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Skywatch for the week of November 2, 2020

skywatch 11-2-2020.mp3

We’ve just passed the final cross-quarter day in our calendar. The quarter days mark each season’s beginning: March 21st for spring; June 21st for summer; September 21st for autumn; and December 21st for winter. The cross-quarter days divide each of these seasons in half: February 2nd, which is winter’s midpoint, called Imbolc by the ancient Celts; and in old calendars it was called Candlemas; not too long ago, it also picked up the name Groundhog’s Day. May 1st is the midpoint for spring, and was called Bealtane. August 1st, the middle of summer, was called Lunasadh or Lammas; and November 1st was All Saints Day in the Church calendar, but also Samhain, the beginning of the ancient Druid year. Samhain is a late harvest occasion, when the last of the crops are brought in before winter’s beginning. Nights are now noticeably longer than the days, as witnessed by our return a couple of days ago to Standard Time from Daylight Savings Time.

Skywatch 11-3-2020.mp3

Tue Nov 3, 2020 FRED WHIPPLE
The American astronomer Fred Whipple was born November 5th, 1906. As a young graduate student he helped to plot the orbit of the newly discovered planet Pluto, and in the 1930’s he showed that meteor showers are the result of particles shed from passing comets. But he is best known for his work in comet theory: in 1950, he came up with the basic model for comet composition that is still in use today. It’s called the “dirty snowball” theory, and it proposes that comets are basically big chunks of frozen ice, mostly water ice, with lots of rocky pebbles and dust grains mixed in. When a comet approaches the sun, the ices melt or sublimate and form an atmosphere or coma, around the comet nucleus; the solar wind and the pressure of sunlight blow this atmosphere out into a long comet tail. When the Giotto spacecraft flew by Halley’s Comet and imaged its 20-mile-wide nucleus during the comet’s last appearance in 1986 it confirmed his theory.

Skywatch 11-4-2020.mp3

Just as we experience daylit and dark periods on earth, so the moon has both day and night. But the moon spins slowly; a lunar day lasts two weeks, followed by two weeks of lunar night. As the moon orbits the earth, even though half of the moon is lit up at any time, we can’t always see the entire illuminated part. The moon’s rotation period matches its revolution, so it rotates once for every one orbit. This is called a tidal or synchronous lock, an effect of the earth’s tidal pull on the moon, which has slowed its rotational speed to match its revolution. Because of this we can only see half the moon (lunar nearside;) the farside of the moon (sometimes wrongly called “the dark side,”) can never be seen from earth. Or as Pink Floyd tells us, ”There is no dark side of the moon; matter of fact, it’s all dark!” But the sun lights up the dark side, sorry, farside, just as much as lunar nearside.

Skywatch 11-5-2020.mp3

Hercules was one of ancient Greece’s most revered heroes. Even the heavens were a veritable picture-book that chronicled his adventures. The zodiac reveals many of his twelve great labors. Soon to set after the sun are the stars of Sagittarius the archer. This centaur is a depiction of Hercules’ teacher, Chiron. Well-placed in the south are a scattering of stars which mark Aquarius, the Water Carrier. This is symbolic of Hercules’ releasing the flood of river waters that cleaned the Augean stables. High in the east is Aries the Ram, a representation of the golden fleece, which Hercules pursued with his good friend Jason while he was between labors. Nearer toward the eastern horizon is Taurus; this was a wild bull which Hercules subdued in a kind of a “capture and release” program. There are more constellations connected with Hercules, but they won’t show up in our evening sky until next month.

Skywatch 11-6-2020.mp3

By now I hope everyone has recovered from our semi-annual lurch in time. After more than half a year of Daylight Savings Time, we’ve finally returned to the more sensible Standard Time. Daylight Savings Time was implemented in the United States in 1918 by the Woodrow Wilson administration, and it has been with us pretty much ever since. Most astronomers I know really hate Daylight Savings Time. It's true that by setting our clocks ahead one hour, we can stretch out the afternoon and evening daylight periods, but for a stargazer, this can be a real nuisance. In the astronomy business, Daylight Savings Time is known as Darkness Wasting Time, because it makes us wait an extra hour for the skies to darken and let us see the stars. Now as nights get longer and daylit periods shorter, and a return to Standard time, at least for now we can get some serious observing done long before the midnight hour.