Skywatch for the week of July 13, 2020
Mon Jul 13, 2020 PRINCIPIA
On July 6 in the year 1686, Principia Mathematica was published in England. Principia was Isaac Newton’s great book on gravity and motion, which became a major breakthrough for our understanding of how the Universe works. His three laws of motion – inertia; force equals mass times acceleration; and action-reaction, plus the relationship between gravity, mass and distance, are still in use today, showing us how we can send rockets to the moon and beyond. Kind of a shame he didn’t pay for the printing of his own book. Edmond Halley paid for its publishing, because he wanted it to help him work out comet orbits. Halley tried to get the Royal Society to pay for it, but they’d tied up all their money in a beautiful book, the History of Fishes, which they weren’t able to sell. Years later, when Halley wanted payment for his duties as secretary, they just gave him a lot of the fish books and suggested he could sell them and make some money that way.
Tue Jul 14, 2020 THREE PLANETS NEAR OPPOSITION, CONJUNCTION
Three worlds of the outer solar system will be sharing a small part of the sky this summer; two of them will be easy to see, but the third one will require a very good telescope in order to find it. They’re all located between the constellations of Sagittarius the Archer and Capricornus the Sea Goat, and all of them will rise at sunset, and remain above the horizon for the rest of the night. Since these worlds are hundreds of millions of miles away, they resemble stars to the naked eye, but telescopes reveal them as round worlds. The brightest of them is Jupiter. To the left of Jupiter is another planet, not as bright, but it has a yellow tint, and that’s Saturn. Now the one you can’t see without a big telescope is Pluto, and it’s a little below and to the west of Jupiter. Because these celestial objects are on the opposite side of the sky from where the sun is, we say they’re at opposition. And since they appear fairly close together, they’re nearly in conjunction.
Wed Jul 15, 2020 THE MILKY WAY AND THE LOCAL GROUP
The Milky Way, part of it at least, can be seen tonight under clear dark skies. It spreads across the eastern sky, from Cassiopeia in the north to Sagittarius in the south. The Milky Way is our home galaxy; we live on a planet orbiting a star about two-thirds of the way out from its center. Other galaxies surround ours, all bound together by gravity. We have satellite galaxies, most notably the Large and Small Magellanic Clouds. And there’s a bigger spiral galaxy about 2 and a half million light years away: its catalog number is M31, but we know it best as the Andromeda Galaxy. Now besides the Milky Way and M31, there are about 30 other, smaller galaxies in the immediate neighborhood (and by “immediate neighborhood,” we mean anything that’s within a million parsecs of here.) This cluster of galaxies is known as the Local Group. Most of them are fairly small and contain only a billion or so stars. M31 and the Milky Way are the Group’s gravitational “anchors”.
Thu Jul 16, 2020 STARS OF THE SUMMER TRIANGLE
The Summer Triangle is made up of three bright stars that are well-placed in the eastern sky after sunset at this time of the year. The highest star, Vega, is the brightest of the three, but the star Altair, below and a little to the south of Vega, is almost as bright. The third star, the northernmost one, is called Deneb, and it’s no match for the brightnesses of the other two. But that’s because, as you may have guessed, Deneb is much farther away from us, so its light is correspondingly dimmer. Altair is a mere seventeen light years away – that’s a little over a hundred trillion miles. Vega is a little farther away, twenty-five light years, but it’s an intrinsically brighter star, and that extra luminosity makes it brighter. But Deneb, which is the dimmest star, is also about a hundred times more distant; if Deneb were as close to us as the other two, it would be bright enough to cast shadows!
Fri Jul 17, 2020 ROBERT HOOKE
Robert Hooke was born on July 18, 1635. He’s best known for his pioneering work in analyzing insects, plants, all manner of things in nature, using a microscope. He made a lot of sketches, and first described the cell-like structure of living organisms. He was also a mortal enemy of Isaac Newton. In fiction, Sherlock Holmes had to combat Professor Moriarty; Superman had to fight Lex Luthor; and Batman had to deal with the Joker. For Isaac Newton, it was this guy – Robert Hooke. Newton had just been made a member of the Royal Society, a group of English and European philosophers and scientists. Newton had built a small reflecting telescope, the first of its kind, and he was persuaded to share his experiments on how the eye sees light. Hooke, who had done some work in this area, strongly criticized Newton, and Newton didn’t like it. Hooke also claimed to have worked out the laws of gravity long before Newton’s published work, Principia. Thus began a life-long battle between the two.