| Apr 24, 2008

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Night Skies - May 2008 Mercury & Astronomy Day by Leo Enright

During May, the hours of daylight noticeably increase as the hours of nighttime darkness decrease, but neither change is by as much as it was in the month of April. In this area, on May 1st, sunset is just a bit after 8:00 p.m. EDT, and by the end of the month it is at 8:40 p.m., an average difference of over one minute per day. Over the same period, the end of evening astronomical twilight moves from 10:00 p.m. to about 11:00 p.m., an average change of about two minutes per evening. This means that, as many skywatchers have already noticed, as spring progresses, there is a noticeably longer wait for the sky to become completely dark. In the morning, on May 1st, astronomical twilight begins at 4:00 a.m., with sunrise at 6:00 a.m., but by the end of May, morning astronomical twilight begins just after 3:00 a.m., and sunrise is at 5:30 a.m.. The hours of complete darkness are indeed becoming fewer and fewer. As was just seen from these numbers, complete darkness in this area on the last day of May is only from 11:00 p.m. to 3:00 a.m., that is, for only 4 out of the 24 hours of the day. Use the above information to determine your times of local sunset, end of twilight and beginning of twilight, and sunrise for each night of the month.

The long periods of twilight in May evenings give us a chance to observe the bright stars of spring as they gradually come into view in the southern sky. As twilight slowly darkens, try to recognize the pattern of stars that our ancient ancestors called Leo the Lion, high in the southern sky, and Virgo, the young lady of ancient mythology, at mid-height in the south-eastern sky. In the constellation Leo, the distinctive pattern has two parts – the outline of a “backwards question mark”above the very bright star Regulus AND to its left a right-angled triangle, with the bright star Denebola as the brightest of the three stars. In this two-fold pattern, the people of antiquity saw the shape of crouching lion facing westward, with the curve of the “question mark” showing its head and mane, the star Regulus marking its forepaws, and the triangle outlining its hindquarters. To the left of the lion, in the middle of the reclining figure of Virgo, is the very bright star Spica, seen by ancient Romans as an ear of corn or a sheaf of wheat held by Astraea, their goddess of justice, though people of other cultures saw it as the central star in the outline of another one of the goddesses in their list of female deities. When looking in the May sky at the three bright stars just mentioned, try to remember that you are observing suns at very great distances from our Earth. Regulus is 69 light years away and Denebola is 40 light years away. Spica, though it is seen to be considerably brighter than Denebola, is actually over six times farther away from us, at a distance of 275 light years! Remember, the next time you see the gleaming white light of Spica, that its INTRINSIC brightness is incredibly enormous – actually 2300 times that of our sun!

If we look to the northern sky during May’s evening twilight hours we may be surprised to notice that the Big Dipper, the most familiar part of the constellation Ursa Major, the Great Bear, is at its maximum height – almost directly overhead. In fact, the ‘pointer stars” in the cup of the Dipper, the ones that always direct us northward and to the Pole Star, are pointing almost directly DOWNWARD to Polaris, the Pole Star.

Among the five bright planets, only four of them may be seen this month, since Venus is in almost the same direction as the sun and the glow of the sun will prevent it from being seen. The month’s “special feature planet” will be Mercury, a planet that is rarely, if ever, seen by most people, but for the first three weeks of May, it puts on its best evening appearance of the whole year, and should be easily seen by anyone who makes the effort, even if the sky is cloudy on some May evenings. This planet, sometimes called “the slippery planet” – like the shiny liquid metal of the same name – was named after the ancient Roman messenger god with the winged feet. It is “hard to catch”, meaning that it is not often seen, because it is never high in the sky. Though it is bright, it must be seen between about 45 minutes after sunset and the time it sets – shortly after that, OR on other occasions when it is seen in the eastern morning sky before sunrise. This month be sure to have a good view of the western horizon and look low in that direction every clear night between May 1st and May 24th – beginning about 45 minutes after the sun sets and for about 30 minutes after that. It may be possible to see it for a few nights after the 24th. See if that is possible, also. Careful observers will notice two things: firstly, that Mercury appears sightly higher each evening before May 12th (if you look at the same time each evening: e.g. 45 min. after sunset), and after the 12th it appears lower each evening, and secondly, that its brightness fades considerably as the month advances. (I would be interested in hearing from readers regarding their number of Mercury sightings during May – if the dates and times are recorded accurately.) The second planet to be noticed in the evenings will likely be reddish Mars. It is well up in the western sky in evening twilight, and in early May, not far from the “twin stars”, Castor and Pollux, the brightest stars in the constellation Gemini. As May advances, Mars will be seen moving upward and into the constellation Cancer which is between Gemini and Leo the Lion. On the evenings of May 21st, 22nd, and 23rd, Mars will pass in front of the stars of the Beehive Cluster (also called M44), a sight not be missed by anyone who owns binoculars or a small telescope. The third planet likely to be seen, Saturn, is very high in the southern evening sky among the stars of the constellation Leo the Lion, and just to the left of their brightest star Regulus. It will take a very careful observer to notice the movement, over the course of the month, of Saturn moving eastward and away from the star Regulus. (At its enormous distance from Earth, Saturn’s movement appears much slower than that of Mars.) Owners of small telescopes will be fascinated by the view of Saturn and its rings and several of its many moons orbiting the planet. Brilliant Jupiter is still considered a morning planet, as it has been for over six months, even though by the end of May it may be seen rising in the east at about midnight. In morning twilight, Jupiter is by far the brightest object in the whole southern part of the sky, (not counting the moon, when present) just as it has been for the past two months. As with Saturn, small telescope owners will be fascinated by the views of Jupiter’s four brightest moons orbiting the planet.

The slender moon’s conjunction with Mercury will be noteworthy after sunset in the western evening sky on May 6th. The lunar crescent near Mars, Castor and Pollux on the evenings of March 9th and 10th will also be a beautiful sight. On May 12th don’t miss the triangle formed by the First Quarter Moon, Saturn, and Regulus. On the morning of May 24th, the waning gibbous moon will appear below the planet Jupiter, also an interesting sight.

International Astronomy Day occurs on Saturday May 10, an occasion that has been used for over 30 years by amateur astronomers to share their interest and views of the sky with the public. Weather permitting, I hope to offer safe solar viewing through a telescope that day at Sharbot Lake Beach near the Medical Centre from 1:00 to 3:00 p.m., and views of lunar craters, planets, and star clusters from 8:30 to 10:30 p.m..

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.

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