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Tue09012015

Last updateSun, 30 Aug 2015 2pm

Astronomy

Spectacular events in September include a close pairing of a waning crescent moon and Venus at dawn on Sept. 10—and a total lunar eclipse in the early evening on Sept. 27.

Venus now rises before the sun, and has become a spectacular morning “star” in the east before dawn. Venus reaches its greatest brilliance this year in the third week of September. Now through mid-October, the crescent phase of Venuscan be seen with just a pair of binoculars—just find Venus on any morning before sunup, and then eliminate the planet’s glare against a darkened sky by simply keeping track of it until sunrise or longer. The mornings of Sept. 10 and Oct. 8, with Venus near a crescent moon, are excellent opportunities to easily locate and observe Venus in the daytime.

Ranking next in brightness after Venus these morningsare the blue-white Dog Star Sirius in the southeast, and yellow Capella northwest of overhead. Look for the Winter Hexagon, in clockwise order from its lowest member: Sirius, Procyon, Pollux (and nearby Castor, not shown on the map), Capella, Aldebaran and Rigel. Folks checking the eastern sky regularly about an hour before sunrise can watch for the first appearances—called heliacal risings—of planets and first-magnitude stars. Regulus and Jupiter will emerge by mid-September.

During Venus’ reign as “morning star” through March 2016, the moon passes by the planet in our skies seven times. The first of these monthly events will be on Sept. 10, when, one hour before sunrise, Venuswill gleam in the eastern sky just 4 degrees to the upper right of a 7-percent sunlit crescent moon, graced by earthshine illuminating its upper non-sunlit side. About 6 degrees to the moon’s lower left is dim red Mars, and 9 degrees farther to the lower left is Regulus, heart of Leo, the Lion. Within 15 minutes later, look 6 to 7 degrees to the lower left of Regulus for Jupiter, just rising. The entire span of objects from Venus to Jupiter takes up just 23 degrees.

Venus on the morning of Sept. 10 appears three-quarters of an arcminute in diameter, compared to the moon’s 30 arcminutes, or half a degree. So viewed through a telescope at 40-power, Venus will appear as large as the moon does to unaided eye! The crescent Venus, then 18 percent sunlit and nearing greatest brilliancy, will be very striking. As morning twilight brightens, the crescent Venus will be resolvable even with 7-power binoculars, and easy to find 4-5 degrees to the moon’s upper right. Predawn or daytime, Thursday, Sept. 10, will be an impressive morning for outdoor astronomy!

Evenings: Golden Arcturus in the west, and blue-white Vega nearly overhead, shine as the brightest stars at dusk. Altair and Deneb complete the Summer Triangle with Vega. The moon can be followed one hour after sunset daily from Sept. 15-28, as it waxes from a thin crescent, through first-quarter phase (half full and 90 degrees east of the sun), to one day past full. Steady Saturn remains in view in the southwest at dusk, and appears not far from the reddish twinkling first-magnitude star Antares, heart of the Scorpion, and outshines that star by half a magnitude. Through a telescope, we get fine views of Saturn’s rings.

On Sept. 15, the moon will be 3 degrees up in the west-southwest 45 minutes after sunset, with Spicavisible in binoculars 3 degrees to moon’s lower left.

For the next 12 evenings, look nightly for the moon within an hour after sunset, and watch it change its phase (fraction illuminated) and move toward the place where it has an encounter with Earth’s shadow on Sept. 27.

On Friday, Sept. 18, an hour after sunset, the crescent moon is in the southwest, with Saturn just 2 degrees to its lower left. Note the red supergiant star Antares, heart of the Scorpion, twinkling 12 degrees farther to the left of Saturn, and a little lower.

The next evening, Saturday, Sept. 19, the fat crescent moon is 9 degrees nearly directly above Antares, while Saturn is 12 degrees to the right of the other two bodies, forming an isosceles triangle. On Sunday evening, Sept. 20, the moon is in the south-southwest one hour after sunset, 17 degrees to the upper left of Antares, and 87 degrees (nearly a quarter of a circle) east of the sun. This evening, the moon is nearly at first-quarter phase and appears slightly less than half full.

On Sept. 20, there is just one week to go until a very special full moon: Early in the evening on Sunday, Sept. 27, there will be a total eclipse of the harvest moon. The partial eclipse will already be under way as the moon rises in the east, just before sunset. Excessive media hype will be given to this eclipse, because it coincides with a so-called “Supermoon,” the closest moon of the year. The moon will be in total eclipse from 7:11 p.m. to 8:23 p.m. Pacific, and the partial eclipse will end at 9:27 p.m.

This full moon is both the faintest of the year (when deepest in Earth’s shadow at 7:47 p.m.) and the brightest (around 10:23 p.m., when just outside the penumbra of Earth’s shadow). Stand between a bright light and a reflectorized road sign so that the shadow of your head is cast upon the sign, and you’ll see a brilliant halo around your head’s shadow. The moon’s surface reflects light in the same manner as the reflectorized sign—very strongly back toward the direction of the light source.

On the next few evenings after the eclipse, you can witness moonrise nightly until it occurs too late for convenient viewing. By Oct. 2, moonrise occurs nearly four hours after sunset.

You can also convenientlyobserve the moon daily about one hour before sunrise beginning Sept. 28, the morning after the eclipse, through Oct. 11. On Oct. 2, the moon will occult Aldebaran after sunrise, covering and uncovering the star at about 6:44 a.m. and 7:18 a.m. in the Coachella Valley—both events visible through a telescope.

Be sure to check the schedule of monthly star parties, observing sessions for special events, lectures and more from the Astronomical Society of the Desert at www.astrorx.org. Also: Subscribe to the Sky Calendar online, at www.abramsplanetarium.org/skycalendar.

Robert C. Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys providing skywatching opportunities for school children in and around Palm Springs. Robert D. Miller, who provided the twilight charts and the planet orbit charts, did graduate work in planetarium science and later astronomy and computer science at Michigan State University and remains active in research and public outreach in astronomy.

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