StarWatch: Moravian College Astronomy

StarWatch for the greater Lehigh Valley

APRIL 2008

APRIL STAR MAP | STARWATCH INDEX | MOON PHASE CALENDAR

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607 APRIL 6, 2008: Astronomy in the Twilight Zone: If you follow this column, then you know that I enjoy tracking the rising and setting positions of the sun and the moon and photographing objects which are close to the horizon. Since I am not yet retired, my observing preferences lean towards the evening hours, but I am not opposed to grabbing quick looks at the morning sky, especially when retrieving the newspaper. Much of my photography is done at dusk. I enjoy capturing a healthy twilight glow because it diminishes the effects of light pollution which are enhanced nearer to the horizon. Recently, I began wondering just how late into the season I could view or photograph some of the winter and spring constellations, so I consulted The Sky, my favorite astronomy computer program to find out. To keep enough twilight glow in my photos, and yet be able to discern the stars I was interested in capturing, I chose to place the sun nearly 12 degrees below the horizon at the very limit of what is called nautical twilight. That is about an hour after sundown. My favorite winter star pattern is Orion the Hunter. While it is cramming the western horizon during the week surrounding April 25, so is Canis Major the Big Dog, Taurus the Bull, and the Pleiades star cluster. Catch all of these star patterns near the horizon in deep twilight around 8:50 p.m. About one month later, during the week surrounding Memorial Day, May 26, Gemini the Twins sets standing up in the WNW in deep twilight, 9:30 p.m. The bright star Procyon will be to Gemini’s left and Capella, to the right. Mars will be located in Cancer the Crab, above and to the left of the twins. When Leo the Lion sets at 9:30 p.m. during the week of July 6, Mars and Saturn will be enjoying each other’s company in the Lion’s belly. The crescent moon will pass Leo on the nights of July 4 through the 9. Maps are online.

608 APRIL 13, 2008: MESSENGER's Magnificent Mercury: NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft, MESSENGER, is revealing a bonanza of high resolution data since its first flyby of the Messenger god on January 14. A cursory look at Mercury shows it to be similar to our own moon, a rocky, cratered planet with little atmosphere, scalded by the sun during the day and frozen by night. MESSENGER is bringing into sharper focus the differences between these two worlds begun with Mariner 10’s three flybys of Mercury in 1974-5. Internally, Mercury possesses a huge iron core, larger than Earth’s moon, and 75 percent the diameter of the 3,100 mile-wide planet. Our moon has virtually no core. This variation in composition suggests that the moon and Mercury had different evolutionary histories, a problem which scientists hope to clarify through MESSENGER’s chemical analysis of Mercury’s surface. Unlike the moon, Mercury possesses a weak magnetic field, implying that part of its core is liquid and conducting electrical currents. Surface features also vary. Mercury has wrinkles across its exterior, suggesting that the planet shrank as it cooled. Nothing like that happened on Luna. Similar to the moon however, Mercury has sizeable areas of heavily cratered terrain and huge basins, like the 960 mile-in-diameter crater called Caloris. But unlike the moon, there are also immense regions of Mercury’s cratered surface which appear to have been partially melted early in its history. MESSENGER’s flyby of Mercury in January will be repeated in October 2007, again changing the spacecraft’s path so it can be positioned for an orbital insertion on March 18, 2011. That is when the real science begins as MESSENGER completes a yearlong study of our solar system’s tiniest world.

609 APRIL 20, 2008: Yellow Saturn, Red Mars: Saturn and Mars take center stage this week as the moon begins to wane from its full phase to last quarter. One hour after sundown, face south and gaze up just over halfway into the sky to see two closely separated starlike objects. The one to the left will be distinctly brighter and should shine with a steadier light. You’ll easily fit these two luminaries into the field of view of binoculars or a spotting scope, allowing their differences to become even more apparent. Yellowish Saturn to the left will shine steadier, while scintillating Regulus, the brightest star of Leo the Lion, will flicker white, with just a tinge of blue. Saturn has been retrograding or moving backwards towards Regulus since December 20, 2007 as the Earth has come around and passed the Ringed World. We see this effect everyday in our own lives. As we walk past trees, cars, and buildings, they seem to move backwards as we go by them. The faster moving Earth has long passed Saturn which takes 29.5 years to orbit the sun, and the backwards or westward motion of Saturn against the starry vault will end on May 3 when Saturn begins to move eastward, away from Regulus. Rotating your gaze about an eighth of a turn to the right, and looking at the same altitude will bring your eyes to three luminaries of nearly equal brightness. The two stars which will be closer together are the head stars of the Gemini Twins, Castor (right) and Pollux (left). Below the twins, but at a slightly greater distance, and forming an inverted “L,” will be a star which does not twinkle. Through binoculars it will have an obvious reddish hue compared to yellowish Pollux and white Castor. If you’re thinking Mars, you’re right on target. By July 10 Mars will have caught up to slower Saturn in the belly of the Lion, a beautiful twilight sight near the western horizon.

610 APRIL 27, 2008: Mercury Leaps into the Western Sky: Last week, I was asked by a fifth grade Muhlenberg student which of the eight planets was my favorite. For a moment she caught me off guard. I thought about Saturn’s splendid system of rings and how majestic they look through an eyepiece. Then my mind raced to Jupiter and its four Galilean satellites, strung out like delicate pearls on a necklace; but I had to confess that neither of these giant planets held the same allure as seeing elusive Mercury scraping the treetops as it nears its setting position. For a person like me, who has had several close encounters with blindness, observing the most difficult of the classical planets is always a wonderful treat which reminds me of how fortunate I have been. The best opportunity of this year to observe the Messenger god in the evening sky is almost upon us. For mid-latitude locations Mercury will be well positioned in the WNW from this Wednesday through the evening of May 23. During this three week plus interval, Mercury will be at least 10 degrees above the horizon, 30 minutes after sundown. Throughout the week of May 11, Mercury will set nearly two hours after the sun, about as good as it gets for this tiny world. Mercury will be highest in the sky on the evening of May 13, but if you’re going to make an attempt at locating it, try finding Mercury the week before the best evening rather than after this period. As Mercury comes around the sun and heads for inferior conjunction with Sol on June 7, its brightness will be fading as less and less of its hemisphere facing us is illuminated by the sun. As an example on May 6, when an ultra thin crescent moon will be passing just two degrees above Mercury, the planet will be nearly five times brighter than when seen on May 20. Regardless of Mercury’s brightness, always carry along binoculars to make its identification easier.

[April Star Map]

[April Moon Phase Calendar]