Night_skies_08-39

Jeff Green | Oct 02, 2008

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At the beginning of October in this area sunset is at about 6:45PM. Eastern Daylight Time, and astronomical twilight ends at about 8:30PM. By the end of October, sunset will be a few minutes before 6PM with astronomical twilight lasting until shortly after 7:30PM. Use these numbers to calculate sunset and end-of-twilight times for each evening of the month.

The increasing hours of darkness mean that more of us can enjoy the wonders of the autumn sky which still features, as it did in the past several months, the great Milky Way passing overhead during the night. However, as many will notice, its direction is changing. Whereas during the summer months, this great river of stars flowed from north-northeast to south, it now sweeps from northeast to southwest in the early night, and in the later night (by midnight and after) it runs from eastward to westward. This enormous band of light, which we are fortunate to see from our relatively unpolluted rural locations, is actually the starlight from the many millions of stars in our home galaxy, with our sun being just one star among those many millions. By now regular readers should know four of the prominent constellations sparkling within the Milky Way: Queen Cassiopeia, in the shape of a large “W” now well up in the northeast; Cygnus, the Swan, also known as the Northern Cross now a bit west of the ‘directly overhead position’ which it occupied last month; Aquila, the Eagle, now in the southwestern sky; and finally, very low in the southwest, in fact, setting there before midnight, the ‘Teapot of Sagittarius’. Now occupying the position above the southern horizon that Sagittarius held for the past two months is the constellation Capricornus, the Sea-goat, a somewhat bizarre creature from ancient mythology. With less active imaginations than their ancient counterparts, modern stargazers tend to see, not a ‘Sea-goat’, but an enclosed “V-shaped” pattern that is usually a reminder of the hull of a boat, as viewed from the front. Elsewhere in the sky, as we move into autumn, the three very bright stars of The Summer Triangle, which dominated the area of the zenith (that is, directly overhead) in the past two months, are now noticeably down into the western sky, but they are not yet down low enough to be confused with brilliant Venus which is even lower in the west this month.

Meanwhile, as the Summer Triangle moves westward, the stellar groupings that we call “the stars of winter” are moving up into the eastern and southern skies in the early and late evening. First among them is the star cluster called The Pleiades, sometimes referred to as “the Seven Sisters”, and in Japan known as “Subaru” – an image easily recognized in the logo of a well-known car from that country. This little cloud of stars can easily be seen in the east after 10PM any clear October evening. On a clear and moonless night people have long used it to test their eyesight – their visual acuity. If your location is not light-polluted, you, too, should be able to distinguish the cluster’s six brightest stars, and with good visual acuity, you can distinguish the “Seventh Sister” or the “Lost Pleiad”. (Young children tend to perform best on this test! Some may see even more than seven!) After doing the ‘test’, take ordinary binoculars, and marvel at the view of the two dozen stars visible in the cluster, and at the bluish nebulosity that may be visible near one of its stars on exceptionally clear nights! By midnight, the outline of Orion, the Great Hunter of the winter sky, may be seen climbing up into the southeastern sky, a foretaste of the brilliant winter star patterns.

Among the five bright planets, four of them may be seen this month, with Mars being out of sight because it is too close to the direction of the sun. As mentioned above, brilliant Venus is to be seen low in the western sky after sunset. Careful observers, if they check its position each evening 30 minutes after sunset, may note that at the beginning of October it is above the horizon by less than the width of a fist held at arm’s length, but by month’s end it is above the horizon by well over the width of a fist.

The second planet to be noticed each evening in October will likely be the second brightest one, Jupiter, the largest of them all and the one with the most known moons (62). It is in the southern sky, far to the left of Venus when it first comes into view. In fact, at the beginning of the month, these two brilliant planets are 64 degrees apart (the width of a fist held at arm’s length is 10 degrees), but during the month ‘Lady Venus’ moves much closer to ‘King Jupiter’, making the distance only 31degrees on October 31, and she continues the “dance toward the king” for yet another month until by the end of November they will be side by side. Begin immediately to make this “two-month-long observation” of Venus’s march to Jupiter – covering about one degree per day for about 61 days. On any clear evening, the owners of small telescopes or good binoculars will be able to see the four largest and brightest of Jupiter’s 62 known moons as they orbit around their planet and change their positions from hour to hour.

The remaining two bright planets, Saturn and Mercury, are seen this month only in the eastern sky in the morning. Saturn rises at the beginning of morning twilight as October begins, and earlier each morning during the month. It is still found among the stars in the constellation Leo the Lion, as it was last year, since this “far out” planet appears to move so slowly in its orbit around the sun. Those who observe it with a small telescope may notice that the angle of Saturn’s rings is tilted even less than it was last year, and, in fact, this angle is now less than it has been in the last 12 years. Mercury may be seen beginning on October 14 by those who search low above the eastern horizon about 30 minutes before sunrise. Both its brightness and its distance above the horizon increase slightly each morning until October 23, and after that its distance above the horizon decreases slightly.

Among the lunar-planetary conjunctions this month there is a “repeat event”. On BOTH the evening of October 1 and October 31, which is Hallowe’en, the very thin young crescent moon may be glimpsed VERY low in the western sky just below the planet Venus by those who look VERY carefully about 30 minutes after sunset. BE SURE to have a good western horizon, and to use binoculars so as not to miss the thin moon. Try not to miss the First Quarter Moon near Jupiter on the evenings of October 6 and 7, and the waning gibbous moon near the Pleiades on the morning of October 17, and also the thin crescent moon in the morning sky before dawn on October 24 when it is between Saturn and the star Regulus, the brightest star in the constellation Leo the Lion.

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.