What's Up in the Night Sky

December 2016 - Vol. 20, No. 12

This Month's Night Sky - NOTE: The next paragraph describes the sky as it appears at 10 pm EST (11 pm EDT) near mid- month. The sky also looks this way at 11 pm EST (midnight EDT) during the beginning of the month and at 9 pm EST (10 pm EDT) by month's end.

Some believe the winter night sky is the most beautiful of the year! By mid-month, misty Pleiades, the famous open star cluster of the constellation Taurus, is visible due S. at 10 pm. Although part of the constellation Taurus, it lies above its "lazy V" asterism whose brightest star, orange-tinted Aldebaran, glows near the point of the lower branch of the "V". Above are the constellations Perseus, Cassiopeia (whose "W" shaped asterism is unmistakable) and Auriga. Lovely Orion, whose asterism reminds me of a slightly lopsided hour glass, moves upwards from the SE. Note its three "belt" stars located at the "pinch" of the hour glass. The hazy object below the middle belt star is M42, the Great Orion Nebula, a region of space where stars are being born. Orion is followed by the bright stars Procyon (Canis Minor) and Sirius (Canis Major). Along with the bright star Betelgeuse (Orion), these three stars form the famous "Winter Triangle". To the E shine the Gemini twins, Castor and Pollux. In the SW, the diamond-shaped Great Square of Pegasus stands on one corner while high in the N, Ursa Major's asterism, the Big Dipper, stands on its "bowl".

MERCURY is well placed for the evening twilight sky until the 23rd. VENUS continues shining in the evening sky. MARS sets in the early evening, moving into the constellation of Aquarius on December 15. JUPITER rises well after midnight, gleaming in the morning sky. SATURN disappearing in the evening twilight, moving toward conjunction with the Sun on November 10. URANUS in the evening sky. The outer planet NEPTUNE sets late in the evening.

Review how to determine Angular Measurement.

Calendar of Events

NOTE: For those observers not in the ET zone, convert the calendar times to your zone's time by subtracting one hour for CT, two for MT and three for PT. Don't forget to adjust for Daylight Savings Time when necessary by subtracting one hour from your planisphere's time. Dawn and dusk times must also be corrected. See your local newspaper, TV news, or cable TV's Weather Channel for sunrise and sunset times or check with the U.S. Naval observatory. Unfortunately some of these events may occur during daylight hours in your area.

DATE	EVENT
03	Venus 6 deg S. of Moon, Mercury is close to Venus on this date.
06	Neptune .7 deg S. of the Moon, occultation observable from Central America, USA, parts of Canada, Greenland, Iceland, and western Europe.
10	Saturn in conjunction, not visible
11	Mercury greatest elongation E.
14	Geminid meteors peak, this storm may generate up to 120 meteors per hour!
12	Moon at perigee, expect large tides.
15	Aldebaran 0.5 deg S. of the Moon occultation from northern Mexico, USA, parts of Canada, south tip of Greenland, northwest Africa, and western most Europe.
17	Solstice, Sun standstill at S. extreme, summer arrives in the southern hemisphere.
18	Regulus 1.0 deg N. of Moon, occultation from southernmost Australia, parts of Antarctica.
22	Jupiter 2 deg S. of Moon. Ursid meteors peak, this storm generates up to 10 meteors per hour.
25	Moon at apogee.
28	Mercury at inferior conjunction.

Lunar Almanac for December 2016

Phases of the Moon	Phase and Date(s)	Best viewed before local midnight
	New 29	Deep Space Objects
	1st. Qtr 07	Planets & Moon
	Full 14	Moon
	Last Qtr 21	Deep Space & Planets

Topic of the month: Celestial Twins

Welcome to the last month in our 20th year! The evening sky welcomes the northern hemisphere to longer nights and views of the celestial vault, while in the south night is shrinking and Summer has arrived. The constellation of Gemini should be visible from a modest site by 10 pm for citizens of Earth in the northern hemisphere. For many human civilizations, the constellation was associated with twins.

Two of the constellations bright stars, Castor and Pollux, are less than 5 degrees apart in the night sky from Earth although they are not gravitationally bound to each other. Pollux, the brighter of the two, shines at 1.14 mag. and is easily the brightest star in the constellation. Despite its brightness, it has been designated as Beta Geminorum (Beta Gem). The star is much more yellow than its "twin" as it has a K0 spectral classification and is considered an orange giant. A mere 52 light years away, Castor (visual magnitude 1.93) was discovered to be a double star in 1678. As astronomical knowledge became more sophisticated, the system was eventually identified as a system of three components, now designated Castor A, B, and C. Castor A and B were studied by William Hershel in 1803, he identified that the stars were actually gravitational bound and thus, Castor A/B is the first true physical binary that was recognized. Castor A and B are separated about 6 arc-seconds and their revolution period about 467 years. Castor B is a pair of stars, reported as type A5 in Burnham's Celestial Handbook. They are of a similar size and each is about 1-1/2 the size of our Sun. Castor C is a pair of red dwarf stars, each about half the mass of our own Sun. This component has been designated YY Geminorum due to its variable nature. Castor A and B have a faint companion (separated 72 arc-seconds) that have the same parallax and proper motion. With six individual stars gravitationally bound together, Castor is considered to be a sextuple star system.

Amateur astronomers will want to look for an object in the constellation of Gemini known as NGC 2392 or the Eskimo Nebula. This object is an excellent planetary nebula. Although it is not quite as large as the more popular "Ring Nebula", M57 in Lyra, NGC 2392 is brighter at 8th magnitude. The central star of this planetary is also much brighter. At 8th magnitude it is easily seen in amateur telescopes, whereas the central star of the Ring is too faint for the small telescope. Don't look at CCD images and expect your view to approach the color and detail of those. The visual observation has its own charm and wonder. On a good night the edges of the Eskimo's parka will thrill the eye.

No discussion of Gemini is complete without mention of the huge star cluster known as M35. This cluster spans an area greater than the full moon and is visible at dark sites with the naked eye. The cluster was known before the time of Messier, but was found in his catalog and so the M designation has remained. The cluster contains at least 120 stars covering an area of 30 light years. Viewing this object with an amateur instrument gives one the impression of many double stars. Also visible as a fuzzy patch of light in the same field of M35, is the rich cluster NGC 2158. This cluster is more distant than M35 and would take a large amateur instrument at high power to resolve into stars.

--See You Under the Stars!
Astra for Astra's Almanac

The star chart above was generated by Stellarium, a free open source planetarium program. The above image was created by Dawn Jenkins, using Stellarium and a graphic editing program to format the image for this web page. Editing was done for educational purposes only. Stellarium offers much more to amateur astronomers and is being used in planetariums and to guide telescopes in the field. Simple charts like the one above can be used on the internet for non-profit, illustration purposes. Proper credit is due of course! Thank you to the makers of this fine program from Astra's Star Gate.