12/31/2008
January Skies - by Dick Cookman
Highlights: Comet Journal, Martian Landers, Planet Plotting, Orbital Puzzles, Meteors, January Moon
Focus Constellations: Ursa Major, Draco, Ursa Minor, Cepheus, Cassiopeia, Andromeda, Perseus, Auriga, Taurus, Gemini
Comet Journal
Comet C/2007 N3 (Lulin) rises in Libra before dawn. It may be barely visible to the naked eye at 6th magnitude and should be easily seen in binoculars. It has a retrograde orbit and passes between the orbits of Earth and Mars in the first quarter of 2009. Current projections are that it may reach 4th magnitude after it passes perihelion on January 15th and reaches closest proximity to Earth on February 24th.
Comet 210P/2008 X4 (Christensen) was rediscovered in December and has brightened to 8th magnitude. In January it circuits east to west along the base of Ophiuchus in the southeast before dawn. It may reach 7th magnitude in early January.
Martian Landers
Preliminary conclusions from data gathered before the Phoenix lander ceased communications indicate that soil samples were composed of wind deposited sand and silt weakly held together in small clumps. These clumps may have been cemented by adhesion of interstitial water molecules or minerals deposited from soil water during times of warmer summers when a higher axial tilt produced more direct rays of solar energy in Martian polar regions. The Moon's gravitational attraction suppresses Earth's variation of axial tilt to 21.5 to 24.5° over a 40,000 year cycle. The absence of a large moon permits much larger variation for Mars.
Ray Arvidson of Washington University in St. Louis, is the lead scientist for the lander’s robotic arm. In view of the above variation and its impact on Martian summer temperatures, he states “There have probably been dozens of times in the past 10 million years when thin films of water were active in the soil, and probably there will be dozens more times in the next 10 million years.”
Opportunity survived the solar conjunction when Earth and Mars were on opposite sides of the Sun. Radio silence ensued on Nov. 30th and ended on Dec. 14th due to the solar conjunction which precluded communication and interrupted the trek toward Endeavour Crater. The rover monitored atmospheric readings and analyzed a small rock during the interval.
Spirit’s solar panels produced 161 watt hours of energy on November 14th (Sol 1730), significantly more than the 89 watt hours produced 4 days earlier during a dust storm. Dust buildup on the panels limited light received by the photovoltaic cells to 30% of that received by the panel array, down from 33% before the storm.
“Spirit is not out of the woods yet,” said Spirit's project manager, John Callas of NASA’s Jet Propulsion Laboratory in Pasadena. “The storm and all its dust have not gone away completely. And this is the time of the Martian year when storms like this can occur..." Planners were conservative and limited activity to recharging Spirit's batteries until the storm ended completely. The rover was instructed to keep some heaters turned off and to conduct only limited observations and communications between Sols 1730 and 1734. Similar low-power sequences then continued during the rest of the month.
The rover team initially planned to delay moving Spirit from its location at the north end of “Home Plate” plateau until the end of the solar conjunction which suspended commands from Earth. The storm reinforced that decision.
Planet Plotting
Mercury (-0.7 to +0.7) is visible after sunset during the first part of January. It passes through inferior conjunction on the 20th when it is between the Earth and Sun and is visible before sunrise at month's end. During early January it will rapidly retreat from its New Year's Eve visit of Jupiter (-1.9) in Sagittarius as it ascends to maximum eastern elongation (19°) from the Sun on Jan. 4th. It will then follow the giant planet as both descend toward the horizon and disappear into dusk in the second week of January. Dim Neptune (7.9) in Capricornus also descends into dusk in mid January and Mercury reappears in predawn skies on the 29th when it rises with Capricornus.
In January Venus (-4.5 to -4.7) tours the western sky from Capricornus to Pisces after dusk. During the early part of the month it oversees Mercury and Jupiter frolicking above the horizon. It reaches greatest eastern elongation (47°) on the 14th and will visit within 1.5° of Uranus (+5.9) on the 22nd.
Saturn (+1.0 to +0.7) rises during the wee hours between Leo and Virgo in the southeastern sky. Its rings are inclined by only 1° relative to Earth and are barely visible. Distant Mars (+1.3) in Sagittarius reached superior conjunction on the other side of the Sun on Dec. 5th and is not visible until latest January when fortunate observers may find it adrift in the glow of sunrise below Mercury.
Orbital Puzzles
January highlights include two events which surprise those unfamiliar with astronomy and the night sky. The first occurs at 7 PM EST on January 3rd when Earth makes its closest approach to the Sun. The perihelion Sun is 91.5 million miles away, over 5% closer than on July 3rd when the aphelion Sun is farthest from Earth. The northern hemisphere tilts away from the winter Sun and toward it in the summer. Currently both seasons are moderated because a nearer perihelion Sun increases northern hemisphere winter temperatures and a more distant aphelion Sun cools summers.
The second event is on January 4, 2009 when the latest sunrise occurs at 40° N. latitude. The latest sunrise does not coincide with the shortest day which was the winter solstice on Dec. 21, 2008. Neither coincide with the earliest sunset which was on Dec. 7, 2008. For people who experience sunset more frequently than sunrise, Dec. 7th to be the shortest day and day length seems to increase slowly from the 7th to the 21st even though it is really decreasing. From the 22nd to Jan. 4th day length for sunset aficionados seems to increase more rapidly than it really does. These variations of apparent day length arise because our clocks measure mean solar time, not apparent solar time (sundial time) and clocks run slower on Dec. 7th than on Dec. 21st. Earth travels faster in its orbit at perihelion thus requiring more than one rotation to return the same geographic location to its solar noon position. As a result mean solar time runs faster than solar time.
Clocks run fastest on Feb. 11 and fast on July 26. They run slowest on November 3rd and slow on March 13th. Solar time and clock time coincide on April 15th, June 13th, September 1st, and December 25th. The deviation from a more direct relationship with Earth's orbital velocity results from solstices and perihelion/aphelion occurring in different locations in Earth's orbit.
Meteors
The Quadrantid meteor shower in the morning twilight of January 3rd has a short lived maximum which occasionally peaks at over 100 meteors per hour. The 1st quarter Moon sets about midnight providing dark skies for this brief but intense shower. The shower appears to emanate from a radiant in a part of the sky between Bootes, Ursa Major, Draco, and Hercules formerly known as the constellation Quadrans Muralis. At dawn it is about 30° above the northeastern horizon.
Watch for a second possible peak in the same area of the sky on the evening of the 3rd and for a minor shower between the 20th and 27th of January in the area of Leo and Coma Berenices.
January Moon
January's Full Moon occurs on the 10th at 10:27 PM EST. The Moon is closest to Earth and is at "perigee" almost 17 hours before Full Moon. As a result, the Moon will appear larger than normal and will be almost as large as it was in December when perigee was slightly more than 5 hours after Full Moon. Residents adjacent to ocean shorelines will also experience higher than normal tides due to the proximity of the Moon.
The Full Moon of January is called the "Old Moon" or the "Moon after Yule". Colonial Americans called it the "Winter Moon" and medieval English referred to it as the "Wolf Moon". Chinese know it as the "Holiday Moon" and to Celts it was the "Quiet Moon". Anishnaabe (Chippewa and Ojibwe) call it "Gichi-manidoo-giizis" (Great Spirit Moon).
The waxing crescent Moon is 15° above Venus on the evening of the 1st and does not move near another planet until midmonth when the waning gibbous Moon is about 6° SSW of Saturn in the predawn sky. During the 30 hours prior to New Moon at 2:55 AM EST on the 26th, the waning crescent Moon passes within 1° of Mars, 5° of Mercury, and within 1° of Jupiter. Unfortunately none of these can be seen very well from North America. At the end of January, the waxing crescent Moon again highlights Venus in the evening sky.
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