SKY HAPPENINGS

The Sky for July 2014
By Faylene Roth

The Milky Way trails high overhead from north to south after astronomical twilight darkens the night sky. Our line of sight—southward past Aquila (bright star Altair) and Sagittarius—directs our gaze towards the galaxy’s center. Turn northward towards Cygnus (bright star Deneb) and Cassiopeia to gaze outward towards the edges of the galaxy. Then follow the constellations of the ecliptic (zodiac) from northwest to southeast: Leo (bright star Regulus), Virgo (planet Mars and bright star Spica), Libra (planet Saturn), Scorpius (bright star Antares), Sagittarius, and Capricornus. The bright red star west of Hercules (overhead) is Arcturus (Boötes).

The days are now getting shorter, but the effect is offset by lingering twilight which provides nearly two hours after sunset before the skies completely darken on the western horizon. The period of daylight decreases 39 minutes this month.

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.

APHELION
On July 3 at 6:14pm the Earth reaches aphelion—its farthest point from the sun—in its elliptical orbit. Since the speed of the Earth slows slightly at aphelion, the summer season in the northern hemisphere is about five days longer than its winter season. The three percent difference in distance is not great enough to affect summer temperatures. The tilt of the Earth’s axis claims that responsibility.

MOON HAPPENINGS
July 5 –First Quarter Moon brightens western sky until after midnight.
July 12 –Full Moon occurs at 5:25am, rises at 8:50pm, and lights the sky through the night.
July 19–Dark evening skies until Last Quarter Moon rises after midnight.
July 26 –Dark skies during the New Moon period.
(The time of moonrise and moonset assumes a flat horizon. Actual time may vary.)

VISIBLE PLANETS
Mars – Three red orbs shine in the night sky this month. Arcturus (Boötes), planet Mars, and Antares (Scorpius). Look for Mars in Virgo to the west of its bright star Spica. A waxing quarter moon passes by Mars and Spica July 5-7. By July 13 Mars has shifted to the east of Spica. It sets soon after midnight. (Magnitude -0)

Mercury – Wait until after July 11 to search for Mercury as nautical twilight brightens the morning sky. Find a high vantage point with a clear view of the eastern horizon and look about 7-10 ̊ below brilliant Venus above. By month’s end Mercury is low on the horizon and lost in the sun’s glare. (Magnitude +0)

Saturn – Face south and look about 30 ̊ east of Mars for Saturn’s golden face. Saturn has been moving westward in retrograde motion since early March. A waxing gibbous moon passes Saturn on the nights of July 8-9. By July 21 it stalls at the far west corner of faint Libra, and then begins its normal eastern progression across the sky. Only the outer planets move in retrograde motion. They do not really change their direction of motion. It just appears that way as the Earth, in its orbit—interior to those of the outer planets, passes by an outer planet. The apparent change in motion is similar to looking at a car in the rearview mirror as you pass it. The car is still moving forward, even though it appears to be receding from your point of view. (Magnitude +0)

Venus – Anyone who is up an hour or more before sunrise will have no trouble finding Venus in the northeastern sky. It gains altitude as the month progresses. On July 1 it appears a few degrees above reddish Aldebaran (Taurus). On the morning of July 23, it appears about 10 ̊ below a waning crescent moon and a few degrees above Mercury. (Magnitude -3.8)

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.

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.

Hold your hand at arm’s length to measure apparent distances in the sky. The width of the little finger approximates1.5̊. Middle, ring, and little finger touching represent about 5 ̊. The width of a fist is about 10 ̊. The fist with the thumb extended at a right angle equals 15 ̊. The hand stretched from thumb to little finger approximates 20 ̊-25 ̊. The diameter of both the full moon and the Sun spans only 0.5 ̊. Adjust for the size of your hand.

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.