June 7, 2020

Astro-Event: A Fine Holiday Conjunction.

The Moon+Jupiter on Dec 25th at 7PM EST.

(Created by the Author in Starry Night).

Looking forward to Christmas? The sky has a present for all on the evening of December 25th. On this date, the 95% illuminated waxing gibbous Moon will make a splendid pairing with the -2.7 magnitude planet Jupiter. Look to the east for the pair at sunset. In fact, this is also a great time to try and pick out Jupiter in the daytime, a feat that is made simpler by the nearby and easy to spot Moon. Use binoculars to guide your eyes to the ghostly disc of Jupiter against the daytime sky, and then attempt a naked eye sighting. The “bluer” the sky, the better chance you have of picking out a low-contrast Jupiter against the background near the Moon… a fun feat to attempt!

A daytime Jupiter as seen on September 8th, 2012.

(Photo by author).

The Moon will actually occult (pass in front of) Jupiter and its retinue of moons for observers in South America and South Africa. The occultation is centered on 00:10 UTC December 26th, and will occur in the daytime for observers west of Brazil. (see graphic below) This is actually part of a short cycle of three Jovian occultations by the Moon occurring over the next few months, with the next ones occurring on January 27th & February 18th of 2013 (also occurring in the southern hemisphere). For North American observers, Jupiter will pass from 29’ arc minutes of the limb of the Moon as seen from Miami (25° 30’ north latitude) to 39’ arc minutes as seen from Montreal (45° 30’ north). This occurs around 6:15PM EST with the Moon at about 30-40° degrees elevation above the horizon. The Moon rises around 3:30PM from middle northern latitudes on the evening of the 25th.

The occultation path for December 25th-26th. (Created by the author in Occult 4.0).

Now for the obligatory “Wow!” factor. The Moon appears about half a degree (30’ arc minutes) across, while Jupiter appears about 1/38th the size with a disk 47” across. But of course, that’s just an illusion; Jupiter is 41 times larger than our 2,084 miles in diameter Moon, but is over 1,600 times farther away.  And that also means the reflected sunlight took only just over a second to leave the lunar surface and enter your eye, but took over 34 minutes to depart Jupiter for your viewing convenience. What madness is this, that tiny rocky satellites can cover gas giant worlds? Sometimes, the universe just makes you say “Wow!”

Jupiter just passed opposition on December 2nd, and will not do so again until January 5th, 2014. Neither Jupiter nor Mars reach opposition in 2013. I’ll leave it to any astute readers to calculate when that last happened… the Moon also reaches Full this week at 10:23 UTC on the 28th. This is the last Full Moon of 2012. The December Full Moon is also known as the Full Frost, Winter or Oak Moon. This is also known as the “Moon before Yule” when it falls before Christmas (which it doesn’t this year) and the “Long Nights Moon” being the closest to the northern hemisphere Winter Solstice. This is also the most northern Full Moon of 2012, (a direct result of the solstice tie-in) with a 19° 49’ north declination. Happy holidays; expect our action-packed look at “all things astronomical” in 2013 before 2012 is out!