StarWatch: Moravian College Astronomy

StarWatch for the greater Lehigh Valley

SEPTEMBER 2015

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

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994 SEPTEMBER 6, 2015: Autumn, Just Around the Corner: Where is our summer going? Earth’s axial inclination is responsible for the seasons. In the summer the Northern Hemisphere tilts towards the sun, even though Earth’s actual distance from Sol is at its greatest. It rises north of east, transits very high in the south, and sets north of west. Its path is long, causing its time above the horizon to be greater than in any other season. The Earth receives more energy than it radiates back into space at night, and temperatures soar; weather patterns stagnate, and our climate feels more tropical. In the winter the sun rises south of east, still transits in the south, but much lower, then sets south of west, causing its path to be much shorter. This translates into fewer hours of sunlight and much less energy reaching the ground. The net result is colder temperatures, and our climate moves towards a sub-Arctic venue. It is the transition times that are the most interesting to me. We are basically two weeks from the autumnal equinox which will happen on September 23 at 4:21 a.m. for the East Coast. The sun will be crossing the equator and heading into the Southern Hemisphere for the next six months, and our days will be growing shorter rapidly and our nights equally longer. On September 6 for 40 degrees north latitude, at any of the major northern longitude circles, such as 75/90/105 degrees west or east longitude (Philadelphia, PA is an excellent example), sunrise will occur at 6:33 a.m. local time, and sunset will take place 12 hours, 51 minutes later at 7:24 p.m. The same time interval beyond the autumnal equinox, 17 days later on October 10, Sol will rise at 7:06 a.m. EDT and set 11 hours, 23 minutes later at 6:29 p.m. That is an 88 minute difference in the shortening of the day and the lengthening of the night. Going farther north will expand this difference while southward, this interval becomes less. So if you have begun to lament the shortening days, the best way of putting it is to say, “You ain’t seen nothing yet!” Happy autumn; it’s almost here!

995 SEPTEMBER 13, 2015: Lunar Coronas: One of my practices after sundown is to make brief excursions outside and look up. It is not as if I am trying to make the great serendipitous discovery of the century, but over the decades, a certain amount of intuitive and practical information about the sky and weather has been garnered which has become invaluable when deciding “whether the weather” will cooperate for an astronomical observation. During what I expected to be an average, bright moonlit evening in late July, I popped out of doors to see how the humidity was improving after the earlier passage of a cold front. It wasn’t. The atmosphere still felt drenched, but the air was very clear, and low, scudding stratus clouds moved rapidly under a veil of much higher nearly transparent clouds that still covered a majority of the sky. It was a weird scene which got more bizarre as soon as the moon was covered by a lower cloud. The high clouds around the bluish moon were red. I was witnessing a lunar corona. Vibrant ones are rare, and so I scrambled indoors for my camera gear and emerged 10 minutes later under a nearly clear sky. I felt betrayed, but I waited and sure enough about five minutes later, both high and low clouds reappeared in tandem from the west, and the corona materialized in all of its vividness. Unlike halos and rainbows which result from the refraction/reflection of light inside hexagonal crystals of ice or raindrops, coronas are fashioned from the diffraction (bending) and scattering of light around extremely small particles of water or ice. In a corona, colors are normally subtle blends rather than the more delineated transitions seen in a rainbow. A corona’s shape and boundaries are also affected by the sizes of the tiny particles diffracting the light. Despite all of the technicalities, a lunar corona is a superb expression of Nature’s beauty. The lower, darker clouds which dimmed the moon and accentuated the saturated red hues around Luna made this corona extra special. A photo is online at astronomy.org.

996 SEPTEMBER 20, 2015: September’s Lunar Eclipse to Dazzle: One of the premier astronomical events of the year, a total lunar eclipse, happens Sunday, Sept. 27. Lunar eclipses occur when the full moon completely enters the shadow (umbra) of the Earth. Depending upon atmospheric clarity around the world, portions of the moon will be bathed in various shades of reds, yellows and oranges if the evening is clear. The eastern half of the US is favored for this event with most of the eclipse occurring before midnight on the 27th, while West Coast observers catch the rising moon partially obscured by Earth’s umbra. There are two other circumstances which make this eclipse rarer than most. First, the moon will be within one hour of its closest distance to Earth for the year. That means Luna will appear to be about 13 percent larger than when it is at its average distance from Earth. Second, this full moon is also the Harvest Moon because it occurs closest to the autumnal equinox which takes place on Wednesday, Sept. 23 at 4:21 a.m., EDT. For the East Coast, the moon enters Earth’s penumbra or secondary shadow at 8:10 p.m., Sept. 27. Viewing from the moon, an astronaut with the proper filters would begin to watch the Earth eclipsing the sun. Less sunlight is reaching the moon, but it will be an additional 30 minutes before Earthbound viewers see this ashen light dimming the left side of the moon. At 9:07 p.m. the moon begins moving into the true shadow, taking one hour and 3 minutes before it is eclipsed completely. Then for the next 74 minutes, the moon is in totality and most colorful. Totality ends at 11:24 p.m., EDT, and for the next 63 minutes, Luna marches from the umbra. At 1:24 a.m., on Sept. 28 the moon exits the penumbra and the eclipse is officially over. Although this eclipse will be beautiful to behold with just the unaided eye, telescopes and especially binoculars or spotting scopes of any kind, will add to the vibrancy of the colors. We do not see another total lunar eclipse until 2019.

997 SEPTEMBER 27, 2015: The Martian: “Home Alone”: When astronaut hero, Mark Watney, of Andy Weir’s book, The Martian, was finally rescued after 549 days on the Red Planet, he stank like a skunk. Little did Weir know how sweet that smell would become. Now The Martian is coming to the big screen this week, and the anticipation among science aficionados and space enthusiasts is becoming almost unbearable. In a Skype interview that I attended at the Mars Society convocation in Washington this past August, Weir told the story of a techie who wanted to write, but just couldn’t find a publisher. Then a light bulb (low energy fluorescent) when off in his brain. “Why not start a website and release chapters online to begin garnering an audience of readers. The idea proved successful, and as new chapters of The Martian came out, Weir’s audience grew to about 3000. Weir was meticulously accurate in how he planned to save Mark Watney from almost certain death after being left behind by crew members when the communications dish of their habitat dislocated, and he was taken for a short, but nasty ride in the dust and darkness of a Martian storm; but he didn’t always get it right. Techies to the rescue... They corresponded with Weir, told him what was wrong, and helped him correct his copy to create a rollercoaster ride of survival and near death experiences for Watney, but also a book that was accurate scientifically in virtually all aspects of the story. Astronauts love it because it portrays how they must not only be smart and nerdy, but also practical, solving life and death problems on a day-to-day basis. NASA loves it because they hope it will excite the public’s vision to one day travel to Mars. And writer, John Schwartz, of the Times feels that the public will embrace The Martian enthusiastically because of the sheer terror of what it would feel like to be completely “home alone” on a hostile world millions of miles from Earth, knowing that each day could be your last. The Martian debuts on Friday, October 2.

[September Star Map]

[September Moon Phase Calendar]