SKY HAPPENINGS

The Sky for May 2013
By Faylene Roth

DAYLENGTH
May’s 50-minute gain in daylight pales in comparison to the 69 minutes gained in April. However, as the earth approaches the far end of its elliptical orbit, the northern hemisphere swings into a more direct face-off with the sun. Canyon walls warm more efficiently. The duration of twilight also increases in May. Civil twilight extends usable light for one-half hour before and after sunset. During civil twilight the sun is within 6° of the horizon. Dim light marks the next half-hour period called nautical twilight, when the sun ranges 6°-12° below the horizon. Astronomical twilight spans another 30-40 minutes in the morning before nautical twilight and again in the evening after nautical twilight. Dark overhead skies and faint light on the horizon mark this period, when the sun sits as low as 18° below the horizon. As the ecliptic (sun’s apparent path across the sky) advances northward, the period of astronomical twilight lengthens, because the smaller circumference of the earth at northern latitudes allows sunlight to extend farther above the horizon.

MOON HAPPENINGS
May 2 – Last Quarter Moon rises several hours after midnight.
May 10 – New Moon occurs at 1:28am.
May 18 – First Quarter Moon sets several hours after midnight.
May 24– Full Moon occurs 10:25pm in Moab and rises at 8:17pm.
(The time of moonrise and moonset assumes a flat horizon. Actual time may vary.)

METEOR EVENTS
Most activity from the Eta Aquarids Meteor Shower occur May 4-7. These long-trained, fast-moving meteors emanate from the region of Aquarius which rises in the southeastern sky a few hours before astronomical twilight brightens the eastern horizon. Leftovers from Halley’s Comet supply the debris for this meteor shower. Expect up to 10 meteors per hour.

SPRING SKIES
Earth’s position in springtime directs our nighttime view out the “top” of our galaxy—through the region around Coma Berenices (below the Big Dipper’s handle). Fewer stars populate the space between our sun and the upper boundary of our galaxy—a mere 1,000 light years away. On spring evenings our familiar view of the Milky Way lies flat below the eastern horizon. Sometime after midnight the earth will have rotated enough to bring it into view. Many more stars become visible as we gaze through the 30,000 light year distance into the galaxy’s center.

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 – The second brightest object in evening twilight (moon excluded) remains visible for most of the month. During the last week of May, Jupiter relinquishes its superior position in the evening sky, disappearing into the sun’s glare, leaving Mercury and Venus to punctuate the evening twilight. (Magnitude -1.9)

Mercury – Mercury rarely moves far enough from the sun to be visible in evening twilight, but a bright apparition and suitable markers make this month an opportune time for viewing. Look for it May 21 about 7° north of Aldebaran (Taurus). Find it May 24/25 north of Venus 1.4° and May 26/27 north of Jupiter 2°north of Jupiter. (Magnitude -1)

Saturn – Look for Saturn low in the southeastern evening sky between Virgo and Libra. Locate 1st magnitude star Spica (Virgo) about 12° above and to its right. 0-magnitude Arcturus (Boötes) hangs about 30° above Saturn. Saturn becomes less a morning feature this month as it sets well before sunrise by month’s end. (Magnitude (+0.2)

Venus – Undeniably the brightest object in the evening sky (moon excluded), Venus remains below Jupiter until very late in the month. May 24-29—during early twilight—Mercury, Venus, and Jupiter appear within 5° of one another—first Jupiter to the left, Mercury to the right, and Venus below. Their positions change over the week until they form a vertical line—Mercury highest followed by Venus then Jupiter. (Magnitude -3.9)

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.

Three stars pierce the darkness of the overhead night sky: 0-magnitude Arcturus (Boötes)—25° below the arc of the handle of the Big Dipper (Ursa Major) and 1st magnitude stars Spica (Virgo)—30° below Arcturus—and Regulus (Leo)—60° ­west of Spica. Jupiter at -2 magnitude outshines the few bright remnants of the winter sky that linger on the western horizon while Saturn at 0-magnitude heralds what is to come on the eastern horizon. Vega (Lyra) then Deneb (Cygnus)—harbingers of summer skies—climb above the northeastern horizon after midnight.

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.