You are hereTHIS MONTH'S SKY - September 2008
THIS MONTH'S SKY - September 2008
THIS MONTH'S SKY
Celestial Events -- September 2008
|The action is still in the west after sunset. Although Saturn has fallen back toward the Sun and no longer visible (it is in conjunction with the Sun on September 3rd), Venus, Mercury and Mars can be found, though with some effort. The trick is to spot brilliant Venus first, then "planet-hop" to the others.
On September 1st a thin crescent Moon joins the trio of planets. Look for it 6½° below Venus (magnitude -3.9) a half hour after sunset. Mercury (magnitude 0.0) will lie roughly midway between the two (see chart 1 at right). Mars (magnitude 1.7) will be 6° upper left of Venus and will require binoculars or a telescope.
As usual the Moon leaves the scene quickly. Venus and Mercury get higher with each night, while Mars is lower. On September 11th Mercury reaches greatest elongation from the Sun. On the same day, Venus passes less than one-third of a degree above Mars. The two planets will be in the same field of a medium-power telescope, while Mercury is another 3° below left (see chart 2 at right).
Venus and Mercury seem to be moving in parallel, but after a few days Mercury begins to fall behind. By the 19th, Mercury is again in conjunction with Mars, a little over 2° below left. Both planets are fading and will probably require binoculars at this point. They will be lost by month's end. Chart 3 shows the planets' movements the first 20 days of September.
While this is the swan song for Mercury and Mars, Venus becomes more prominent each week as it slowly pulls away from the Sun. She will have more adventures in the coming months.
|Mark the evening of September 12-13 on your calendar. On that night the Moon will pass in front of the planet Neptune, blocking it from view for a few hours. At almost the exact same time, Uranus (the next planet inward) is at opposition -- at its closest to Earth and up all night.
The Moon rises that night just before 8 PM, followed a few minutes later by Neptune. The two are in the constellation Capricornus. Early on they are about 1½° apart. The Moon, over 90% lit, closes in on Neptune, and the planet disappears behind the leading dark edge of the Moon at 8:51 PM (all times for NYC). Neptune will reappear at 10:03 PM.
Since the Moon is very bright and Neptune is of eighth magnitude, very clean optics will be required to see the actual occultation, as the event is called. Your binoculars or telescope must not reflect light internally. If you don't have top-notch equipment, you can still have fun looking a half hour or so before the occultation and the same amount of time afterwards, when Neptune will be sufficiently far from the Moon to be seen. If you didn't know how to find Neptune, this is a good opportunity. It is noticeably blue-green and will show a small ball, unlike a pointlike star.
At almost the same time, the seventh planet Uranus will be at opposition in the constellation Aquarius. At opposition, a planet is opposite the sky from the Sun. Alternatively, Sun-Earth-planet form a straight line, with Earth in the middle. This means the planet is up all night and relatively close.
If the sky is cloudy that evening, don't fret. The actual day of opposition is not very important -- Uranus will be essentially as big and as bright for several weeks. Like Neptune, it appears as a blue-green orb, slightly larger than Neptune (since it is closer).
|On September 19th at 9:09 PM the waning gibbous Moon rises, about three-quarters lit. It is in the midst of the Pleiades star cluster. Already the star Electra is hidden behind the Moon. In a little over an hour our moon will pass over several of the Seven Sisters.
Maia disappears at 9:38 and Taygeta only three minutes later. Electra comes into view at 9:59 and Taygeta reappears at 10:01. Alcyone is swallowed up at 10:16, but Maia is uncovered at 10:23. Alcyone reappears at 10:36.
A movie of the occultation is at right, along with the times of disappearance and reappearance of the Pleiades stars.
PLANETS IN SEPTEMBER
MERCURY (magnitude 0.0 to 2.1, diameter 6.1" to 9.9", phase 70% to 12% lit) is visible with effort the first half of September in the western sky at dusk, as described above. It is at greatest elongation from the Sun on the 11th, the same day it is in passed by Venus. The following day Mercury is in conjunction with Mars. Soon its decreasing phase makes it difficult to see, and Mercury will be lost in bright twilight.
VENUS (magnitude -3.8 to -3.9, diameter 10.8" to 12.0", phase 92% to 86% lit) continues ever so slowly to pull away from the Sun, its slow movement a consequence of being relatively far from Earth. On the 11th, it is spectacularly close to Mars, only 19' (arc minutes) or 0.3°. See the article above for details.
MARS (magnitude +1.7 to +1.6, diameter 3.9" to 3.8") is swallowed up by the Sun this month. Nevertheless, it can be found early in the month with binoculars, using bright Venus as a guide. The two planets will be in the same telescopic field on the 11th (see above).
JUPITER (magnitude -2.5 to -2.3, diameter 43.5" to 39.9") is in the Teaspoon of Sagittarius. It is stationary on the 7th, resuming direct motion (eastward among the stars).
SATURN (magnitude +0.8 to +1.0, diameter 16.1" to 16.2") is invisible most of the month, as it is in conjunction with the Sun on the 3rd. By the end of September is can be spotted in the east before dawn.
URANUS (magnitude +5.7, diameter 3.7") in Aquarius is at opposition September 12. See above.
NEPTUNE (magnitude +7.8, diameter 2.3") is in Capricornus, moving retrograde (westward among the stars). The Moon will occult it on the 12th (above).
PLUTO (magnitude +14.0, diameter 0.1") is in northwestern Sagittarius. It is stationary, resuming direct motion, on the 9th.
A very young MOON is near Mercury, Venus and Mars on September 1st (above). On the 6th it is 2° away from Antares and reaches First Quarter phase the following day. The Moon is 4° below Jupiter on the 9th. On the evening of the 12th it occults Neptune (above). Two days later it passes above Uranus. Full Moon is on September 15th. On the evening of the 19th, the Moon occults several of the stars in the Pleiades. Last Quarter occurs three days later and New Moon on the 29th.
THIS MONTH'S EVENTS
(Times in EDT)
|SEPTEMBER 1||A DRAMATIC GATHERING OF THREE PLANETS AND THE MOON LOW IN THE WEST AFTER SUNSET. THE VERY THIN CRESCENT MOON, VENUS AND MARS ALL LIE WITHIN A 4° CIRCLE CENTERED ON MERCURY. SEE ABOVE FOR MORE DETAILS.|
|September 3||Saturn is in conjunction with the Sun, entering the morning sky.|
|September 7||First Quarter Moon at 10:04 AM.|
|Jupiter is stationary, resuming direct motion (eastward against the stars).|
|September 9||Pluto is stationary, resuming direct motion.|
|SEPTEMBER 11||VENUS IS ONLY 0.3° ABOVE MARS THIS EVENING. MERCURY IS ANOTHER 3° LOWER LEFT OF MARS. MERCURY IS ALSO AT GREATEST ELONGATION EAST OF THE SUN TODAY.|
|SEPTEMBER 12||NEPTUNE IS OCCULTED BY THE MOON THIS EVENING.|
|URANUS IS AT OPPOSITION.|
|September 15||Full Moon at 5:13 AM.|
|September 19||Falling back toward the Sun, Mercury is in conjunction with Mars again, 4° below the Red Planet. Use binoculars.|
|THE MOON OCCULTS THE PLEIADES TONIGHT (above).|
|September 22||Last Quarter Moon at 1:04 AM.|
|Autumnal Equinox at 11:44 AM.|
|September 29||New Moon at 4:12 AM.|