

Celebrate Aphelion

Akkana Peck

Get out your heavy coats, because the earth hits aphelion (its farthest point from the sun) on the morning of July 4. Well, okay, maybe you won't actually need that coat; here in the northern hemisphere, our tilt toward the sun more than compensates for the slightly greater distance. So keep the coat in the closet, and have a cold iced beverage to celebrate aphelion!

Mars rules the southern sky throughout July. It passed its closest approach to us on June 21, but throughout July it remains closer -and therefore appears larger to us, and shines brighter at magnitude -2 - than any time since 1990, even at oppositions. And past opposition, Mars rises earlier and thus is easier to observe in the early evening.

Another bonus of this year's opposition is that Mars passes near quite a few deep sky objects; the view of Mars next to other objects can be a lovely view and a novel target at public star parties. Through July, Mars will stay fairly near the globular cluster M19.

The spring equinox in the Martian southern hemisphere was mid-June, so the southern polar cap should be prominently visible now and should shrink over the next few months. Both polar caps are a bit less obvious now, with the planet's equator pointed at us, than at recent oppositions when one polar cap or the other has been tilted toward us; and during July Mars' northern hemisphere will tilt a bit more toward us, making the southern pole still more challenging. Still, on nights of steady seeing we may be able to see the size of the south polar cap, and monitor it as it shrinks.

The planet is low in the southern sky for us northern observers, so to see detail on Mars we're very dependent on waiting for nights of steady seeing. If you want to see detail, try to look as often as you can, so you'll catch steady nights when they happen. I've heard early reports of local observers being able to see hints of the Tharsis volcanos and bits of Valles Marineris, as well as the usual large albedo features like the dark features Syrtis Major, Acidalia, and Margaritifer, and light features like Hellas, Chryse and Xanthe. Sinus Meridiani seems darker and more prominent than it was in the last opposition; perhaps this will be a good opposition to look for the "eye of Mars", a part of Meridiani which looks like a human eye). It will be well placed in the early evening in the first few weeks of July. As the month progresses, the difference in Mars' day and our own will gradually bring Syrtis Major and Hellas to early-evening visibility, and then, by month's end, we'll start to see Olympus Mons and the Tharsis area (the volcanos themselves are usually not visible from here, but often one can see orographic clouds or other atmospheric features caused by the volcanos, changing from day to day as the Martian weather changes).

Mars isn't the only planet near opposition. Neptune, in Capricornus, reaches opposition on the night of July 29, at eighth magnitude, easily within reach of binoculars. Brighter Uranus, in the same constellation, won't reach opposition until the middle of August, while faint Pluto, joining Mars in Ophiuchus, was at opposition last month and is now ideally placed for observers tired of bright Mars and looking for a fainter planet to observe.

About 3 a.m., the morning planets begin to rise: Venus and Saturn, joined later in the morning sky by Mercury. Venus shows a gibbous disk, shrinking as the month progresses, while Mercury begins July as a crescent but waxes rapidly toward greatest elongation on the 17th, to become nearly full (and difficult to locate in the sun's glare) by month's end. Halfway through the month, Jupiter pulls away from the sun to join them in the predawn sky.

The crescent moon will make nice groupings with the morning planets for several days late in the month. On July 18, it passes between Saturn and Jupiter, and a day later, use it to locate Mercury well below Jupiter in the twilit sky. But the real show happens a few days before that, during daylight on the morning of July 17, when the crescent moon passes in front of Venus. The planet will disappear behind the bright limb of the moon at about 10:08am, reappearing from behind the dark side at around 11:42. The event should be visible in binoculars or even to the naked eye for people with sharp vision; in a telescope, the size of the Venusian disk means that disappearance and reappearance will take a bit over half a minute, giving plenty of time to watch the event and perhaps to share some views. This might be a good chance to interest your coworkers in astronomy - take a telescope to work, or to a favorite lunch site!

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

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