NIGHT SKY HAPPENINGS

The Sky for April 2017
By Faylene Roth

The star-studded winter sky (Orion’s belt, dog stars of Canis Major and C. Minor, twins of Gemini) sinks into the western horizon during early hours of twilight. Leo, Virgo, and Boötes claim dominance over the springtime sky. Their respective 0 and 1st magnitude stars— Arcturus, Regulus, Spica—mark three points of an isosceles triangle spanning over 60 ̊ of the overhead sky. Earth’s orbit around the sun provides a different view each season of the stars in our galaxy. With spring, the faint tresses of Coma Berenices (which intersect the northern side of the triangle) direct our view toward the north pole of our spiral galaxy. Its boundary with the universe beyond lies a few thousand light years away which means fewer visible stars compared to what we see in winter and summer when we look through the central plane of our galaxy.

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 –visible on the ecliptic as evening twilight darkens SE horizon; high overhead at midnight (in Virgo); outshines 1st magnitude Spica; attains maximum brilliance as it reaches opposition (opposite side of Earth from sun) on April 7. (Magnitude -2.5)
Mars –red disk below 1st magnitude red star Aldebaran (Taurus) on WNW horizon at twilight. (Magnitude +1.7)
Morning (At Twilight)
Jupiter –remains visible low on W horizon. (Magnitude -2.5)
Saturn – rises after midnight in Sagittarius and overhead at morning twilight. (Magnitude +1.3)
Venus – visible shortly before civil twilight, near head of Pisces south of the Square of Pegasus. (Magnitude -4.5)

MOON HAPPENINGS
April 3 – Waxing first quarter moon lights the evening sky then sets soon after midnight.
April 11 – Full moon (12:08am) rises at 8:28pm (closer to full on April 10 rising at 7:30pm).
April 19 – Dark evening skies return with the waning last quarter moon rising after midnight.
April 26 – New moon (6:16am) yields dark skies for several nights.
(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.)

(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.

CELESTIAL HAPPENINGS
Canyon Country manifests its own version of alpenglow against high desert walls, alpine peaks, and high cirrus clouds. Long wavelengths of red and orange light project high into the sky before sunrise and after sunset. Light reflected off water and dust particles in the atmosphere intensifies the vivid reds, pinks, and purples of the local rocks.
Comet 41P/Tuttle- Giacobini- Kresak could reach naked eye visibility (5th magnitude) in early April. In early April it passes through Draco’s tail halfway between the cup of the Little Dipper (Ursa Minor) and the handle of the Big Dipper (U. Major). By April 20 it grazes Draco’s head and moves on towards Lyra. Go to http://www.cometwatch.co.uk/ for updates.

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.