December 14, 2017

Astro-Event: Aphelion, an Occultation, a Launch and a Close Double.

Sunday night’s occultation path across the US…(Credit: NASA/IOTA).

We bring you this week’s edition of astronomy events a tad early to point your eyes skyward towards an interesting event; the occultation of a 10th magnitude star constellation Virgo. The asteroid is 52 Europa, and the event will last for up to 18 seconds for viewers from Montana across the central US down to our very own neck of the woods in Central Florida. This occurs from 11:00-:05 PM EDT the night of July 3rd depending on your location and I would plan on familiarizing myself with the star field the night before, weather permitting. Observers can video the occultation or simply watch it and note the duration through the eyepiece; if enough data is received, a silhouette of the asteroid’s profile can be constructed…satellite rocks have even been found this way! NASA will also hold a chat at 9:30 PM EDT the same night to discuss efforts underway to observe this event; the International Occultation Timing Association wants to hear from you!
Next up, the aphelion of our home world the Earth occurs on July 4th at 15:00 UT. You wouldn’t know it, but the planet is currently at a distance of 152,102,200 km from the Sun and is now continuing its long approach towards perihelion in January. Aphelion only falls in the middle of northern hemisphere summer during our present epoch and moves around the modern calendar once every 26,000 years thanks to our friend, the precession of the equinoxes. Eccentricity of our orbit, in turn, oscillates between 0.005 and 0.058 on a roughly 100,000 year cycle, of which we’re currently on the low end of the scale with an eccentricity value of about 0.017.
The next big ticket event this week is the final launch of the space shuttle program on July 8th at 11:26AM EDT. This is the final flight of Atlantis, after which NASA faces the brave new world of life aboard the ISS without a shuttle work horse on hand. It’ll be a bitter-sweet moment when that orbiter touches down one last time, with the next date for a manned mission from US soil unknown… unfortunately, the geometry for viewing the last shuttle flight over the continental United States could be worse, although viewing at the tail end of the 12 day mission may improve a bit if it slips a couple of days… follow us on twitter (@astroguyz) for all of the latest updates!
Finally, while you’re awaiting Sunday night’s occultation, I wanted to draw your attention to a fine double star in the constellation Virgo; Porrima, or Gamma Virginis. Located at R.A. 12:41:40 and Dec. -1 deg 27’, many have remarked lately as to “that bright star near the planet Saturn…” this is a fascinating double star system that is currently well placed for summer evening viewing. At a current separation of 1.7” and widening, this +3.5 magnitude pair of stars are at about equal brightness, and on an orbit that will change applicably in our lifetime of about 169 years. I like to use Porrima as a good test of the sky conditions and the optics of my instrument; a clean split means good seeing conditions. I’ve been able to reliably split the pair since 2008… When will you be able to do the same? The components of the Porrima system are two F0 stars each at about 1.4 solar masses about 39 light years distant.

The astro-word for this week is Periastron. This is the periapsis or closest point in a star’s orbit about a primary star in the system. In the case of Porrima, periastron occurred in 2005 with a physical distance of 4.8 astronomical units (about the distance from Earth to Jupiter on closest approach) and an apparent separation of 0.4”. The system will continue to widen until 2088, when a physical separation of 81 astronomical units (Earth to Eris on its way out to aphelion) with 6” arc seconds of separation, an easy split!

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