SKY HAPPENINGS

The Sky for July 2012
By Faylene Roth

DAYLENGTH
The days get shorter now that we are past the summer solstice, but not at an even monthly rate. We lost three minutes of daylight from the solstice to the end of June. The month of July sheds an additional 39 minutes, August drops 66 minutes, September dispenses with 71 minutes, and October withdraws an impressive 72 minutes. The rate of loss of daylight then slows. November loses only 51 minutes, while the period of daylight from December 1 to the December 21 winter solstice decreases by a mere 12 minutes more before the reversal begins.

Note the lingering effect of summer twilight. Twilight occurs in three stages. Civil twilight marks the period right after sunset when ambient light remains adequate for most outdoor activities. Nautical twilight begins as color and shapes disappear from the landscape. Astronomical twilight continues as the overhead skies darken and residual light on the western horizon fades to black. In summer the western horizon does not darken until well after 10:00pm.

APHELION
At 10:00pm on July 4 the earth reaches aphelion—the farthest point from the sun in its elliptical orbit. The earth’s speed slows by about 0.6 miles per second (1km/sec) at the far end of its orbit which makes the northern hemisphere’s summer season about five days longer than its winter season. The three percent difference in distance between aphelion and perihelion (nearest point in orbit which occurs in January) isn’t enough to cause seasonal changes. Earth’s tilted axis claims responsibility for that.

METEOR EVENTS
The last week of July offers meteor activity from the radiants of Capricorn and Aquarius. Both constellations will be low in the southern sky after midnight. Best viewing is after moonset July 27-31. Up to 15 meteors per hour are common from the Aquarids Shower and 20 can be expected from the Capricornid Shower, which often includes bright yellow fireballs.

THE MILKY WAY
Summer is the best time to enjoy the Milky Way. Trace it from north to south through Cassiopeia, Cygnus, and Aquila. Then look into the center of our galaxy as your eyes follow the Milky Way to the southern horizon. The dense nebula of stars and star dust spreads across the southern sky through Sagittarius and Scorpius. When you gaze into the western region of Sagittarius, you are looking into the dense bulging center of our galaxy, nearly 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.

MOON HAPPENINGS
July 3 – Full Moon occurs at 12:52pm and rises at 8:44pm.
July 10 – Last Quarter Moon rises soon after midnight.
July 18 – New Moon occurs at 10:24pm.
July 26 – First Quarter Moon sets about one and one-half hours after midnight.

(The time of moonrise and moonset assumes a flat horizon. Actual time may vary.)

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 fist with the thumb extended at a right angle equals 15 degrees. The hand stretched from thumb to little finger approximates 20-25 degrees. The diameter of both the full moon and the sun spans only 0.5 degree. Adjust for the size of your hand.

Primary Sources: USGS; U.S. Naval Observatory;
Your Sky at http://www.fourmilab.ch/yoursky/

To find out when the space shuttle and International Space Station are visible
from your location, go to: http://spaceflight.nasa.gov/realdata/sightings/index.html and click on Sighting Opportunities.

VISIBLE PLANETS
Jupiter – Rises in the eastern sky about one hour before astronomical twilight brightens the morning sky. Jupiter appears on the eastern horizon about three minutes earlier each day, so by month’s end look for it between 2:00am and 3:00am depending upon your viewshed. On the morning of July 14 a cluster of stellar objects appears in the east around 4:00am: the Pleiades, a waning crescent moon, Jupiter (about 0.5 degree south of the moon), Aldebaran (Taurus’ bright orange star), with Venus on the horizon. (Magnitude -2.0)

Mars - Moves from Leo to Virgo this month. Find its small red glow low on the western horizon at evening twilight. Mars sets by midnight from most viewsheds. On July 24 it appears 4.2 degrees north of the waxing crescent moon. (Magnitude +1.0)

Mercury – Reaches its greatest eastern elongation on July 1, so Mercury will be fairly easy to see during the first few days of the month. Look for a bright object below Leo (in faint Cancer) in the WNW sky during nautical twilight. Binoculars can be helpful. (Magnitude +0.6)

Saturn - Low in the western sky and setting soon after midnight. 0 magnitude Saturn outshines 1st magnitude Spica (both in Virgo). On July 25 Saturn is six degrees north of the moon. (Magnitude +0.6)

Venus – Rising with Taurus and the Pleiades in the morning sky about one-half hour to one hour after Jupiter. On July 9 Venus appears almost one degree north of red star Aldebaran (Taurus). On July 15 it is 3.6 degrees south of the moon with Aldebaran to the right of the moon and Jupiter above.. (Magnitude -4.4)


Note: Apparent magnitude values range from -4 to +6 for most planets and visible stars. The lower the value the brighter the object. A decrease of 1.0 magnitude is 2.5 times brighter.

Dominant feature of the summer sky—the Summer Triangle—is formed by 0 magnitude Vega (Lyra), and 1st magnitude stars Deneb (Cygnus) and Altair (Aquila). Cygnus the Swan appears to fly south through the Milky Way, its long neck pointed toward the center of our galaxy in the region of Sagittarius. The squarish body of Hercules stands out between Vega and Arcturus (Bootes).

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 sky from astronomical twilight to midnight. As the night and month progresses, the constellations shift toward the northwest. The celestial equator is measured in hours (h).
The ecliptic is measured in degrees.