Night_skies_08-34

Jeff Green | Aug 28, 2008

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With the approach of September skywatchers are usually pleased both with the increased hours of darkness for late evening observing and with the decrease in hazy conditions that often follow the hot days of July and August. Locally sunset on September 1st is at about 7:45 p.m. Eastern Daylight Time, and evening astronomical twilight ends at about 9:30 p.m., that is, over an hour earlier than it did just a month ago. During September each sunset will be about 2 minutes earlier than the previous one, until by the end of the month it is at about 6:45 p.m., with twilight ending at about 8:30 p.m.. (From these numbers you can readily calculate the sunset and “end of twilight” times for each day of the month.) Morningside there is also a change of over a half-hour in the times of “beginning of twilight” and sunrise. On September 1st, astronomical twilight begins at about 4:45 a.m. and sunrise is at about 6:30 a.m.. Together these facts mean that, over the entire month, there is over an hour and a half decrease in the amount of daylight, and of course, an equal increase in the amount of darkness. And so, that much more time to enjoy the fascinating views of the late summer and early autumn sky!

The beautiful Milky Way that so dominated the dome of the heavens in the summer still passes overhead during the late evening hours. The constellation Cygnus the Swan, also named The Northern Cross, is still seen “flying” almost directly overhead as evening twilight ends. This “bird of stars” flying right along the Milky Way and within the great trio of stars called The Summer Triangle (individually known as Deneb, Vega, and Altair) may remind us of the coming southward migration of our Canada Geese and other birds.

Of the five bright planets that may normally be seen with the unaided eye, three of them form an unusually close cluster this month, one continues to dominate the evening sky, and one is hidden until late September when it rushes into view in the morning sky. Venus, Mercury and Mars are the trio that appear in the very same part of the sky for almost the entire month. However, “that part of the sky” is what will make seeing them a challenge for many people. They are very low in the western sky, and for most of the month can be seen only from about 30 minutes to about 60 minutes after sunset. A good view of the western horizon is definitely required. Binoculars will definitely help – even if they are old ones that have not been used for many years. The first three evenings of September will be an excellent time to try to see the trio of planets and the slim crescent moon just above the western horizon beginning about 30 minutes after the sun sets. On September 1, the extra-slim lunar crescent will be just below the planets that form the shape of a “V” with Mercury at the bottom, Mars at upper left and bright Venus at upper right. On September 2, the lunar crescent will have moved to the left of the planetary “V”. On the 3rd, it will be somewhat further to the left of that “V”. The moon will move on, but the planetary group will stay, though with changes since Venus will move to the left and pass by Mars on September 11, forming a new “V” – this time with Venus on the upper left and Mars on the upper right. After September 10, observers may notice a distant background star near the planetary grouping. It is the star Spica, brightest star in the constellation Virgo, and it will be to the left of the grouping of planets, but, day by day, will gradually get closer to them. After September 20, careful observers will notice Mercury fading, becoming fainter than the star Spica. Except for bright Venus the group will appear slightly lower each evening, and become very difficult to observe after September 25, but Venus will move higher into the evening sky, and by September 30, Venus will be visible for over an hour before setting. Following this close grouping for most of the month will be well worth the effort of finding a location that gives a good westerly view, such as over a large stretch of water. Just as was the case last month, the planet Jupiter continues to be the glaringly bright object in the southern sky during the late evening and it is easily seen in the southwest until about midnight. Owners of good binoculars will enjoy watching the ever-changing positions of the four brightest of its many moons as they orbit the planet, night after night. Extremely careful observers may note that Jupiter appears to move slightly westward (to the right) among the stars that are just above the Handle of The Teapot in Sagittarius until September 8, but after that it moves slightly eastward (to the left) among those stars. It has now ended what astronomers call its retrograde motion, something that we see only because our planet Earth, with its ‘inside’ orbit, has passed Jupiter. Jupiter, for many months, will now appear to move forward, or eastward, among the stars. After September 16, Saturn should be easily visible in the early morning among the stars of the constellation Leo the Lion. Look for it low in the east beginning an hour before sunrise. By the end of the month, it may be seen for up to two hours before sunrise.

Several lunar-planetary conjunctions are notable this month. First of all, review the information above about the slim crescent moon near the three-planet grouping on the first 3 evenings of September. Don’t miss the First Quarter Moon near the star Antares in the southern evening sky on September 6, and passing the planet Jupiter on the 3 evenings of September 8, 9, and 10. Well worth seeing will be the waning crescent moon passing the star Regulus in the eastern sky an hour before sunrise on the morning of September 26 and also passing the planet Saturn at the same time on the following morning, September 27. Remember that the Full Moon on the night of September 14, being the one closest to the Autumnal Equinox, is called The Harvest Moon. Try to take note of why farmers over the centuries have appreciated the celestial geometry of its rising – rising a mere 20 minutes or so later from night to night during the following week (instead of the usual or average 50 minutes) thus giving them increased time to complete the harvesting of the crops “under bright conditions”. On the night of September 19, the moon will occult, or move directly in front of, several of the bright stars of the famous Pleiades star cluster. Starting at 10:00 p.m. use your binoculars to see how many disappearances and reappearances of bright stars you can observe, and record the precise times of these events.

More information about observing the spring and summer sky is available in the book entitled “The Beginner’s Observing Guide” which is now available at Sharbot Lake Pharmacy.