Mars Cleans Up Its Act / September Opens With A Binocular Comet

The dust storm has altered the appearance of some of Mars’ features. I’ve marked two: the giant canyon Valles Marineris (arrows) and the big, orange patch. The length of the canyon’s floor is now yellow with dust. Look around and you’ll see more changes. Damian Peach

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.

Unscheduled conjunction of the waning gibbous moon and a jet this morning, Sept. 1. Bob King

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.

This map shows the sky as you face northeast at 11 p.m. tonight just before moonrise when comet 21P/Giacobini-Zinner stands about 10° high. Luminous Capella makes finding the fuzzy blob easy. The comet goes around the sun once every 6.6 years. This time around, it will pass relatively close to the Earth and show up nicely in binoculars from a dark sky. Amateur telescopes will reveal a bright head and fainter tail pointing to the west. Stellarium

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.

On August 22, comet 21P/Giacobini-Zinner displayed a bright head, compact bright center and a tail more than ½° long. Michael Jäger

Take a pair of binoculars (8×40, 7×50, 10×50 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.

Use this finder chart to track 21P/G-Z through mid-September. The comet’s location is shown daily at 0h Universal Time (UT) with positions marked every 5 days. 0h UT is the same as 6 p.m. Eastern Daylight Time, 7 p.m. Central Time and so on. To convert UT to your local time, click here. Stars are plotted to magnitude 7.5 as are the brighter Messier objects. North is up. Click for a large version and print it out to use outdoors. SkyMap software with additions by the author

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.