SKY HAPPENINGS

The Sky for April 2012
By Faylene Roth

DAYLENGTH
On April 19, when the sun rises at 6:36am, we will have regained the hour of morning sunlight lost in the switch to daylight saving 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.

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 up to 20 meteors per hours may be seen. Look for its radiant, the constellation Lyra, in the eastern sky around midnight. Best viewing occurs between 2:00am and 5:00am when the constellation is overhead. A new moon provides prime viewing conditions. Be alert during the last two weeks of the month for April fireballs, large random meteors with no known radiant. .

ALPENGLOW
The red-orange glow that reflects from the rims and cliffs just before sunrise and after sunset are intensified by the red rocks of Moab. As the sun sets, its light rays are projected through the atmosphere at an angle. The shorter wavelengths of blues and greens are easily absorbed and scattered. The long red and orange wavelengths penetrate deeper through the atmosphere and tint the peaks, rims, and clouds that rise above the sun’s horizon. While the term alpenglow, refers to the pink light reflected off mountain peaks, it is the same physical phenomenon that illuminates our red cliffs and creates red, pink, and purple clouds at sunset.

MOON HAPPENINGS
April 6 - Full Moon occurs at 1:19pm and rises at 8:09pm
April 13 - Last Quarter Moon rises several hours after midnight.
April 21 - New Moon occurs at 1:18am.
April 29 - First Quarter Moon sets several hours after midnight.

(The time of moonrise and moonset assumes a flat horizon. Actual time may vary.)

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 - Find Jupiter low on the western horizon (below Venus) as nautical twilight fades. Jupiter outshines the faint constellation Aries in which it now appears. By month’s end, Jupiter has moved too close to the sun to be visible. (Magnitude -2.1)

Mars - The red planet continue to shines prominently along Leo’s belly. On April 2 it appears to the left of Regulus (Leo) and a nearly full moon. Even though Mars now shines brighter than Saturn, it appears smaller because of its relative size. (Magnitude -0.5)

Saturn - Saturn’s rising on April 1 (in Virgo) marks the end of astronomical twilight. On April 15, when Saturn reaches opposition (far side of earth from sun), it rises as the sun sets. Since opposition marks its closest point to earth, Saturn shines at its brightest for this year. By month’s end it joins Arcturus (Bootes) and Spica (Virgo) as dominant features of the eastern sky. Saturn appears to the left of Spica and shines yellow compared to Spica’s blue-white light. (Magnitude (+0.3)

Venus - On the night of April 2 Venus passes across the star-studded Pleiades cluster to the north of Taurus on the western horizon. Its brilliance increases as Venus enters its crescent phase. The inner planet is approaching its annual pass between the sun and the earth. Venus appears above, and outshines, Jupiter on the western horizon. On April 24 a slim waxing crescent moon appear s to the left of Venus. (Magnitude -4.4)


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.

Spring skies are less densely populated with stars because our nighttime view is upward and away from the central plane of the galaxy; hence, the rich star-studded band of the Milky Way barely extends above the horizon in our evening skies..

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.