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Sky Happenings January 2008
Moab UT (at City Hall)
38O34’ N Latitude 109O33’ W Longitude
4048 ft - 1234 m
The Sky for January 2008
By Faylene Roth
Sunrise and Sunset Times for
January 2008

DAYLENGHT
The length of daylight has increased a few minutes since the winter solstice, but the time of sunrise does not change during the first eleven days of January. Since the declination (angle) of the sun above the horizon determines the amount of daylight, sunrise should occur earlier after the winter solstice just as sunset occurs later. However, the Earth’s distance from the sun influences the time of sunrise. On January 2, Earth reaches perihelion, its closest point to the sun. If the Earth’s orbit were a perfect circle, a solar day (not always 24 hours like a clock day) would equal one rotation of the Earth’s axis. Since the orbit is an ellipse, the sun’s gravitational force causes the Earth to move faster as it approaches perihelion. The result is that the sun is not directly overhead after a rotation of 360 degrees. Instead, the sun is east of the meridian (point directly overhead) because the Earth has moved farther along in its orbit which changes its position relative to the sun. Between November and February each solar day is longer because of the extra time required for the sun to reach the meridian. Noon arrives a few seconds later each day. A late noon delays sunset. The effect was noticed when the sun began to set later after December 13, before the winter solstice. Delayed sunsets also delay the following sunrise which explains why the sun is not rising earlier during the first few weeks of January. By January 12 the declination of the sun is high enough in the sky to overcome the effect of a longer solar day.

MOON HAPPENINGS

January begins with a waning moon high in the morning sky at sunrise. Just before dawn on January 4 look for a crescent moon forming an isosceles triangle with Venus and Antares, a 1st magnitude star in Scorpius. New Moon occurs on January 8 at 4:37am. On January 17 the moon grazes the star cluster Pleiades just before midnight. On January 19 a waxing gibbous moon hovers above Mars. A nearly full moon rises on January 21 at 4:36 pm. Full Moon occurs in the morning twilight of January 22 at 6:35am. On January 24 the moon and Saturn both rise shortly after 8:00pm with Regulus, a 1st magnitude star in Leo just below. Best viewing will be soon after 9:00pm.

MAGNITUDE OF THE STARS
Astronomers use the term magnitude to measure the brightness of stars. Absolute magnitude measures actual differences in size and intensity of a star’s light if all stars were the same distance from Earth. Apparent magnitude, more useful to stargazers, ranks a star’s brightness based on what we actually see. A small star near Earth appears brighter than a large more distant star. The original magnitude scale had six ranks. The brightest stars were ranked 1st magnitude. Magnitude 1 stars are 2.5 times brighter than magnitude 2 stars; magnitude 2 stars 2.5 times brighter than magnitude 3 stars; etc. A magnitude 1 star is 100 times brighter than a magnitude 6 star, which is the faintest level that can be viewed without magnification. Well-marked constellations, like the Big Dipper, Cassiopeia, and Orion, are defined by 1st and 2nd magnitude stars. Fainter constellations, like Pisces and Cancer, contain only 3rd or 4th magnitude stars. The current magnitude scale has been extended to -26.8 to accommodate the brightness of the sun and to +27 to accommodate the faintest objects seen with the largest telescopes. Some of the brighter stars like Rigel, Capella, and Vega are now designated magnitude 0. Sirius, the brightest of all stars, is -1.6; Venus ranges from -3.9 to -4.7.

METEOR SHOWERS

Look low in the northeastern skies for meteors from the Quadrantid Meteor Shower during the first week of January. The Quandrantids, renown for their abundant though faint meteors, peak on the night of January 3 between midnight and early twilight on the morning of January 4.


COMET VIEWING

Be sure to check the progress of Comet 17P/Holmes as it passes through Perseus during December. Its outburst of dust and ice on October 24, 2007, increased its brightness by a million times in just a few days. By mid-November the diameter of the dust and ice cloud forming its coma was larger than the diameter of the sun. Comet Holmes went through a similar outburst and increase in brightness in 1892 when it was first discovered. Normally it is no brighter than the planet Pluto. The comet is fading in brilliance as it expands, but it could repeat its performance of 1892 when the first outburst was followed by another a few months later. Comet Holmes has an orbital period of seven years. Another Comet, 8P/Tuttle, is due to rendezvous with the sun around the beginning of the new year. Look for it as it passes through Cassiopeia and Andromeda during December. Its brilliance is similar to that of a dim star when viewed with the unaided eye. Comet Tuttle, with a period of 13.6 years, is the source of the Ursid Meteor Shower.

VISIBLE PLANETS
Jupiter - returns mid-January shining at magnitude -1.9 below Venus in the morning sky; on January 31 both Venus and Jupiter rise within a few minutes of 5:30am.
Mars - centered amongst four of the brightest stars in the night sky—Capella to the north, Betelgeuse to the south, Pollux to the east, and Aldebaran to the west; its reddish light shines brighter (magnitude -1.5) than any star in the night sky other than Sirius (bluish star to the southeast at magnitude -1.6); by month’s end, Mars loses half its apparent brilliance as it moves farther from Earth.
Mercury - visible in the southwestern sky from a high plateau about 40 minutes after sunset January 12-28; on the 9th look for it near the crescent moon.
Saturn - rises mid-evening at magnitude 0.7 with the constellation Leo; still visible in the morning sky to the west.
Venus - bright morning star at magnitude -4; rises around 4:30am on January 1, then about one minute later each day, rising around 5:30am by the end of the month; Venus can sometimes be spotted during the day; note its location relative to the sun at sunrise, then look for a white spot in the blue sky to the west of the sun later in the day.
Uranus - shining at magnitude 5.9, its blue-green tint is barely perceptible to the unaided eye in dark sky; look for it in Aquarius..

COMET VIEWING
Comet Holmes remains in Perseus throughout January. Its brilliance has diminished as its coma (dust and ice surrounding its solid core) expands. Comet Tuttle makes its closest approach to Earth on January 2. Look overhead between Aries and Pisces. Tuttle, as dim as a faint star, moves quickly through the night sky because it is so near the Earth, reaching the southern horizon by the end of January. Use binoculars for best viewing of both comets.

 

Winter skies feature more bright stars than other seasons. Sirius in Canis Major, Rigel and Betelgeuse in
Orion, Capella in Auriga, Vega in Lyra,
Procyon in Canis Minor, Pollux in
Gemini, Aldebaran in Taurus,
Deneb in Cygnus.

Hold the star chart high above your head and
match the compass directions to the
direction you are facing. Adjust
the star chart by orienting
Ursa Major (Big
Dipper) to match
its position
in the sky
.

The star chart approximates the night sky from astronomical twilight to midnight. As the night and the month progress, the constellations will shift toward the northwest.



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