SKY HAPPENINGS

The Sky for April 2013
By Faylene Roth

DAYLENGTH
On April 18, when the sun rises at 6:37am, we will have regained the hour of morning sunlight lost in the switch to daylight time on March 11. Evening activities continue to benefit from the three phases of twilight. Civil twilight, in which the sun drops six degrees below the horizon, extends adequate light for most activities for one-half hour. During the next half hour, nautical twilight signals the loss of color in the landscape and darkening of the overhead sky as the sun drops to twelve degrees below the horizon. In the final stage, astronomical twilight, residual light on the horizon fades to darkness by the time the sun drops to eighteen degrees below the horizon. The reverse progression applies to dawn.

MOON HAPPENINGS
April 3 – Last Quarter Moon rises several hours after midnight.
April 10 – New Moon occurs at 3:35am.
April 18 – First Quarter Moon sets several hours after midnight.
April 24 – Full Moon occurs 10:25pm in Moab and rises at 7:11pm.
(The time of moonrise and moonset assumes a flat horizon. Actual time may vary.)

METEOR EVENTS
Light meteor activity occurs throughout the month in all directions, but the Lyrids Meteor Shower provides the most reliable viewing. Activity ranges from April 16-25 with peak activity on April 21/22 when 10-20 meteors per hours may be seen. Look for its radiant, constellation Lyra, in the eastern sky after midnight. Best viewing occurs between 2:00am and 5:00am when the constellation is overhead and the waxing moon has set. Be alert during the last two weeks of the month for April fireballs, large random meteors with no known radiant.

ZODIACAL LIGHT
Imagine sunlight streaming through a window and illuminating dust particles floating in the room. In the same way, the setting sun illuminates dust particles present in the inner solar system. Near the vernal equinox, the sun’s rays shoot high into the sky after sunset, when we can see this display. It is called the zodiacal light. As astronomical twilight fades from the western horizon, look for a cone of diffused light originating from the point where the sun set and leaning a little to the south. The pyramid of light extends as high as 50-60 degrees above the horizon. With a high vantage point and a clear view of the eastern horizon, you may also see the zodiacal light right before astronomical twilight begins at dawn.

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 – Face southwest and look above the V-shaped head of Taurus the bull to find Jupiter, the brightest object (moon excluded) in the early evening sky shining with a steady silver-white light. Farther south (below Orion) the second brightest object in the evening sky—Sirius the dog star (Canis Major)—twinkles with a spectral array of colors at -1.4 magnitude. Both set before midnight, along with most of the brilliant winter stars, leaving behind the more faintly studded spring sky. (Magnitude -2.1)

Saturn – Once Jupiter sets on the southwestern horizon, shift your focus to the southeastern horizon where Saturn’s steady golden light rises in the dim space between Libra and Virgo (bright star Spica). Saturn shines throughout the night and remains visible on the southwestern horizon through early morning twilight. (Magnitude (+0.3)

Asteroids – With excellent vision or a pair of binoculars, you can peer into the horns of Taurus and view the asteroids Ceres and Vesta. Extend the northern horn of Taurus to the star it shares with the lower end of Auriga (opposite bright star Capella on the upper end of Auriga). On April 6 Ceres is less than one-half degree south of this shared star. Vesta will be on a line about one-half to two-thirds the distance from Ceres and the faint star at the end of the south extended horn of Taurus. (Both asteroids shine with +8 magnitude, Ceres slightly brighter.)

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.

Follow the arc of the Big Dipper’s handle eastward to 0-magnitude Arcturus (Bootes). Continue the arc towards the horizon to Spica (Virgo). A loop up and back to the handle passes through Regulus (Leo) dominant in the overhead sky.

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.