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Skywatch for the week of December 21, 2020

skywatch 12-21-2020.mp3

Skywatch Monday Dec 21.2020

The sky is filled with invisible lines that trace out the paths of the sun, moon and planets, the extensions of earth coordinates and the patterns of the constellations. While constellation pictures are imaginary, it’s important to note that the astronomer’s coordinate lines are not imaginary: they’re real, but they just happen to be invisible. These “ghost lines in the sky” help us to make sense and order of the Universe. One such line traces out the division between the earth and the sky, and is called the horizon. Another line, the celestial meridian, begins due north, reaches the top of the sky - the zenith - and ends in the south. The earth’s equator projected out into space traces the line of the celestial equator across the heavens, while the ecliptic, the path of the earth’s orbit about the sun, serves as a calendar in the sky, showing us where the sun should be on any given day of the year.

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Skywatch Tuesday Dec 22

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 an hour of twilight at dawn and another hour of twilight at dusk - 24 hours in all.

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Skywatch Wednesday Dec 23

The astronomer Johannes Kepler was born on December 27th in the year 1571. Kepler believed in Nicholas Copernicus’ theory that the earth was not the center of the universe, but instead orbited the sun. But while Copernicus had a beautiful idea, in terms of predicting where the planets would be it didn’t work any better than the geocentric theory; both theories were riddled with errors, bad observations and mistaken assumptions. Copernicus held on to the ancient idea that the orbits of planets were perfectly circular, but the data that Kepler used, obtained from the painstaking observations of the Danish astronomer Tycho Brahe, didn’t support that notion. Unlike past theorists, Kepler refused to toss out the data. Instead he got rid of the theory and introduced a new one: the orbits of planets are elliptical. Once elliptical orbits were calculated, the motions of the planets became understandable and predictable.

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Skywatch Thursday Dec 24

Sir Arthur Eddington was born on this day, December 28, in 1882. It was Eddington who proved Einstein’s theory of general relativity. He was a great scientist, and he knew it. When someone praised him as being one of only three people who understood Einstein’s theory, he replied, “Well, there’s me, and there’s Einstein. Who else is there?” Einstein’s theory of general relativity said that the gravity of a massive object, like the sun, could bend any light waves that came near it. So any stars that were along the same line-of-sight as the sun would seem displaced by its gravity. You can’t ordinarily see stars near the sun, because it’s too bright. But during a total solar eclipse, you can. And during a solar eclipse in 1919, observations by Eddington found that stars near the sun in Taurus, a constellation that’s visible in our southern sky this evening, were displaced – he had proved Einstein’s theory.

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Skywatch Friday Dec 25

Sir Isaac Newton was born on January 4th, 1643. He was also born on Christmas Day, December 25th, in the year 1642. Newton has two birthdays because when he was born, England was still using the old Julian calendar. The Gregorian calendar was adopted long after he died, and when it was put in place, eleven days had to be added to all the old Julian dates, which would reckon his birthday to be January 4th. Isaac Newton invented calculus so that he could develop his three laws of motion, describing such things as inertia, force and acceleration, plus the famous third law – for every action there is an equal and opposite reaction – and that’s how rockets work. Newton discovered mathematical laws which described gravity, and he reasoned that it was universal, that is, that gravity works everywhere in the same way. In addition to carrying out investigations into the nature of light, Newton also built the first reflecting telescope, called the Newtonian reflector in his honors