What's Up in the Night Sky

December 2017 - Vol. 21, 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 Aldeberan, 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". Look for the northern cross (Cygnus) standing upight on the western horizon on Christmas day after sunset.

MERCURY observable early in the month in the evening sky, will reach inferior conjunction and return to the morning sky late in the month. VENUS can be seen in morning twlight,but will vanish in the Sun, not to reappear until next year. MARS in the morning sky moves into the constellation Libra on the 21st. JUPITER in the morning sky passes 1 deg N of Alpha Librae, the beautiful green star Zubenelgenubi, mag 2.8, and its binocular companion of 5.3 mag. SATURN will not be visible this month.. URANUS and NEPTUNE are both visible in the night sky before midnight.

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	Aldebaran 0.8 deg S. of the Moon occultation from most of central and north Asia, north Greenland, and northwest North America. This month's full moon is also called the Cold Moon, that is when the days are the shortest and the temperatures drop in the northern hemisphere where the full moon names come from. Because this moon phase is close to perigee, the lunar disk will appear somewhat larger than average. Check it out!
04	Moon at perigee, expect large tides.
08	Regulus .7 deg S. of Moon, occultation from northeast and central Europe, north Greenland, north Asia, and northern Micronesia.
12	Mercury at perihelion.
13	Mercury at inferior conjunction. Mars 4 deg S. of Moon.
14	Geminid meteor peak. This meteor shower is unusual in that it is associated with the 3200 Phaethonan asteroid that orbits the Sun every 1.4 years. Most showers are associated with periodic comets. The Geminid shower is one of the most prolific meteor shower that may produce about 120 meteors per hour at its peak. Ths year they are best observed after midnight when the waning moon is setting. As always, meteors are best observed at a dark site. Jupiter 4 deg S. of Moon. Vesta .2 deg N. of Moon, occultation from parts of Polynesia, Chile, and Argentina.
19	Moon at apogee.
21	Solstice Saturn in conjunction with the Sun.
22	Ursid meteor peak. This shower generates about 10 meteors per hour.
31	The second occultation this month for the first magnitude star Aldebaran. It will be 0.8 deg S. of the Moon occultation fot from most of North America, Greenland, and Europe, except in south and west Russia.

Lunar Almanac for December 2017

Phases of the Moon	Phase and Date(s)	Best viewed before local midnight
	New 18	Deep Space Objects
	1st. Qtr 26	Planets & Moon
	Full 03	Moon
	Last Qtr 10	Deep Space & Planets

Topic of the month: Vesta

Asteroids are small, rocky objects that orbit the sun. These small objects are believed to be material left over from the formation of the solar system. One of the largest asteroids is Vesta, named after the Roman virgin goddess of home and hearth. The asteroid was noted this month in the daily almanac as it will be occulted from some locations on Earth on December 14. This is the second occultation in a series of 4, occuring for the next few months. Unfortunately, the occultations will be visible in far southern locations.

Today, Vesta is considered a minor planet. It was discovered on March 29, 1807 by Heinrich Olbers and became the fourth known asteroid referred to as 4 Vesta. It is the brightest asteroid and also the easiest to observe. The largest asteroid and the first detected by observers on Earth is known as Ceres. That body was large enough that it was pulled it into its spherical shape by its own gravity. Ceres is classified today as dwarf planet, as is Pluto.

Asteroids are believe to orbit the Sun in a area of the solar system called the asteroid belt, a ring of widely spaced objects orbiting around the Sun. The main asteroid belt orbits between Mars and Jupiter, about 2.3 to 3.3 A.U. from the Sun. It is composed mostly of small bodies made up of rocky and metallic minerals. They range in size from hundreds of kilometers in diameter to microscopic dust particles. Asteroids are classed in groups and families based on their orbital characteristics. Some have "moons", smaller asteroids that orbit larger asteroids.

As members of the solar system, the orbit of these small bodies can be affected by planets due to gravitation. The planet Jupiter being a principle player. But as Asteroids drift into the inner solar system Mars, Earth, Venus and even Mercury can change an asteroid's orbit. Asteroids that have been attracted by Earth and cross its orbit are called "Near Earth Objects". Some asteroids are monitored because they come close to Earth, these are called "Potentional Hazardous Objects" or "PHAs". Find out more about these objects on Astra's Near Earth Asteroids page.

Asteroid Vesta mosaic from Dawn Mission space probe Vesta is 530 km in diameter and the second most massive object in the belt. It makes up about 9% of the total mass of the asteroid belt. Luckily this massive object is far enough away from our planet that the Earth is safe. Vesta and Ceres were explored by the Dawn space craft. The Dawn mission studied Vesta from July 2011 to September 2012. The image posted here was created by integrating many images from the Dawn probe. The Dawn space probe has been orbitting Ceres since March of 2015. In October of 2017, NASA was given permssion to extend the Dawn Mission. The space probe may end its days in orbit around the asteroid long after communications with the probe are not possible.

The asteroid Vesta is almost a dwarf planet, it is nearly spherical and the materials making it have differentiated into crust, mantle, and core similar to Earth. Key features on the asteroid are 3 overlapping craters Minucia (21.5 km), Calpurnia (50 km) and Marcia (58 km). Seen in the upper left section of the image, these three craters are affectionatley know as the "Snowman." Vesta has a bulge around its equator, but its southern pole has been blasted by a collision with another object. The huge impact basin is 500 km in diameter. Its central peak rises 15 km above the mean surface, almost as high as Olympus Mons. Occuring about a billion years ago, Vesta lost some 1% of its mass due to the impact. Fragments spewing out from this event have traveled all the way to earth. They have been identified as HED (Howardite-Eucrite-Diogenite) meteorites.

The magnitude of Vesta is variable, the best opposition tops out at 5.4 but a good opposition would be 6.2 or 6.3, just beyond the limit for naked eye observers. A telescope is required to detect the asteroid.

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