StarWatch: Moravian College Astronomy

StarWatch for the greater Lehigh Valley

APRIL 2016

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

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

Current Solar X-rays:

Current Geomagnetic Field:

1024 APRIL 3, 2016: Mercury Ascending in the West: Mercury, the most elusive of the wanderers, is now ascending in the west for its vernal debut after sundown. Just like the spring moon rockets into our western sky during February, March, and April, so do the inferiors, those planets that are closer to the sun than the Earth—Mercury and Venus. All of the major objects in our solar system travel around the sun near Earth’s orbital plane, the ecliptic, so when the ecliptic is tilted at a steep angle to the western horizon, as it is in the spring, a planet like Mercury passing behind the sun rapidly climbs in altitude to become visible in the western sky after sunset. Mercury reaches its greatest angle of eastern elongation from the sun on Monday, April 18 when it will be nearly 20 degrees away from Sol. That translates to nearly 11 degrees above the WNW horizon, 40 minutes after sundown, when it is dark enough to make Mercury an easy target to the unaided eye, and effortless if binoculars are used. There are advantages to viewing Mercury earlier than later anytime it is visible in the western sky, and that is because it is more fully lit by the sun and considerably brighter. On April 4 when Mercury will only be about a degree off the horizon, 40 minutes after sundown, its vividness will rival Sirius the Dog Star in the SSW, the brightest luminary of the night. By Friday Mercury will have climbed to five degrees above the horizon, but at the same time, it will be only 2/3rd as bright as Sirius. The advantage here is its altitude, making it easier to find. In addition, there will be a razor thin crescent moon only eight degrees to Mercury’s upper left. Both will be visible with the unaided eye if a good enough western horizon is available and sky conditions are clear; but again, if you own binoculars, the view will be so much more impressive. On Monday, April 18, Mercury will be found nearly 11 degrees above the WNW horizon 40 minutes after sundown, but it will now be a full magnitude, 2-1/2 times fainter than Sirius, but still a beautiful sight.

1025 APRIL 10, 2016: Mercury: Tiny and Strange: The small world of Mercury is prominent this week, visible about 10 degrees above the western horizon, 40 minutes after sundown. Until its remote exploration by Mariner 10 in the mid-1970s and Messenger (2004-2015), Mercury was a bright “star” occasionally seen in the morning or evening twilight. We knew it was the smallest of the planets, and its orbit was the most eccentric (after Pluto). Mercury made exactly two revolutions for every three rotations, a condition called spin-orbit coupling. Since Mariner and Messenger, we know its surface resembles the moon’s barren terrain with basins and craters made by large and small impacts. Like the moon, volcanism has played a major role in shaping Mercury’s final topography, but its composition was different, with a large iron core and a weak but active magnetic field that was offset from the planet’s center. Mercury also contracted as it cooled, losing nearly 10 miles in its diameter, buckling its surface, resulting in titanic “earthquakes” that would have dwarfed any on the Earth. Also unique to Mercury was the mystery of the “hollows,” shallow irregular depressions in some craters that seem to have resulted from Mercury losing surface material. Another big surprise was that Mercury had water ice deposits on or near its surface in its polar regions. Because its axis is vertical to its orbital plane (Earth’s axis is tilted by 23.5 degrees), deep craters near Mercury’s poles never were sunlit and continued to preserve deposits from icy comets that bombarded the planet early in its history, similar to crater-shadowed water ice deposits in the polar regions of our moon. Finally, there were the volatiles, materials that melt and boil at low temperatures. Water is one such substance, but so is potassium and sulfur when planetary surfaces are analyzed. These elements are found on Mercury in the highest abundances of the four inner planets, adding more intrigue to an object that has been baked by the unforgiving sun for over 4.5 billion years.

1026 APRIL 17, 2016: Vernal Sky Blossoming: The East Coast is finally warming after a topsy-turvy late winter and early spring. There have been so many clear nights lately that it feels more like I’m living in a dream state than the normally unsettled Lehigh Valley clime. The winter sky is almost gone, with Orion bending forward into the western twilight, Sirius the Dog Star settling rapidly into SW, and all of the other winter star patterns crowding into the western sky ready to bid adieu to us until next year. The vernal heavens are all abuzz, and although its patterns are not as spectacular as the summer or winter sky, because we are looking away from the plane of the Milky Way Galaxy, there are some bright objects and patterns that are worth noting. Negating the moon, which will reach its full phase early on April 22, there is a really bright “star” center stage in the south. That’s Jupiter, and above it is Leo the Lion, created by a backwards question mark for his head and mane (above Jupiter’s right) and three stars that form a triangle for this back (above Jove). It really does look like a Lion, “lying” in the grass or the Egyptian Sphinx close to Khufu’s pyramid at Giza. Turn around to face north and look high; you’ll see the Big Dipper, still climbing but nearly upside-down. The two stars to its left in the bowl point towards the North Star, the pivot around which the sky circles. The center luminary in the Dipper’s handle, Mizar, is actually a double star with a visual component, Alcor, close and below. A small telescope at low magnifications will split Mizar into two stars, and each of these and Alcor are actually doubles, making Alcor-Mizar a sextuple system. Yes, there is really more than meets the eye heavenward. End your quick perusal of the early spring sky by following the sweep of the Dipper’s handle to the bright star Arcturus and onward to spike Spica. Arcturus is an aging orangey giant star, fourth brightest of the night, while Spica numbers 16, and is a young, hot blue supergiant to boot. Sounds like a soap opera… Happy spring observing!

1027 APRIL 24, 2016: Draco: Never at Rest: One of my favorite constellations of the northern sky is Draco the Dragon. In late April its tail begins just under the bowl of the Big Dipper. The BD’s scoop and handle are nearly at their culminating (highest) positions in the north at 10 p.m. Then Draco’s tail curves downward over the cup of the Little Dipper and falls to the right of Polaris, the North Star. Following the sinuous tail of the Dragon is like trekking along on the winding Yellow Brick Road in the Wizard of Oz. The eye just seems to hop, skip, and tumble naturally from one star to another, but then there is a breakdown that occurs where the tail meets a small cadre of five faint stars that outline the tiny body of this fearsome sentinel. Where do you go from here? The secret during late April and early May is to look to the right and above the body stars to find an additional four luminaries that create a trapezoidal figure. That’s the head of Draco. By connecting the star of the head that is closest to the star of the body, the picture of Draco—tail, body, neck, and head—is completed. Some people see Draco as a turtle, camel, alligator or a snake. Whatever works is fine; however, for well over two decades, Draco has been my mystery constellation when I instruct classes under the real or electronic sky. I simply outline Draco’s stars without any comment and let my audiences guess at the pattern. Whether the group is composed of kindergarten kids or seniors, someone always, and I mean always, comes up with the dragon. There has never been a miss. Since Draco is a north circumpolar star pattern, it never sets, a fitting punishment for his lack of astuteness in guarding the Golden Apples of the Hesperides, Hera’s wedding gift to Zeus. Hercules, with the assistance of dim-witted Atlas, stole them from right under Draco’s nose causing Hera in a fit of rage to swing Draco by his tail skyward, to fall among the northern stars. Because he can never set, he is doomed to be ever vigilant, never to sleep, eat, or drink again, one unhappy and cranky beastie.

[April Star Map]

[April Moon Phase Calendar]