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The Sky This Week, 2020 June 23 - 30

Early evenings with the waxing Moon.
Moon with Earthshine, 2020 April 28, Alexandria, Virginia.
Moon with Earthshine, 2020 April 28, Alexandria, Virginia
HDR image made with an Explore Scientific AR102 10.2-cm (4-inch) f/6.5 refractor
and a Canon EOS Rebel SL2 DSLR.

The Moon returns to the evening sky this week, waxing through her crescent phases as she courses her way through the spring constellations.  First Quarter occurs on the 28th at 4:16 am Eastern Daylight Time.  Look for the bright star Regulus, the heart of Leo, the Lion, about five degrees west of the Moon on the evening of the 25th.  On the 28th and 29th she will bracket the bright star Spica in the constellation of Virgo, and she ends the week just two degrees north of the second-magnitude star Zubenelgenubi on the night of the 30th.

The year’s latest sunset occurs on the 27th for folks living in northern temperate latitudes.  Here in Washington Old Sol slips below the horizon at 8:38 pm EDT that evening.  Most of us probably won’t notice the Sun’s gradual return to earlier set times for a couple of weeks; he won’t set before 8:30 pm until July 20th.  On the other end of the day, though, sunrise is now four minutes later than it was on June 13th, so the days are indeed beginning to gradually get shorter.  Rest assured, though, since there is lots of summer yet to pass.  Currently it’s our longest season, about 93.6 days and gradually lengthening.  It will reach its maximum duration, just under 94 days in length, sometime around the year 4000CE.  As a consequence, boreal winters are gradually getting shorter, currently about 89 days’ duration.  The difference and slow variations are due to long-term changes in the Earth’s orbit known as Milankovich Cycles.  Over long time intervals, perturbations by the Sun, Moon, and other planets cause Earth’s rotation angle and the eccentricity of its orbit change, causing all four seasons to vary in length.  The closest they will come to equal length in the foreseeable future will be inn about 30,000 years, when the eccentricity of Earth’s orbit is very nearly circular.

With darkness not fully enveloping the sky until after 10:30 pm, the Moon offers us a nice target for evening stargazing time.  I have always enjoyed my time spent gazing at Luna’s distant surface even though it never changes.  However, while the features stand in mute testimony to eons of little change, no two views of any particular part of the Moon are ever the same.  The stark lighting, unhindered by an atmosphere, create highlights and shadows that provide unique views each time I point the telescope her way.  As she waxes through her crescent phases a detail that is often overlooked is the subtle blue tint of “Earthshine” lighting the part of her disc not directly lit by sunlight.  If you were to stand on the Moon and look back at Earth, you would see the complement to the phase we see from home, a gibbous Earth hanging in a jet-black sky.  Earth reflects about 37 percent of the light that it gets from the Sun, and its disc is four times larger in diameter in the lunar sky.  I am always amazed that Earthshine is as bright as it appears since the Moon only reflect about 12 percent of the incident light that hits it.  As the Moon waxes in our sky Earth wanes in the lunar sky, so the effect of Earthshine becomes difficult to see once the phase reaches first quarter.  

In addition to the subtleties of lighting, the Moon offers an astonishing variety of landscapes that gradually change as the terminator line slowly spreads across the disc.  Tailor-made for the small telescope, it’s a great way to spend a summer evening in your yard.

We are now just a few weeks from opposition for Jupiter and Saturn, when the two giant planets will be in the sky all night.  They currently rise about 20 minutes apart, with Old Jove cresting the horizon before 10:00 pm.  By midnight they are well up in the southeastern sky, just east of the distinctive “Teapot” asterism in Sagittarius.

Mars is still best seen before dawn in the southeastern sky.  The red planet is gradually growing brighter as he drifts eastward through the faint stars of Pisces.  You should have no trouble spotting him in gathering morning twilight as he continues his eastward trek along the ecliptic.

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