October 22, 2017

Astro-Event: A Difficult Occultation.

Looking west from Tampa, Fl at about 5:40PM EST. (Created in Starry Night). 

   This week, interesting planetary goings-on are afoot low in the western dusk skies, if you have the patience to observe them. On the evening of Monday, December 6th, visual athletes will want to try and spot an extremely thin crescent Moon occult a fading planet Mars. Now, here’s the tough part; the actual occultation is only visible over a thin swath of the US southeast and central America, and the only hope to acquire it visually is in the 15 minute span directly after sunset. The Moon will be only 28 hours past New, itself a difficult but not impossible target. Mars will be at about magnitude +1.3, and folks located further south (such as Astroguyz HQ on the Florida peninsula) will have a better shot at seeing the disappearance, as the ecliptic is unfavorably angled into the weeds this time of year. Local sunset around the Tampa Bay area will be 5:34 PM EST, and the occultation will begin at 5:45 PM with both the Moon and Mars just 8 degrees above the horizon. Moonset occurs at 6:33 PM, and viewers farther west along the track will see a reverse re-enactment as Mars re-appears in the twilight skies. Interestingly, reader Ed Kotapish points out that Mars will pass in front of the M8 nebula on December 9th, but with the Sun a scant 14 degrees away, this will be impossible to observe. And don’t forget tiny Mercury! Fresh past greatest eastern elongation on December 1st, the innermost world passes within one degree of Mars on December 13th. Start following the pairing now to know just where to look. A good pair of binocs and a clear western horizon will be your friends following these tough to observe events…good luck, and let us know what you see!

The Astro-word for this week is astronomical twilight. This is the phase of time between sunrise or sunset and the center body of the Sun reaching a position of 18 degrees below the local horizon. During this period, the atmosphere scatters sunlight across the terminator of the Earth, causing a gradual decent out of or into darkness. Areas near the equator experience a very brief twilight; areas in higher northern or southern latitudes may experience a twilight that is permanent throughout the day or at least several hours long at various times throughout the year.  Of the three technically defined twilights (Civil or Legal, Nautical, and Astronomical), Astronomical Twilight is the “darkest” of the three… how late or early can you notice the presence of the Sun?              

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