NIGHT SKY HAPPENINGS

The Sky for July 2017
By Faylene Roth

The Milky Way dominates the July night sky. In late evening twilight it forms a north-to-south arc east of the meridian (overhead) stretching from Cassiopeia to Sagittarius. Gaze into Sagittarius and imagine the black hole at the galaxy’s center some 26,000 light years away. Later each night, the Earth’s counterclockwise rotation changes the view for us earthbound stargazers which makes Sagittarius and Scorpius drift across the meridian into the southwestern sky. The same thing occurs as the month progresses and the Earth’s counterclockwise revolution around the sun leaves the galaxy’s center in our wake. Look for Hercules directly overhead during most of July. By month’s end Lyra has claimed that space.

Lyra’s bright 0-magnitude star Vega shines at the edge of the Milky Way. It forms a triangle with 1st magnitude Deneb (Cygnus) and 0-magnitude Altair (Aquila) to form the asterism called The Summer Triangle which fills the overhead portion of the Milky Way. The Big Dipper (an asterism in Ursa Major) hangs high in the western sky dangling downward from its handle. Follow the arc of the handle to red-star Arcturus (Boötes) in the western sky. Towards midnight watch for the appearance of Pegasus over the eastern horizon with both the constellation and the galaxy of Andromeda trailing from its northeastern corner. Capricornus follows Sagittarius into the southeastern sky.


Twilight extends the period of daylight in three stages at each end of the day. Morning twilight begins with astronomical twilight—about 1-1/2 hours (nearly 2 during summer months) before sunrise—as the eastern horizon brightens. Nautical twilight continues—as the overhead sky turns blue and color returns to the surrounding landscape—for another 30-40 minutes. The final stage—civil twilight—provides adequate light for most outdoor activities for the half hour before the sun crests the horizon. The opposite progression occurs after sunset.

VISIBLE PLANETS
Evening (Before Midnight)
Jupiter –The brightest planet in tonight’s sky can be found high in the western sky at twilight about 12 ̊ northwest of 1st magnitude blue-star Spica in Virgo. Jupiter sets before midnight by month’s end. (Magnitude -2.0)
Saturn – Look for Saturn below Ophiucus the Serpent Bearer and between Sagittarius and Scorpius. Its yellow orb contrasts brilliantly with red-star Antares about 15 ̊ west in Scorpius. Both are directly south around midnight and set several hours later. (Magnitude +1.1)

Morning (At Twilight)
Venus – The “morning star” rises in Taurus about 3:30 am, an hour before astronomical twilight brightens the eastern sky. (Magnitude -4.0)

MOON HAPPENINGS
July 1 – Waxing first quarter moon lights the evening sky then sets soon after midnight.
July 8 – Full moon (10:07pm) rises at 8:24pm.
July 16 – Dark evening skies return with the waning last quarter moon rising after midnight.
July 23 – New moon (3:46am) yields dark skies for several nights.
July 30 – Waxing first quarter moon lights the evening sky then sets soon after midnight.

(The moon rises later each day—as little as 30 minutes to as much as one hour. Time of moonrise and moonset may also be delayed in mountainous terrain.)

Twilight is often the best time to look for Venus and Mercury because they frequently rise or set within one-half to one hour of sunrise or sunset. Twilight transitions between night and day in three stages at each end of the day. Morning twilight begins with astronomical twilight as the eastern horizon brightens —about 1-1/2 hours (nearly 2 during summer months) before sunrise when the sun is 18 ̊ below the horizon. Nautical twilight takes over for another 30-40 minutes—as the sun passes 12 ̊ below the horizon and the overhead sky turns blue and color returns to the surrounding landscape. The final stage—civil twilight—begins when the sun ascends to 6 ̊ below the horizon and provides adequate light for most outdoor activities for the half hour before the sun crests the horizon. The opposite progression occurs after sunset. Civil twilight covers the period after sunset during which daytime light quality persists for about one-half hour. Color then fades from the landscape during the 30-40 minute period of nautical twilight during which the overhead sky darkens while the western sky retains color. Astronomical twilight then transitions to night skies that are now darkened along the horizon.

NO MAJOR METEOR EVENTS IN JULY
Minor meteor showers from radiants in Lyra, Pisces, Capricornus, and Draco can add up to 20+ meteors per hour during the last half of July. Fireballs are common. The new moon period beginning around July 20 will provide dark skies between midnight and morning twilight when best viewing occurs.

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.

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

Note: Hold your hand at arm’s length to measure apparent distances in the sky. The width of the little finger approximates 1.5̊. Middle, ring, and little finger touching represent about 5̊. The width of a fist is about 10̊. The fist with the thumb extended at a right angle equals 15̊. The hand stretched from thumb to little finger approximates 20̊-25̊. The diameter of both the full moon and the sun spans only 0.5̊. Adjust for the size of your hand.