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The Sky This Week, 2015 April 28 - May 5

Bright objects to rival the Flower Moon.
Jupiter & Io, 2015 April 26, 01:26 UT

The Moon waxes in the evening sky this week, brightening the overnight hours as her phase increases to Full Moon, which occurs on May 3rd at  11:42 pm Eastern Daylight Time.  May’s Full Moon is popularly known as the Flower Moon, Milk Moon, or Corn-planting Moon.  Luna begins the week beneath the springtime constellation of Leo, the Lion, and ends up passing among the stars of the summer constellation of Scorpius.  Look for the Moon near the bright star Spica on the evening of May 1st.  On the evenings of the 4th and 5th she rises in the southeast in the company of the planet Saturn.

May 1st is one of the so-called “cross-quarter” days that mark the mid-points of the astronomical seasons.  Just as there are four seasonal markers in the form of the equinoxes and solstices, these are dates which have been traditionally observed by many cultures around the world.  Here in the U.S. we still more or less unwittingly observe two of these dates as Groundhog Day and Halloween.  May Day is still a popular observance in many countries in Europe with roots in various pre-Christian traditions.  Perhaps the most widespread of these was Bealtaine, a Celtic festival celebrating the greening of the landscape after the long winter.  Feasting, dancing, and general merrymaking accompanied this festival, which was then adapted to celebrate feasts of various Christian saints, eventually mostly devoted to the Virgin Mary.  The tradition was imported to the U.S. by European immigrants and was quite widely observed in the late 19th and early 20th Centuries.  Its popularity waned after the 1920’s but has seen a rebound with increasing interest in old “pagan” traditions. 

Moonlight washes out many of the fainter stars in the rising spring constellations, but there’s one star of the season that shines through the brightness of the Moon-washed sky.  Arcturus is that star, and you’ll find it high in the east at the end of evening twilight.  The star’s distinctive rosy tint is caused by the star’s gradual expansion as it evolves into its stellar “twilight years”.  Hydrogen is now burning in a shell around a helium core in the star’s center, and as the shell expands the outer surface cools, emitting a redder color of light.  Arcturus is one of the closest bright stars to the solar system at a distance of just under 37 light-years, and its luminosity, some 120 times that of the Sun, makes it the fourth-brightest star in the sky.  It has a very high “proper motion” relative to the Sun, moving the equivalent of Jupiter’s angular diameter in about 25 years.  Since the time of the ancient Greeks the star has moved about one degree across the sky.  Because Arcturus is so bright and so close it has been extensively studied as a standard star for its class, and its light was used as the trigger to open the 1933 Chicago World’s Fair.  It is the only star other than the Sun that I have seen with the naked eye during the daytime, but I had help from being on the summit of Mauna Kea in Hawai’i at the time!

The fading evening twilight hour offers us an opportunity to spot the elusive planet Mercury, which is now making its second favorable apparition in the evening sky for 2015.  Mercury may be found about 20 degrees below the bright glow of Venus and 10 degrees to the right of the star Aldebaran.  You’ll probably want to use binoculars to locate him initially, but by 9:00 pm he should be easily visible to the naked eye.  On April 30 the American MESSENGER spacecraft, which has been orbiting Mercury for over four years, will be deliberately crashed onto its torrid surface, the first emissary from Earth to do so.

Venus is now at her best for evening observations, standing high in the sky as evening twilight fades to darkness.  The dazzling planet sets a full two hours after the end of evening twilight, and gives Jupiter a run for his money as lord of the night.

Jupiter still dominates the later evening sky, shedding his bright cream-colored glow from the dim constellation of Cancer, the Crab.  The giant planet reaches quadrature, forming a 90-degree angle between itself, the Earth, and the Sun, on May 4th.  At this time the planet shows a very slight gibbous phase, and the moons cast their shadows well to the east of their discs.  Jupiter’s angular diameter is now less than 85% of what it was at opposition, but it still shows a bigger disc than any other planet for owners of small to moderate telescopes, so he’s well worth watching on calm spring nights.  Jupiter’s most famous feature, the Great Red Spot, will rotate across the planet’s disc on the evening of May 1st, and its innermost large moon Io will drag its shadow across Old Jove’s face on the evening of the 4th.

Saturn can now be glimpsed low in the southeast in the late evening sky.  You’ll find him in the company of the Moon on the evenings of the 4th and 5th.  You’ll probably want to wait a few hours for a good look at the ringed planet, but he’ll reach opposition in another month and spend the entire night gracing the sky.