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Sky Happenings September 2007
Moab UT (at City Hall)
38O34’ N Latitude 109O33’ W Longitude
4048 ft - 1234 m
The Sky for September 2007
By Faylene Roth

Day length shortens by one hour and five minutes this month. The times for sunrise and sunset are calculated for a flat horizon, so actual times may vary depending upon the surrounding landscape. Twilight progresses in three stages. Civil twilight occurs approximately one-half hour before official sunrise; you can easily function without artificial light. Nautical twilight, about one hour before sunrise, may require artificial light to assist movement. Astronomical twilight occurs about one and one-half hours before sunrise. A faint glow appears above the horizon. The same progression applies to dusk.

The shortened days and waning moon of early September provide excellent conditions for star gazing through the middle of the month. On September 3 a third quarter moon rises with the Pleiades in the eastern sky at 11:32pm. New Moon occurs September 11. A waxing crescent returns to the western night sky September 15 to the west of Libra. The moon can be a guide for locating many constellations. On September 17 find it near Scorpius; on the 19th it begins to move through Sagittarius; on the 22nd it is in the center of Capricornus; on the 24th in Aquarius; on the 27th in the middle of Pisces; on the 28th in Aries; and on the 30th in Taurus. Full Moon occurs September 26.

An imaginary line called the ecliptic traces the path of the sun across the sky relative to the background stars. Earth’s orbit around the sun creates the seasons because the equator is tilted from the plane of the ecliptic at an angle of 23.5 degrees. An extension of Earth’s equator into the celestial sphere creates the celestial equator. The fall equinox occurs at the point in the earth’s orbit where the plane of the ecliptic intersects the celestial equator. Neither the northern hemisphere nor the southern hemisphere tilts towards the sun. As a result, the direct rays of the sun fall perpendicular to the equator on this day. Sunrise will be due east and sunset will be due west. The autumnal equinox occurs September 23 at 3:51am MDT. The length of day and night should be equal, but according to the sunrise/sunset table, daylength is seven minutes longer on September 23. This occurs because the atmosphere refracts sunlight around the curvature of the Earth. At sunrise we see the sun before it reaches a horizontal plane with where we are standing. At sunset we continue to see the sun after it dips below the horizon. On September 26 the sunrise/sunset table does show equal periods of day and night. By this time, the days are actually shorter than the nights.

Colliding Galaxies

Astronomers are currently watching four galaxies collide in the region of Ursa Major. Three are elliptical galaxies of approximately the same size as our own Milky Way; the fourth is ten times larger. The collision, as seen today, occurred five billion years ago. In real time, the event would be long over. Two other colliding galaxies are being watched in the southern sky in the region of Capricornus. These two galaxies are rich in gases which are spawning new stars. In another five billion years the Milky Way Galaxy is expected to collide with the Andromeda Galaxy. The gravitational forces that interact in these collisions sling many stars into intergalactic space. Some of these stars will be pulled back into new positions within the galaxy, while others will remain as isolated stars in intergalactic space.

Jupiter - the only planet visible in the evening
sky; located in the southwest near Antares
in Scorpius, easily identified by its brightness.
Mars - visible between midnight and sunrise in the eastern sky;
near Taurus in early September and in Gemini by the end of the month.
Saturn - rises in the eastern sky each morning following Venus by one-half to one hour; located in Leo near the bright star Regulus; in opposition (opposite side of the earth from the sun) to Earth on September 9, which marks its closest point to Earth.
Uranus - best appearance in early September; barely visible as a blue-green speck in Aquarius.
Venus - rises to the east in the early morning hours before Saturn; reaches maximum brightness as this year’s morning star on September 12; binoculars reveal its crescent phase.

Ursa Major
Ursa Minor


Corona Borealis
The Andromeda Galaxy can be seen as a faint blur in the constellation Andromeda. Look for the slightly bent line
of four stars to the south of Cassiopeia. Find the
second star from the north end, then follow the
two stars that form a right angle with it. The
galaxy is adjacent to the last star.

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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