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The Sky This Week, 2010 September 21 - 28

Autumnal Equinox and Harvest Moon


Jupiter, 2010 SEP 21, 03:32 UT
Imaged with USNO's historic 12-inch Clark/Saegmüller refractor

The Moon begins the week big and bright, then starts to wane as she slowly makes her way into the morning sky as she wanes through the gibbous phases. Full Moon occurs on the 23rd at 5:17 am Eastern Daylight Time. This happens to be the Full Moon closest to the autumnal equinox, so it is almost universally known as the Harvest Moon. The reason for this particular appellation has to do with the curious geometry of Luna’s orbit for Northern Hemisphere observers at this time of year. If you’ve been following the Moon’s phases this month you’ve probably noticed how close to the southern horizon her path has been. This means that her orbital plane intersects the eastern horizon at a shallow angle. During most of the year her distance with respect to the horizon increases by 10 to 12 degrees each night, resulting in her rising about an hour later from one night to the next. At this time of year, though, Luna’s orbit intersects the horizon at a very shallow angle, so her nightly depression below the horizon is only a few degrees. This means that successive Moon rises around the time of Full Moon differ by 20 minutes to half an hour at our latitudes, which in the "old days" gave farmers a little extra light to work their fields to bring in the harvest. At high northern latitudes the Moon actually rises earlier on successive nights!

This year’s Harvest Moon takes place within hours of the autumnal equinox, which falls on the 22nd at 11:09 pm EDT. At that instant the center of the Sun’s disc will be located directly over the Equator just west of Papua New Guinea, passing from the northern hemisphere of the sky into the southern hemisphere. Since the Sun subtends a tangible disc, though, the actual time when we see exactly 12 hours between sunrise and sunset won’t occur until a few days after the equinox. Here in Washington that phenomenon occurs on the 26th. This is also the time of year when the change in length of daylight occurs at its most rapid rate. There is no mistaking that the days are getting shorter!

The evening twilight hours still find Venus and Mars chasing each other through the southwestern sky. Venus is still dazzlingly bright and can be easily spotted in the twilight glow within a few minutes of sunset. Mars, on the other hand, is some 100 times fainter than Venus and will probably require binoculars to see before he sets. The red planet may be found about seven degrees above and to the right of Venus.

Jupiter has just passed opposition, and now rises at sunset, sets at sunrise, and is visible in the sky all night long. If Old Jove appears particularly bright to your eyes this year there’s a good reason for that: he’s now closer to Earth than he’s been at any time since 1963. On the 20th at around 5:00 pm EDT he was a mere 367,540,544 miles (591,499, 169 kilometers) from Earth! His apparent disc is now about as large as it ever gets, so be sure to get a look at him through the telescope. Fortunately he will remain in fine form for viewing for the rest of the year, but we’ll have to wait until the fall of 2022 to see him this close to us again. As it happens, another planet reaches opposition at just about the same time as Jupiter. Distant Uranus is currently located less than a degree to the north of the giant planet, and may be seen as a faint star-like object with binoculars close to Jupiter. A small telescope will reveal Uranus’ tiny greenish-grey disc. Look for the nearly Full Moon six degrees above Jupiter on the night of the 23rd.

Early risers have the rest of this week to see the best morning appearance of fleet Mercury until the beginning of next year. The swiftly moving planet may be found due east in morning twilight about 12 degrees below the rising bright star Regulus in the constellation Leo. Mercury moves about a degree per day toward the horizon, but he is also growing brighter. Look for him with binoculars about half an hour before sunrise.

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