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skywatch for the week of January 8, 2024

Skywatch Monday 1-8-2024.mp3

Mon Jan 8, 2024 GALILEO'S MOONS

On January 7th in the year 1610 Galileo wrote: "…at the first hour of night, Jupiter presented itself to me. Beside the planet there were three starlets, small indeed, but very bright. Returning on January eighth I found a very different arrangement. On the thirteenth of January four stars were seen by me for the first time." Galileo concluded that the four star-like objects were moons going around Jupiter. To see what Galileo saw, look for Jupiter high in the southern sky this evening; to the unaided eye it will look like a very bright star. But a telescope will let you see Jupiter as a small, round disc, and its moons will appear as tiny stars, lined up on either side of the giant planet’s equator.



Our calendar is based on repeating patterns in the heavens – the earth’s rotation, the phases of the moon, the earth’s orbit about the sun. The calendar has its origins thousands of years ago, from Egypt. By observing the sun’s progress through the sky, ancient Egyptians were able to accurately measure the length of the year, and knew it was about 365 and a quarter days long. Their calendar had 12 months of 30 days each, which worked out to 360 days total. Then they had five extra days or “empty” days, known as heiru renpet, which they used as a holiday at the end of the year. The new year began with the predawn rising of a star they named Sothis, which appeared in the east just before sunrise. Sothis is still shining up there; we call it Sirius, the brightest star in the night, which appears below and to the left of the constellation Orion.


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On January 1st, 1801, the Italian astronomer Father Giuseppe Piazzi discovered Ceres, the first of the asteroids. Piazzi saw it in a place where there was no star recorded, so he marked its position on his star chart. Several nights later, he found that it had moved. Obviously, this was no fixed star in the heavens, but a wanderer in our solar system that was close enough for us to observe its orbital motion around the sun. He informed the scientific community, and named his new world Ceres, after the Sicillian goddess of the harvest. In the years that followed, other astronomers found more asteroids (literally, “star bodies,” And now, 223 years later, we’ve discovered over a million of these minor planets!


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On January 11, 1787 William Herschel discovered Uranus' moons Titania and Oberon. Herschel was a church organist in Bath, England, but his hobby was astronomy. He built his own telescopes, and was so good at it that colleagues were amazed to find that his handmade instruments were far superior to the ones commercially available at the time. It was with just such a telescope that he became the first person in history to discover another planet telescopically, in 1781. He suggested naming it George, after the king of England. But eventually it became known as Uranus – this is better? And six years later, his improved observations led to the discovery of its two largest moons.


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A medium-power telescope can show you four tiny-looking stars buried within the Great Orion Nebula. These stars arranged in the geometric form of a trapezoid, are called the Trapezium. The brightest of these stars lights up the cloud, and ultraviolet radiation ionizes the nebula gas, illuminating it like a giant neon sign. The gas is mainly hydrogen and helium, but it contains a whole chemistry set of elements and compounds in smaller amounts too. The Orion Nebula is about 1300 light years, or 7,800 trillion miles away. So it’s not exactly next door. The cloud is trillions of miles across, and inside it, stars are being made as gravitational contraction heats the hydrogen and helium in the cloud. With the Hubble and the Webb space telescopes, we can even see places where solar systems are forming.