The Sky This Week
U.S. Naval Observatory's Weekly Blog

Marking the Summer Solstice (We've been at it for a long time!)

by Geoff Chester, USNO Public Affairs
15 June 2021

The Moon, imaged 2021 May 19, 02:00 UT from Alexandria, Virginia
with an Explore Scientific AR102 10.2-cm (4-inch) f/6.5 refractor, 1.6X Antares Barlow lens,
and a Canon EOS Rebel T2i DSLR.

The Moon waxes through her crescent phases this week as she wends her way through the late spring and early summer constellations.  First Quarter occurs on the 17th at 11:54 pm Eastern Daylight Time.  Luna starts the week among the stars that form the “head” of Leo, the Lion, then works her way southeastward through Cancer, Virgo, and Libra before finishing the week among the stars of Scorpius.  Look for the Moon to the north of the bright star Spica on the evening of the 19th.  On the 22nd she passes to the north of the ruddy star Antares.

The summer solstice occurs on the 20th at 11:32 pm EDT.  This is the moment when the center of the Sun’s disc reaches an ecliptic longitude of 90 degrees.  It is also when Old Sol reaches his highest northern declination, providing us with our longest days and shortest nights.  The opposite case is true for our friends in the southern hemisphere, who are experiencing the start of their season of astronomical winter.  The observation of the solstice has been a focal point in the sky calendars of many ancient cultures, and we see remnants of these calendars scattered throughout the world.  Perhaps the most famous of these is Stonehenge on the Salisbury Plain in southwestern England.  This famous megalithic structure has seen the annual rise of the Sun on the day of the solstice for millennia.  The iconic stone circle that most of us are familiar with sits on a site that had been in use for well over 1000 years for marking this annual event.  The earliest phase of the monument was constructed around the year 3100 BCE, about the time that the Old Kingdom of ancient Egypt arose.  The Neolithic and Bronze Age people who worked the site left no written records, but it is clear that they aligned the monument to the summer solstice.  Stonehenge sits among many other ancient structures on the plain and is thought to be ceremonially linked with similar “henges” throughout southern England.  Other megalithic structures associated with the solstice are scattered throughout the United Kingdom, Ireland, and much of northern Europe.  Here in the U.S. we also find ample evidence of the importance of the solstice to Native Americans.  Throughout the North American high plains are dozens of “Medicine Wheels” that once served as focal points for religious ceremonies and marking the seasons.  In the American Southwest there are a number of petroglyphs and angled boulders that cast “spears” of light onto carved spiral patterns at the time of the solstice.  

The passing of the solstice means that we now experience our shortest nights here in the Northern Hemisphere.  Here in Washington we have a mere 5 hours and 8 minutes of true darkness between the end and beginning of astronomical twilight.  Fortunately the week’s waxing Moon gives us a nice target for our telescopes during the evening hours.  I often say that the Moon is “looked over, then overlooked” by many of today’s amateur astronomers, but it nonetheless remains one of my favorite targets for viewing on warm summer evenings.  Even though Luna keeps the same face constantly turned in our direction, its features, frozen for eons of time, can always show a detail of light and shadow that make it worth the time to study them.  I can safely say that each time I point my telescope at our nearest celestial neighbor I spot something that I had never noticed before.

The bright glow of Venus continues to be a fixture in the western twilight sky.  You should be able to easily find the dazzling planet by around 9:00 pm local time, and she remains above the horizon for just over an hour before setting.  Venus owes her brilliance to her highly reflective atmospheric clouds, which reflect about 65 percent of the sunlight that strikes them.  

You will now find Mars plodding steadily eastward among the faint stars of the rather nondescript constellation of Cancer.  If you have a pair of binoculars you can track his daily progress as he closes in on the star cluster known as the Praesepe or “The Beehive”.  He will pass through the heart of the cluster early next week.

Saturn and Jupiter are steadily making their way into the evening sky.  By the end of the week both planets rise before midnight, and should be well-placed for viewing during the early morning hours.  The best time to view them is during the morning twilight hours; at 5:00 they straddle the meridian in the southern part of the sky.
 
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