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

DAYLENGTH
The period of daylight begins to decrease in July. Sunrise is 22 minutes later and sunset 16 minutes earlier by the end of the month. Sunrise and sunset are calculated for a flat horizon. Actual times may vary by one-half hour or more depending upon the terrain. With the sun still high in the northern sky, astronomical twilight illuminates the eastern sky nearly two hours before sunrise. Nautical twilight begins about one hour before sunrise as shapes and color appear in the landscape. Civil twilight begins approximately one-half hour before sunrise as details become clearly visible. The reverse progression applies to dusk, with astronomical twilight lingering until after 10:00pm throughout July.

MOON HAPPENINGS
July begins with a fading moon that becomes a new moon on July 2. On July 5 a slivered crescent moon reappears low in the western night sky in a celestial chain with Regulus (Leo’s brightest star), Mars, and Saturn. Find a high vantage point by 10:00pm with a long view of the western horizon for viewing this event. On July 6 the moon rises about one-half hour later and hovers 3.5 degrees south of Saturn, Mars, and Regulus. On July 9 a first quarter moon appears in Virgo near its bright star Spica. On July 13 the rising gibbous moon is found near Scorpio’s bright star Antares. On July 16 look to the east as Jupiter trails within 2.6 degrees of a nearly full moon which rises soon after 8:00pm. On July 17 a nearly full moon lags behind Jupiter in the eastern sky. The moon becomes full two hours after midnight. On July 18 a slightly waning moon rises at 9:11pm. At month’s end, look high in the eastern sky in the early morning twilight of July 27 to see the waning crescent moon paired with the Pleidaes star cluster. By July 29, the waning crescent moon disappears into the morning twilight and sets well before sunset. As with sunrise, actual times may vary depending upon the landscape.

METEOR SHOWERS
Look for meteor activity from the Delta Aquarids shower from July 12 to August 19. This shower peaks on the night of July 27. It can produce up to 20 meteors per hour. Best viewing time for meteor showers is around 3:00am. The radiant point for these meteors is the constellation Aquarius which will rise soon after midnight and be overhead by 3:00am. A faint crescent moon, rising about one hour after Aquarius, should cause minimal interference.

APHELION
Earth reaches its farthest distance from the sun at 2:00am on July 4. It is a coincidence that the earth reaches this point, called aphelion, so soon after the summer solstice. Three major cycles affect the position of Earth relative to the sun. Aphelion and perihelion mark the far and near distances from the sun during the earth’s annual orbit about the sun. During its 95,000 year cycle of eccentricity, the earth’s orbit changes from slightly elliptical to nearly circular and back. Another cycle, which lasts about 42,000 years, tracks the change in obliquity of the earth’s axis to the plane of its orbit. The earth’s axis is currently tilted at an angle of 23.45 degrees, but it can vary between 22.1 and 24.5 degrees. Because of this angle, the northern hemisphere tilts towards or away from the sun at different times of the year which is responsible for our changing seasons. The precession of the equinoxes maps the change in orientation of the north pole in space. Over a 21,000 year cycle the direction of the north pole makes a circular rotation from Polaris to Vega (in Lyra) and back. Precession changes the time of year that the seasons occur. For information on how these cycles may interact to affect Earth’s climate, read about the Milankovitch cycles.

MILKY WAY
The Milky Way stretches north to south across the summer sky. Follow this dense cloud of stars from Cassiopeia in the northeastern sky through the long neck of Cygnus (the swan) all the way to Sagittarius in the southern sky. The Milky Way is the flattened spiral galaxy in which we reside. The sun and our companion planets are situated about two-thirds out one of the spiral arms of the galaxy. When you gaze into the western region of Sagittarius, you are looking into the dense bulging center of our galaxy, about 30,000 light years distant. A very dense mass in the center, referred to as Sagittarius A*, is most likely a supermassive black hole. As the night and the month progress, Sagittarius and the Milky Way sweep across the southern sky from east to west.

Note: Hold your hand at arm’s length to measure apparent distances in the sky. The width of the little finger approximates 1.5 degrees. Middle, ring, and little finger touching represent about 5 degrees. The width of a fist is about 10 degrees. The hand stretched from thumb to little finger equals 20 degrees. The diameter of both the full moon and the sun spans only 0.5 degree.

VISIBLE PLANETS
Jupiter - visible from dusk til dawn during most of July; rises with Sagittarius; shines at minus 2.7 magnitude, its brightest for the year; reaches opposition on the night of July 9, when it is directly opposite the sun from our viewpoint on Earth; is closest to Earth on the night of July 10.

Mars - moves closer to Saturn during the first 10 days of July; on the night of July 9, after midnight, Mars appears to be less than 1° from Saturn; appears farther east from Saturn after July 9 as it rises a few minutes later each night.

Saturn - shining at magnitude 0.75, its yellow hue contrasts with the bright red-orange light of Mars (magnitude 1.68) as both separate themselves from the twinkling yellow-white light of Regulus (magnitude 1.4), the star at the heart of Leo.

Venus
- returns to the night sky after two months in its orbit behind the sun when its light is lost in the sun’s glare; reappears, at magnitude minus 3.9, low on the western horizon soon after nautical twilight during the last week of July; west of Leo.

Note: Apparent magnitude values range from -4 to +5 for most planets and visible stars. The lower the value the brighter the object.

MAJOR CONSTELLATIONS OF FEBRUARY

Overhead
Hercules
Lyra
Ophiucus

Northward
Cassiopeia
Cepheus
Draco
Ursa Major
Ursa Minor

Eastward
Aquarius
Aquila
Capricornus
Cygnus
Pegasus

Southward
Libra
Sagittarius
Scorpius

Westward
Bootes
Corona Borealis
Virgo

High in the eastern sky is the Summer Triangle formed by Vega (Lyra), Deneb (Cygnus), and Altair (Aquila). To find Hercules draw a line from Arcturus (Bootes) to Vega. The squarish body of Hercules is easily seen to the west of Vega.

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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