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The Sky This Week
U.S. Naval Observatory's Weekly Blog
Marking May Day
by Geoff Chester, USNO Public Affairs
27 April 2021
The Full Pink Moon, imaged 2021 April 27, 02:44 UT from Alexandria, Virginia
with an Explore Scientific AR102 10.2-cm (4-inch) f/6.5 refractor telescope, 1.6X Barlow lens,
and a Canon EOS Rebel SL2 DSLR.
The Moon dips into the southern reaches of the ecliptic this week, wending her way through the rising summertime constellations. Last Quarter occurs on May 3rd at 3:50 pm Eastern Daylight Time. Luna passes just over three degrees north of the ruddy star Antares on the morning of the 29th. Two mornings later she passes through the “top” of the “teapot” asterism in Sagittarius. Early risers on May 4th will find her in an attractive triangle with the planets Jupiter and Saturn.
May 1st marks another of the so-called “cross-quarter days” that were once incorporated into traditional agricultural calendars. These dates marked the mid-point of the astronomical seasons, but to many ancient cultures they represented seasonal beginnings. “May Day” is still observed in many parts of Europe, where the tradition has been handed down since Roman times. Its earliest incarnation was the Roman festival known as the Floralia, a day to honor the gods Dionysus and Aphrodite. As with many such celebrations it was observed over several days around May 1st and involved various forms of merrymaking. Its observance dates back to at least the First Century BCE, when Cicero mentioned his participation in the festival. Originally intended as a celebration of fertility, by the end of the First Millennium CE it had evolved into more of a religious festival. Early Christian Germanic people observed it as Walpurgisnacht, commemorating the canonization of the abbess Saint Walpurga on May 1, 870 CE. Celtic peoples observed the date as Beltaine, a night of bonfires, dancing, and leaping through fire to bless their cattle as they headed to high mountain pastures. Many of these traditions can still be found in modern May Day festivals. Feasting, the crowning of “May Queens”, and dancing around a May-pole are traditions that have persisted over the millennia.
As the Moon recedes into the morning sky this is probably your final week to get a good look at the last of winter’s constellations. The seasonal showpiece, Orion, begins to set at the end of evening twilight, and by the late evening all but the bright stars Capella, Castor, Pollux, and Procyon are left hanging in the west. He only star of spring that can rival the bright luminaries of winter. Arcturus is the brightest star in the Northern Hemisphere sky and is outshone by only three other stars, all of which are best seen from more southerly climes. It has a distinct rose-pink tint to my eyes, and this color betrays something about its nature. It is evolving toward the end stages if its life, and current estimates place it at an age of about 7 billion years, much older than our Sun’s 4.5 billion. It is slightly more massive than the Sun, so it gives us something of a preview of Old Sol’s ultimate fate. Arcturus is a “red giant” star, one that has exhausted the hydrogen in its core and now fuses hydrogen into helium in a core-surrounding shell. This, in turn, causes the star’s girth to expand; the diameter of Arcturus is about 25 times that of the Sun. As its surface area expands its radiative layer cools, giving it its characteristic reddish tint. Arcturus has probably been studied with large telescopes more than any other star except the Sun. Because it is so bright its light can be spread out into a highly detailed spectrum that reveals the chemical makeup of its surface layers and atmosphere. It is used as a standard calibration star for mapping the spectra of other stars. From its spectrum we can determine that it is moving in our direction at around 5 kilometers (three miles) per second. Don’t worry, though; its closest approach won’t happen for some 4000 years, and it will never get closer than about 36.6 light-years.
The bright planet Venus and the elusive Mercury share the limelight in evening twilight for the next few weeks. This week you will find them low on the western horizon about half an hour after sunset. Mercury puts some distance between the two as the week progresses, becoming a bit easier to see each passing evening. On the evening of the 3rd he passes just 2 degrees south of the Pleiades star cluster. You will want to use binoculars to see the cluster’s brightest stars.
Mars is now at his most northerly point in the sky, riding high among the stars of Gemini. He begins the week just to the northeast of the bright star cluster Messier 35, then marches resolutely eastward. He is now four full magnitudes fainter than he was at opposition last October, and observers in urban locations might have a little difficulty finding him. That said, his ruddy tint should betray his location southwest of Castor and Pollux.
Jupiter and Saturn continue wo work their way toward the evening sky, but they are still best seen in the hours before dawn. By the onset of morning twilight they should be easy to spot in the southeastern sky among the faint stars of Capricornus. The waning Moon joins them on the morning of the 4th. This should be a nice photo opportunity for early risers.
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