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The Sky This Week, 2020 June 16 - 23

Line 'em up!
Sunset alignment, Mollusk, Virginia.
"Osprey-henge", Mollusk, Virginia
Make your own "horizon calendar" by noting where the Sun rises or sets
on your local horizon throughout the year.

The Moon passes through the last of her crescent phases in the pre-dawn sky this week before returning to grace the early evening sky.  New Moon occurs on the 21st at 2:41 am Eastern Daylight Time.  At this time Luna will pass between the Sun and Earth, producing an annular solar eclipse that will be visible along a narrow path stretching from central Africa to the western Pacific Ocean.  This will be a so-called “ring-of-fire” eclipse since the tip of the Moon’s umbral shadow doesn’t quite reach the Earth’s surface.  Viewers along the central path will see the disc of the Moon centered on the Sun at maximum eclipse, but they will still need to use proper eye protection to view it.  Unlike a total eclipse, the sky doesn’t become dark and the solar corona is not visible.  Early next week, look for the Moon to return to the evening twilight sky.  On the 23rd she will pass just north of the binocular star cluster Messier 44, popularly known as The Beehive.

Astronomical summer officially arrives on the 20th at 5:44 pm EDT.  This is the moment when Old Sol reaches an ecliptic longitude of 90 degrees, and it also happens to be the time when he reaches his highest northern declination, with the center of his disc perched over the Tropic of Cancer.  This is the longest day for residents of the Northern Hemisphere, but the length of day varies widely with latitude.  Here in Washington we will have 14 hours 54 minutes between sunrise and sunset.  Residents of Hawai’i will only see 13 hours 20 minutes of daylight, while residents of Fairbanks will have 21 hours and 50 minutes to enjoy the Sun along with twilight to fill the remaining hours.  

The solstice marked a very important date in the calendars of many ancient cultures as exemplified by the profusion of megalithic remains that align to the solstice sunrise.  Perhaps the most famous of these is Stonehenge, a site occupied and built over the course of a millennium.  Starting as a simple ditch dug into underlying chalk circa 3100 BCE, it had a break in the earthworks that aligned to midsummer sunrise.  Over the next thousand years the site was expanded and the construction became monumental, with huge stones brought to the Salisbury plain from hundreds of kilometers away.  It served as a ceremonial center for the Bronze Age people in the area until around 1600 BCE, when the culture vanished as mysteriously as it came.  Similar structures have been found throughout the British Isles and northern Europe.  Here in the United States we have found scores of circles pecked into flat rocks or excavations aligned to the solstice throughout the Midwest and western states.  Some of these structures look like large compasses etched in stone, while others take advantage of natural clefts in the rock to cast beams of light from the rising Sun onto the centers of spiral petroglyphs.  The power inherent in marking the passage of time seems to be endemic in humans and is probably the key force that allowed us to develop societies and civilization.  You can replicate the actions of these early people by simply noting the point on your local horizon where the Sun rises on the 20th.  Return to that spot once or twice a month over the course of a year and you will have a “horizon calendar” of your own and the power to tell time.

Our evening sky now waits for Jupiter to rise to keep us company through the short summer nights.  He now rises shortly before 10:30 pm in the southeast and scuds along the southern horizon in the company of Saturn, who rises about 20 minutes later.  These two giants of the solar system will stay with us throughout the summer, offering fine telescopic sights for the duration.

Mars crosses the boundary between the dim stars of Aquarius and the even dimmer stars of Pisces.  Early in the week early risers may see an attractive triangle formed by Mars, the lonely star Fomalhaut, and the second-magnitude star Diphda in the gathering morning twilight.  Mars will be the most obvious by far, with his striking ruddy tint contrasting nicely with the pale white of the stars.