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Sky This Week for September 9 to September 18

Asteroid 2 Pallas bright enough to be seen through a telescope, the Harvest Moon rises, and other amazing things to look for this week.
Zodiacal Light
Zodiacal light
WikiMedia Commons

Friday, September 9

First Quarter Moon arrives at 7:49 a.m. EDT. Our satellite won’t rise until around 2 p.m. local daylight time, however, so observers in the Americas won’t see it precisely half-lit. As darkness falls tonight, the Moon appears about 55 percent lit and lies nearly 10° above Mars. The Red Planet is worth exploring in detail any night this week. It shines at magnitude –0.2 and, when viewed through a telescope, shows a 10"-diameter, orange-red disk with several subtle dark markings. 

Saturday, September 10

The Moon’s absence from the morning sky these next few days provides observers with an excellent opportunity to see the zodiacal light. From the Northern Hemisphere, the time around the autumnal equinox is the best for viewing the elusive glow before sunrise. It appears slightly fainter than the Milky Way, so you’ll need a clear moonless sky and an observing site located far from the city. Look for a cone-shaped glow that points nearly straight up from the eastern horizon shortly before morning twilight begins (around 5 a.m. local daylight time at mid-northern latitudes). The Moon remains out of the morning sky until September 14, when the waxing gibbous returns and overwhelms the much fainter zodiacal light.

Sunday, September 11

Asteroid 2 Pallas reached opposition three weeks ago, but it remains a tempting target in the evening sky. The second-largest object orbiting between Mars and Jupiter glows at magnitude 9.4, bright enough to show up through almost any telescope. You can find it in the dim constellation Equuleus the Little Horse, which lies just west of its big cousin, Pegasus. This evening, the asteroid resides 1° due west of Equuleus’ brightest star, magnitude 3.9 Alpha (a) Equulei. The two objects climb nearly two-thirds of the way to the zenith in the southern sky around 11 p.m. local daylight time.

Ursa Major
The constellation Ursa Major the Great Bear
WikiMedia Commons

Monday, September 12

Although Neptune appeared at its best at opposition 10 days ago, its visibility hardly suffers this week. The outermost major planet lies low in the southeast as darkness falls and climbs highest in the south around midnight local daylight time. Neptune glows at magnitude 7.8, which is bright enough to spot through binoculars if you know where to look. The trick is to find the 4th-magnitude star Lambda (l) Aquarii, which lies about 10° southeast of Aquarius’ distinctive Water Jar asterism. This week, Neptune appears some 1.5° southwest of the star. When viewed through a telescope, Neptune shows a blue-gray disk measuring 2.4" across.

Mercury reaches inferior conjunction at 8 p.m. EDT. This means the innermost planet lies between the Sun and Earth and remains hidden in our star’s glare. It will reappear in the east before dawn in about 10 days — the start of its finest morning apparition of 2016.

Tuesday, September 13

The constellations Ursa Major the Great Bear and Cassiopeia the Queen lie on opposite sides of the North Celestial Pole, so they pivot around the North Star (Polaris) throughout the course of the night and the year. In the first half of September, these two constellations appear equally high as darkness falls. You can find Ursa Major and its prominent asterism, the Big Dipper, about 30° above the northwestern horizon. Cassiopeia’s familiar W-shape, which currently lies on its side, appears the same height above the northeastern horizon. As the night progresses, Cassiopeia climbs above Polaris while the Big Dipper swings below it.

Wednesday, September 14

Saturn remains a gorgeous sight in the evening sky all week. It stands some 20° high in the southwest as twilight fades to darkness and doesn’t set until almost 11 p.m. local daylight time. The ringed world resides among the background stars of southwestern Ophiuchus, just 6° north of Antares, the brightest star in neighboring Scorpius. The yellow-hued planet shines at magnitude 0.5 and appears significantly brighter than the ruddy star. When viewed through a telescope, Saturn shows a 16"-diameter disk surrounded by a dramatic ring system that spans 37" and tilts 24° to our line of sight.

Thursday, September 15

Uranus reaches opposition one month from today, but it already has become a tempting evening target. The ice giant world rises before 9 p.m. local daylight time and climbs some 30° above the eastern horizon by 11 p.m. The magnitude 5.7 planet lies in Pisces, 2.4° north of magnitude 4.8 Mu (m) Piscium. Although Uranus glows bright enough to see with the naked eye under a dark sky, binoculars make the task much easier. A telescope reveals the planet’s blue-green disk, which spans 3.7".

harvestmoon
Yoshikaza TAKADA / Flickr

Friday, September 16

Full Moon occurs at 3:05 p.m. EDT, but our satellite will look completely illuminated all night. You can find it rising in the east around sunset and peaking in the south by 1 a.m. local daylight time. The Moon lies among the dim background stars of Pisces, due south of the Great Square of Pegasus. As the Full Moon closest to next week’s autumnal equinox, this is also the Harvest Moon. In early autumn, the Full Moon rises about half an hour later each night compared with a normal lag close to 50 minutes. The added early evening illumination supposedly helps farmers bringing in their crops. The Full Moon also passes through Earth’s outer shadow today, bringing a penumbral lunar eclipse to observers across Europe, Asia, Africa, and Australia. At the peak, which occurs at 18h54m UT, 91 percent of Luna will be immersed in the shadow and viewers will see a distinct dusky shading over the Moon’s northern half.

Saturday, September 17

Viewers with an unobstructed horizon toward the west-southwest should keep an eye out for Venus this week. Although the planet stands just 6° high 30 minutes after sunset, it shines so brilliantly (magnitude –3.9) that it stands out against the bright twilight. Target Venus through binoculars this evening and you should also spot the 1st-magnitude star Spica, which lies just 3° south (lower left) of the planet.

Sunday, September 18

The variable star Algol in Perseus reaches minimum brightness at 5:42 a.m. EDT. If you start watching it in late evening, you can see its brightness diminish by 70 percent over the course of about 5 hours as its magnitude drops from 2.1 to 3.4. This eclipsing binary star runs through a cycle from minimum to maximum and back every 2.87 days. Algol passes nearly overhead shortly before morning twilight commences.

The Moon reaches perigee, the closest point in its orbit around Earth, at 1:00 p.m. EDT. It then lies 224,872 miles (361,896 kilometers) away from us.

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