StarWatch: Moravian College Astronomy

StarWatch for the greater Lehigh Valley

MAY 2013

MAY STAR MAP | MOON PHASE CALENDAR | STARWATCH INDEX | NIGHT SKY NOTEBOOK

Print Large Sky Charts For 10 p.m. EDT: NORTH | EAST | SOUTH | WEST | ZENITH

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Farewell to a wonderful Moravian astronomy class: Gary A. Becker photography...

872 MAY 5, 2013: Planet Rendezvous in “Lusty” May: “It's May! It’s May, the lusty month of May! That lovely month when everyone goes blissfully astray…” That’s Guinevere singing about one of the joyful perks regarding life at Camelot, in mythical England where by law the seasons had regimented time slots, could never be too severe, and the rain could never fall till after sundown. No wonder the empire fell. No astronomer in his right mind would ever want to live in a place like that. Yet in the real world, May produces some of the most spectacular nights of the year. Gorgeous warm days with turquoise skies blend seamlessly into transparent, temperate nights where the stars seem so close you can almost reach out and touch them. I’m hoping for that kind of an evening on Friday, May 10 when a very skinny, razor sharp crescent moon will be a scant two degrees from the goddess Venus. The rendezvous takes place in the WNW with the moon only 11 degrees from the sun, so the event will only be visible in a rather strong twilight and with the use of binoculars. While the Pleiades will only be about six degrees from the moon at this time, I doubt that the bright sky conditions will allow anyone to view this star cluster. Although the East Coast is the favored location, by the time of sunset along the West Coast, the moon will be just over a degree higher in the sky, making its visibility a little more probable. The key to assuring success is to have a location which has a flawless western horizon. This is where owning a penthouse apartment on the Upper West Side of Manhattan with a clear shot across the Hudson would come in handy. Most of us, unfortunately, will simply have to hunt down an appropriate site. Be there with binoculars by sundown and begin to scan the horizon to the left of sunset. You should see Venus first. To Venus’ left and slightly below, the moon should emerge 10-15 minutes later. Clear skies to all!

873 MAY 12, 2013: The Eclipse that Almost Isn’t: On the evening of May 24/25, one of the most ephemeral of astronomical events takes place. It is a partial penumbral eclipse of the moon. Lunar eclipses occur when the sun, Earth, and moon are aligned so precisely that the moon passes through one or both of the shadows of the Earth. The Earth, or for that matter any astronomical body, produces two types of shadows, an umbra and a penumbra, derived from the Latin and meaning shade and light shade respectively. Any portion of the moon that is inside the Earth’s umbra would be a location from which an astronaut on the moon would see the Earth completely blocking the sun. That same onlooker positioned within Earth’s penumbral shadow would notice, using the correct filters, only part of the Earth covering part of the sun. Because of the sizes of the umbra and penumbra at the moon’s distance, four types of lunar eclipses are possible. The moon can pass completely into Earth’s umbra creating a total lunar eclipse or the moon can pass partly into the umbra creating a partial lunar eclipse. The moon can also pass completely into the penumbra, producing a total penumbral eclipse, or partly into the penumbra with the rest of the moon in sunlight. This would be called a partial penumbral eclipse. Because the moon’s orbit is tilted by just over five degrees to the Earth’s orbital plane, the full moon usually passes either above or below Earth’s shadow, and no eclipse happens at all. Three lunar eclipses arise in 2013 and they are all losers, but the worst of the lot, “visible” from the Americas starts at 11:53 p.m. EDT on May 24 and ends 33 minutes later at 12:26 a.m. May 25. Only four percent of the diameter of the moon penetrates into the Earth’s penumbral shadow, an event that will go unnoticed by even the keenest of observational eyes. Still for the records, it has to be listed as a lunar eclipse, the eclipse that almost isn’t.

874 MAY 19, 2013: Magnificent Planet Gathering: Throughout this week three planets will be moving into position for what will become, early next week, the most stunning planetary grouping of the year. The assemblage takes place low in the west during evening twilight. The triad includes Mercury and Venus becoming more prominent because of their increasing distance from the sun, and Jupiter, which because of its slow orbital motion, will be overtaken and in conjunction with the sun on June 19. The week commences with the most difficult planet to see, Mercury, only nine degrees from the sun and setting about 52 minutes after sundown; Venus reaches the horizon only 15 minutes later. Jupiter, because of its distance from Sol, will pose no problem in viewing. Forty minutes after sundown, Venus will still be four degrees above the horizon, and Jupiter will be 10 degrees in altitude. They should be easily seen if a good western horizon is available. Scan just above the horizon 30 minutes after sunset. Binoculars will help Venus and Jupiter stand out more easily against a bright horizon. By Saturday, May 25, the scene has changed completely. All three planets will be found in a tight little triangle about three degrees in diameter and six degrees above the horizon, 40 minutes after sundown. Binoculars will easily contain the three in the same field of view with ample space to spare. Venus will be brightest and lowest to the horizon, followed by Jupiter, which will be farthest to the left. Faintest and farthest to the right will be Mercury. All three members will be bright and easily visible to the unaided eye if sky conditions are transparent enough and the observing location unobscured enough to permit objects only a few degrees above the horizon to be seen. On May 26, the grouping is at its tightest with the largest separation among the three planets less than 2.5 degrees. The 27th finds Venus and Jupiter separated by just one degree.

875 MAY 26, 2013: Polaris: The Guiding Light: One of the two coordinates that allows you to find your position on the Earth’s surface is latitude, the angular measure north or south of the equator. Polaris, the North Star, can easily allow you to observe your change in latitude if your summer travel plans take you in a northerly or southerly direction, like the Caribbean or northern Europe. This is because Polaris lies in the same line of sight as the extension of the Earth’s axis into space. At the North Pole, 90o north latitude, the North Star is essentially overhead, while at the equator, 0o latitude, Polaris lies along the horizon. Its altitude or angular height above the horizon gives an observer a good approximation of his or her latitude position. Follow the Pointer Stars of the Big Dipper downward to Polaris. Presently, they are farthest to the left in the cup and are cresting in the north after darkness. For a good swath of the US, the North Star’s elevation is approximately 40o. That’s equivalent to four clenched fists held at arm’s length (thumb on top), stacked one on top of the other. Let’s say that you’re traveling to Disney World this summer. You’re out by the pool after dark, and the kids are swimming before bed. Polaris will be about three fists high, 28o above the horizon. Traveling to southern Europe? The change will be negligible. Rome (42o N) and Athens (38o N) are too close to our latitude to note any differences by using your fist. However, London is about 52o N, five fists high for Polaris, and southern Alaska is about 60o N. That’s six fists. In Alaska measurements are impossible to make until later in the summer because you are too far north, and the sun never makes it far enough below the horizon for complete darkness to occur. Finally, if Hawaii or a Caribbean cruise is in the offing, Polaris will hug the horizon at two fists or less. To see the change, make sure that you first take a measurement from where you live.

[May Star Map]

[May Moon Phase Calendar]