The Sky This Week, 2012 September 25 - October 2

Jupiter, Europa, & Ganymede, 2012 SEP 25, 09:56 UT Worth getting up for!

The Moon brightens the evening sky throughout this week, climbing northward among the faint stars of autumn’s dim constellations. Full Moon occurs on the 29th at 11:19 pm Eastern daylight Time. Since this Full Moon occurs closest to the autumnal equinox it is almost universally known to Northern Hemisphere residents as the Harvest Moon. The name derives not only from the season but also from a curious quirk of orbital geometry. For viewers in mid to high northern latitudes the Moon seems to skip along the eastern horizon as it rises from night to night around the time of the full phase. In fact, at latitudes north of the Arctic Circle the Moon rises earlier on successive nights! For most of us, though, the difference in moonrise times is somewhere between 15 and 30 minutes for a few days around Full Moon, depending on latitude. Thus the effect is one where the light of the rising Moon adds to the light of twilight, enabling farmers to squeeze a little extra time into their harvest activities each day. Out of the many different names that sky lore ascribes to the year’s various Full Moons, the Harvest Moon is probably the most widely recognized by the general public, followed by next month’s Hunter’s Moon. Of course, in the Southern Hemisphere the opposite case applies; their "Harvest Moon" would fall near the vernal equinox in March.

Even though the bright light of the Moon washes out all but the brightest stars in the sky, you can still enjoy the bright constellations of the summer thanks to another fall phenomenon. During this time of the year as we pass the equinox the times of sunrise and sunset change at their most rapid rate. Most of us will notice this at sunset as Old Sol disappears almost two minutes earlier each night. While a star will transit the meridian four minutes later every day, it will set only two minutes later when referenced to the Sun at this time of year, so the summer constellations seem to linger in the early evening sky for several weeks. You can therefore see the bright stars of the Summer Triangle at nearly the same time relative to local sunset high in the south for several weeks to come. Vega, Deneb, and Altair are quite easy to spot from urban skies during the days around Full Moon and serve as convenient guide-posts to other interesting sights. I like to spend the time around Full Moon observing double stars if no bright planets are available, and two of my favorites are easy to find with the help of these stars. Smack in the middle of the triangle is Alberio, which looks like an ordinary third magnitude star with the unaided eye. Turn a small telescope on it, though, and you will be rewarded with a beautiful pair of stars with a stunning color contrast of blue and gold. I like to call this pair the "Navy Double"! Now take your binoculars and look at Vega, the brightest of the Triangle, nearly overhead at around 8:00 pm. Just to the east of Vega you’ll see a close pair of blue stars. Point a telescope of three inches aperture or better at this pair and you’ll see each component resolved into a very close pair of stars. The "Double-Double" has long been used as a test of good telescope optics and steadiness of the night air.

Ruddy Mars is the only planet that’s still relatively easy to find in the early evening sky. He’s best seen at around 8:00 pm in the southwest. He spends this week between the constellations of Libra and Scorpius, gradually closing the gap on the stars Antares. This star’s name means "rival of Mars", and over the next few weeks you’ll have a great chance to make your own comparison.

Jupiter is making steady progress into the evening sky. By the end of the week he rises shortly after 10:00 pm, and by midnight you can sneak a peek at him at a respectable distance above the horizon. The best time to look at him is still just before dawn; at this time he’s just about overhead. Early morning is a great time to give him a look since the air is usually very calm and steady. When the weather prospects are good I’ve been getting up early a few times each week to enjoy a view of Old Jove in the telescope and the surrounding stars of the Great Winter Circle with the unaided eye.

If you don’t want to rise to see Jupiter in darkness you should still make a point of getting up before dawn to watch Venus as she closes in on the bright star Regulus. Venus is easy to find above the eastern horizon, and over the course of the week she advances steadily on the star. On the morning of October 3rd she will be just 1/6th of a degree from Regulus in one of the closest planetary appulses of the year. You may need binoculars to easily see both objects, and it should be a spectacular photo opportunity.