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The Sky This Week, 2020 April 28 - May 5

The Moon and the stars. What more could you want?
The Moon among the stars of Gemini, imaged 2020 April 28 from Alexandria, Virginia.
The Moon among the stars of Gemini, imaged 2020 April 28, 01:29 UT from Alexandria, Virginia
HDR composite image made with a 10.2-cm (4-inch) f/6.5 Explore Scientific AR102 refractor
and a Canon EOS Rebel SL2 DSLR. Seven exposures (range 1/800 - 10 seconds) at ISO 200
processed for High Dynamic Range (HDR)

The Moon waxes in the evening sky this week, coursing her way through the last of winter’s stars and then plunging southward through the springtime constellations.  First Quarter occurs on the 30th at 4:38 pm Eastern Daylight Time.  Look for Luna a few degrees north of the bright star Regulus in the constellation of Leo, the Lion on the evening of May 1st.  If you have binoculars watch the Moon glide just under a degree south of the third-magnitude star Eta Leonis that same evening.  By the week’s end you’ll find the Moon near the bright star Spica.

This is another great week to get to know our nearest neighbor in space.  The Moon’s high northerly declination places her nearly overhead during the evening’s prime hours, and you can have a ringside seat at the telescope eyepiece as you watch the terminator line gradually reveal wonderful alien landscapes on each successive night.  You don’t need a big telescope to enjoy the Moon’s wonders; some of my favorite views have been through my 3- and 4-inch refractor telescopes.  These instruments are often at an advantage during the fickle conditions of the atmosphere at this time of year.  Smaller apertures aren’t as subject to bad “seeing” caused by active jet streams that tend to plague larger instruments and cause fine details to become fuzzy.  Once you start looking at the Moon, you will probably want to identify the various features that you’re looking at.  There are many smart phone apps and online resources to help you find your way around.  A good lunar atlas will give you information on the sizes of the major features, and it will quickly become apparent that most of the impact craters you see are truly enormous by terrestrial scales.  Luna’s surface reflects the earliest history of the solar system’s formation, bearing mute testimony to a billion years’ worth of bombardment by the myriad objects that accreted to form the planets.

Two bright stars dominate the evening hours.  One is getting ready to set in the southwest while the other is rising to prominence in the east.  The first is Sirius, the brightest star in the night sky.  This luminary, with its icy blue tint, is the brightest star in the constellation of Canis Major, the Greater Dog, often depicted as one of Orion’s hunting dogs in mythology.  Its brightness is due in part to its proximity to the solar system, located just 8.6 light-years away.  It is a middle-aged star like the Sun, but it is somewhat more massive and therefore fuses more hydrogen into helium in its core, making it about 25 times more luminous than Old Sol.  It was a very important star to the ancient Egyptians, since its first appearance just before dawn occurred just before the annual flood of the Nile.

Our next star is Arcturus, which is now climbing to prominence with each passing night.  Arcturus is the fourth-brightest star in the sky and beams down on us from about 37 light-years’ distance.  Its rosy tint contrasts nicely with the blue of Sirius, and it gives us a clue as to what’s happening inside its glowing surface.  Arcturus has evolved away from “normal” hydrogen core fusion and now creates helium in a shell around a helium core.  This, in turn, causes the star’s girth to grow and its luminosity to rise; Arcturus is some25 times bigger and 170 times brighter than the Sun.  This “red giant” phase will eventually be the fate of the Sun when its supply of hydrogen in its core is depleted.  But don’t worry, this won’t happen for another billion years!

Venus is still prominent in the western sky as darkness falls.  The dazzling planet is now at her brightest for the current evening apparition, so she is virtually impossible to miss.  She is currently drifting through the stars of Taurus, the Bull, but if you’ve been watching her progress against the stars over the past few months you will see that she is beginning to slow her frantic pace.  Over the next few weeks she will approach the star El Nath, the tip of the Bull’s northern horn, but by mid-May she will stop and reverse course, never quite getting near the star.

You will find Jupiter, Saturn, and Mars low in the southeast before dawn, making their way through the constellations of late summer.  Jupiter is the brightest of the three, and you’ll find Saturn just five degrees to the east.  Mars, however, is relentlessly plowing through the faint stars of Capricornus about twenty degrees east of the gas giants.  Comparable to Saturn in brightness, his ruddy tint should make him easy to identify as the first rays of dawn begin to brighten the eastern horizon.

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