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December Skies (2007)


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December (2007) Skies

Highlights: Comet Journal, Martian Landers, Planet Plotting, Meteors, Winter Solstice, December Moon
Focus Constellations: Pegasus, Andromeda, Perseus, Cassiopeia, Cepheus, Draco, Ursa Minor, Ursa Major, Gemini, and Auriga


Comet Journal

Comet 17P (Holmes) is best observed in binoculars. After sunset on December 1st, it will be about 4° west of Mirfak in Perseus and will approach 1° in diameter, appearing twice the size of the full moon! When the 2.2 mile diameter comet nucleus exploded in a super—outburst on Oct. 24th, the comet brightened by over 500,000 times. Starlike to the naked eye, through binoculars it appeared as an extended, slightly fuzzy object with a distinctly yellow/orange color from reflected sunlight. In early November, some observers reported a peak magnitude brighter than 2.4 with gaseous jets and a blue fluorescing ionized carbon monoxide tail visibly projecting from the nucleus in CCD images. Since November 18th the comet has not been visible without binoculars.

The cause of the explosion is unknown. The most likely hypothesis yet proposed is that solar heating of substances within the nucleus during passage through perihelion last June resulted in formation of gases in the interior. These may have caused a fragment of the nucleus to break off, disintegrating in a blast which produced about 20% as much dust as was generated by the eruption of Mt. St. Helens in 1980. The expanding central coma of dust and green fluorescing molecular carbon gas was more than 870,000 miles in diameter by November 9th and replaced the Sun as the largest known object in the solar system.

The dispersed dust and gas of the coma was more than 1.86 million miles in diameter by the 21st and the comet appeared as a faint nebulous cloud in Perseus. It will continue to expand and dim unless another explosion occurs similar to the one transpiring in 1892, 75 days after the initial explosion which led to the discovery of Comet 17P (Holmes).

At the beginning of December, Comet 8P/Tuttle will be about 5° southwest of Polaris. Observers estimated it at 11th magnitude on Nov. 22nd and expect it to reach 5th or 6th magnitude when it is closest to Earth at the end of 2007. It will pass Gamma Cepheus on Dec. 10th and continue southward to Andromeda by month's end. In early January, it will pass the celestial equator and disappear from northern hemisphere skies as it approaches perihelion in the southern hemisphere constellation Fornax, near Eridanus on January 27th.

Martian Landers

Upon arrival at Victoria Crater on Sol 993. Opportunity embarked on a clockwise circuit around the crater in search for an access route to the crater. On Sol 1160, after completing about 1/4 of the circuit, mission scientists decided to backtrack to an access near the original point of arrival. The return trip to Duck Bay was completed by Sol 1293 when the rover entered the crater and proceeded to examine the light colored rock layers exposed in the upper part of the wall of Victoria Crater. These layers are thought to represent the original surface of the Meridiani Planum region of Mars before the meteorite impact that excavated the crater. Initially the layers were referred to as alpha, beta, and gamma but were renamed in honor of Nicholas Steno, William Smith and Charles Lyell, the prominent geologists who helped found the science of stratigraphy, the study of rock layers, in the 17th and 18th centuries.

Steno, the uppermost layer, was the first layer examined in detail. On Sol 1327, the rover moved down to Smith where it is continuing with extensive observations as of Sol 1354 (Nov. 15, 2007.) In January, the encoder that determined when the rock abrasion tool contacted the target failed and mission scientists eventually were able to devise a procedure which permitted abrasion to proceed without the encoder. However, the recent failure of a second encoder forced engineers to rely on current limits and contact switches to know when grind teeth come into contact with a rock surface. Unfortunately, during a test of the new mode of operation, Opportunity was mistakenly commanded to rotate the brush in the wrong direction, causing it to bend. This must be mitigated in order to grind deeper into Smith and collect compositional data about the rock using the alpha-particle X-ray spectrometer.

While Spirit examined the boulder field at Site 3a on the Home Plate Plateau from Sol 1334 through Sol 1346 (Oct. 17, 2007,) the rock abrasion tool failed after brushing the surface of a boulder called Humboldt Peak. Since a technique which requires two days for any activities using the rock abrasion tool overcame a similar problem on Opportunity, Spirit was directed to proceed toward Site 4 on the southwestern corner of the plateau.

After moving a distance of 22 meters in two days, the rover diverted to an alternate destination when mission scientists decided that dust on the solar panels will jeopardize adequate supply of the rover's electrical requirements during the coming winter. The dust cut electrical energy to levels below that experienced prior to the onset of winter in 2006. To compensate, the rover must be positioned at a steep angle toward the north so as to receive more solar energy. Spirit was subsequently rerouted to Winter Haven 3 on the north margin of Home Plate Plateau, a site chosen because it provides a 25° northern slope whereas north slopes on the southern part of the plateau are limited to 20°.

On Sol 1362 (Nov. 2, 2007) Spirit embarked on a journey to Winter Haven 3 with an immediate goal of reaching Site 5 to investigate targets with the robotic arm before resuming the northward trip. Upon arrival at Site 5 on Sol 1363 (Nov. 3, 2007) the rover studied a target called Pecan Pie, then imaged El Dorado Dune Field and a mosaic of West Valley View before proceeding with the journey. As Spirit attempted to reach a rock outcrop at Site 7 on Sol 1378 (Nov. 18, 2007), the inoperable right front wheel, which the rover drags behind as it backs across the plateau, hung up on a buried rock forcing a diversion of the rover into a keep-out zone. As of Sol 1383, Spirit was still wallowing around in the sand trying to extract itself so as to continue the trek to Winter Haven 3 before solar panel output drops to levels to low for travel. Through Sol 1380 (Nov. 20, 2007), Spirit’s total odometry was 7,428.01 meters (4.62 miles).

Planet Plotting

Jupiter is lost in the sunset sky in Sagittarius and reaches conjunction with the Sun on the 23rd as does Pluto on December 20th.

Neptune (7.9) in Capricornus and Uranus (m = 5.8) in Aquarius are binocular objects in the south-southwest after sunset. The waxing crescent Moon highlights Neptune on the 13th. On the 16th, Uranus is near the first quarter Moon.

Glorious Mars (m = -1.3 to -1.6) in Gemini rules the December skies and is brighter than any star. It will be brightest when it makes its closest approach to Earth on the 18th, only 58,783,000 miles away. At that time, the disk will span almost 16 arcseconds and will fill about one half of the field of view in a medium power (100X) telescope eyepiece. Mars will be near the Full Moon on Dec. 23rd and reaches opposition on the 24th when it is visible from sunset to sunrise. Because the opposition is in December when Earth is tilted away from the Sun, Mars is higher above the southern horizon than it was during recent oppositions. Surface features may be easier to discern due to less distortion by horizon effects even though the planet is not quite as close nor will appear quite as big as it did during oppositions in 2001, 2003 and 2005. We will have to wait until 2016 before it is again as close.

Saturn (m = 0.7) rises in Leo on Dec. 1st at midnight and at 10 PM EST on the 31st. It is dimming as the inclination of the rings to our line of sight decreases, makes them appear narrower, and reflect less light. The 3rd quarter waning Moon is ~ 3° below Saturn at dawn on the 1st. The waning gibbous Moon passes Saturn at 3 PM EST on the 28th.

Venus (m = -4.2) moves through Libra in December and is brighter than Mars. It rises about 3 AM EST on Dec. 1st and about 5 AM EST on the 31st. The two planets will be magnificent beacons in the eastern and western skies in the hours before dawn. The waning crescent Moon will be within 9° of Venus at dawn on the 4th.

Meteors

The waxing crescent Moon sets early and provides a dark sky to observe the Geminid Meteor Shower on the night of the 13th and 14th. Since the shower peaks during the day on the 14th, it will build throughout the night and may approach 100 meteors per hour in dark skies unpolluted with streetlight, yardlight, and urban/suburban skyglow.

Winter Solstice

The longest night of the year is the night of the 21st-22nd. The solstice occurs at 1:08 AM EST on the 22nd.

December Moon

The December Full Moon is at 8:16 PM EST on the 23th. It is known as the "Moon before Yule" or the "Long Night Moon." The Colonial Americans called it the "Christmas Moon," and the Chinese refer to it as the "Bitter Moon." To the Medieval English it was the "Oak Moon," to the Celts it was the "Cold Moon," and the Anishnaabe (Chippewa and Ojibwe) call it "Manidoo-gizisoons" (Small Spirits Moon.)




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