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Mars Cleans up Its Act / September Opens With a Binocular Comet

OK, Mars isn't completely clear, but the dust is finally settling, making the planet's dark surface markings much easier to...

OK, Mars isn't completely clear, but the dust is finally settling, making the planet's dark surface markings much easier to see. I noticed this two nights ago when I hauled out my 10-inch scope for a look. The night was calm and no leaf stirred, often a sign of "steady seeing," when planets appear sharp even at higher magnifications.  It was also heartening to see that the south polar cap was still large enough to stand out at 125x. When you look at Mars in your telescope, you might notice that the cap's northern edge is bordered by a narrow, dark line. These are dark dune fields that become exposed as the carbon dioxide ice (dry ice) vaporizes, causing the cap to shrink with the coming summer season.

Mars is full of surprises, but I'm glad the dust storm is dissipating so we can see what's been happening on the surface since early June. The redistribution of dust planet-wide has clearly altered the appearance of some Martian surface features. Any new telescopic maps of Mars will have to take these temporary changes into account. Mars was closest to the Earth a month ago; it will soon be making its closest approach to the sun with perihelion on Sept. 15. Southern hemisphere summer starts on Oct. 16.

You've no doubt noticed that the moon is rising later and later. Currently near third-quarter phase, it rises around 10:30-11 p.m. local time. That leaves the skies dark for viewing the Milky Way, which now stripes the sky all the way from the southwestern horizon to the overhead point and into the northern sky through the W of Cassiopeia.

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If you stay up to about 11 p.m., look low in the northeastern sky for a bright, twinkly star. That's Capella, the brightest star in the stick-figure-house of a constellation called Auriga the Charioteer. You can also find it by starting with the W of Cassiopeia, located much higher up in the northeast, and drop down through Perseus to the star. Capella's return to the evening sky in early September is a quiet announcement of the coming winter; in January Capella will shine from overhead on a snowbound landscape.

Take a pair of binoculars (8x40, 7x50, 10x50 and similar will do), point them at the star and carefully focus. If you look just a little above and right of Capella and see a small, fuzzy patch of light, pat yourself on the back. You've just found comet 21P/Giacobini-Zinner . Now glowing at magnitude 7.5, it's the brightest comet in the sky this month and next.

21P/G-Z should continue to brighten to magnitude 7 or better in the next few weeks. It passes closest to the Earth at 36.4 million miles (58.6 million km) on September 10-11, the same date it's also closest to the sun. And while the moon will brighten the sky late at night into next week, it will soon thin to a crescent and stop bothering the stars soon. For now, you can spot the comet in a dark, moonless sky around 11-11:30 p.m. Having Capella nearby really helps.

But 21P/G-Z is on the move and will soon leave the star — and the evening sky — behind as it transitions to the morning sky. Starting Wednesday the 5th through Sept. 23 your best bet is to get up a little before dawn when the comet climbs way up in the eastern sky. Clear of the horizon haze and a glaring moon, it should be easy to see in binoculars and show nice detail in a telescope including a tail fanning to the west.

For more on the comet, including additional maps and details of its remarkable conjunction with the bright star cluster M35 in Gemini, please check out my recent article on Sky & Telescope's website.

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