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Skywatch for the week of September 28, 2020

Monday Sep 28 ARCTURUS AND BOOTES
 
If you look off to the northwest after sunset tonight, you’ll find a star low in the sky. That northwestern star is named Arcturus, which means, “bear guard” or “bear chaser.” That’s because Earth’s rotation causes this star to follow or chase the constellation of Ursa Major, the Great Bear in the Sky, to the north of Arcturus (you’ll recognize part of the Great Bear as the Big Dipper.) Arcturus is the fourth brightest star in the night sky; it’s about 36 light years away – that’s roughly two hundred and sixteen trillion miles from earth - in the constellation Boötes, the Shepherd. This is an agricultural constellation that ancient farmers used to keep track of when to plant and harvest the crops. In the springtime, Boötes can be found in the eastern sky after sunset; now, half a year later, the shepherd has gone over to the other side of the sky, a celestial reminder of harvest time.

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Tuesday Sept 29, 2020 PLACES IN THE SKY 
 
Can you identify the thirty-third largest constellation? It is bordered on the north by Ophiuchus the Serpent Bearer, on the south by Lupus the Wolf, Norma the Level and Ara the Altar, on the west by Libra the Scales, and on the east by Sagittarius and the Southern Crown. Its tail dips into the Milky Way, and there are many nebulae and star clusters within its borders. This constellation’s brightest star is Antares, a red giant hundreds of times larger than the sun. In the South Pacific it’s called Maui’s fishhook, while old Greek myths identified it as the animal that killed the hero Orion the Hunter, but it is kept in check by Sagittarius’ arrows. Just a few thousand years ago the Romans turned its claws into Libra the Scales. Tonight the crescent moon rests above its head and the planet Jupiter can be found above its tail. Can you name this star figure, the eighth constellation of the zodiac? The answer is Scorpius, currently visible in the southern sky after sunset.

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Wednesday  Sep 28, 2020 DECANAL STARS, INVENTION OF THE HOUR
 
Ancient Egyptians accurately measured the length of the year, and knew that it was about 365 days long. Their year began with the appearance of the bright star Sirius at sunrise. With it came the annual flooding of the Nile River, which brought fertile topsoil and water to their fields. The Egyptians also kept a list of “decanal” stars. These were bright stars, although nowhere near as bright as Sirius, which were fairly evenly spaced through the sky. About ten days after the dawn appearance of Sirius, another bright star took its place. Ten days later another decanal star rose with the sun, and so on until the sun returned to Sirius’ position. During the summer, when the nights were short, an Egyptian astronomer could see 12 different decanal stars throughout the night. The night was divided into twelve hours. The day was ten hours long, plus there was

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Thu Oct 1, 2020 SPUTNIK, SATELLITES
 
On October 4, 1957, the world’s first artificial satellite, Sputnik, was sent into earth orbit from a launch site in the Soviet Union. A few months later, the United States successfully launched Explorer 1, and another satellite now revolved about the earth. Today, there are thousands of satellites in orbit; and every so often, you can see one passing overhead. It looks like a moving star, or like a light from a high-flying jet, but the satellite moves along at a pretty good clip, crossing the sky in only a matter of minutes, and yet you can't hear any sound coming from it. These satellites reflect sunlight down to the darkened earth, and so are visible for a couple of hours after sunset or a couple of hours before sunrise, a time when we are in earth's shadow, but the satellite is just outside it. Satellites typically travel from west to east, except for those in polar orbits which move along a north-south path.

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Fri October 2, 2020  How Many Stars 
 
How many stars are there in the Universe? Well, on a clear dark night you can see a couple thousand up there above you. The best estimates of the number of stars in the Milky Way suggest there are over 200 billion stars in our home galaxy. Beyond the Milky Way there are other galaxies, hundreds of billions of them, each containing billions or trillions of stars. So, how many stars? Here’s a good way to get an idea. Next time you’re at the beach, count the number of grains of sand you can hold in your hand. You’ll be at it a while; there’s roughly 10,000 sand grains in each handful. Now count all the grains of sand on the entire beach. Follow that up by counting all the grains of sand on all the beaches of Florida, and then for extra credit, count all the grains of sand on all the beaches of the world. There are more stars than that in our Universe. Of course, if those stars have planets that have sandy beaches, that’s really a lot of sand!