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Skywatch for the week of January 6, 2020

Mon Jan 6, 2020              PERIHELION

The earth's orbit is slightly elliptical, so its distance from the sun changes through the year. The earth reached perihelion at 3 a.m. on January 5th. Perihelion is the point in its orbit when earth is closest to the sun. On average, we're about 93 million miles from the sun, but right now, we are just a little under 91 and a half million miles from it. So if we're a million and a half miles closer to the sun, how come we're having winter? Well, not everyone on earth is having winter; summer has just begun for folks who live below the equator.  Our seasons are not caused by our slight variation in distance, but by the 23 and a half degree tilt of our planet as it orbits. The rotating, revolving earth is like a gyroscope, with the north axis aimed nearly at Polaris, the North Star. Now our north hemisphere is tipped away from the sun; this puts the sun lower in our sky, and with less direct sunlight we get cooler temperatures.



Lately I’ve been looking very carefully at Betelgeuse, which is a red giant star in the shoulder of the constellation Orion the Hunter. For hundreds of years, astronomers have used Greek letters to designate the relative brightness of a star, compared to other stars in a particular constellation. Betelgeuse is Alpha Orionis, while Rigel, a blue giant star in the hunter’s knee, is designated Beta Orionis. The thing is, Rigel is actually brighter than Betelgeuse. It’s possible that astronomers long ago mistakenly thought Betelgeuse was brighter because of its red color. It’s also possible that maybe Betelgeuse actually was brighter than Rigel way back then. I mention this because for the past several months, Betelgeuse has been getting dimmer. Toward the end of a giant star’s life, its brightness fluctuates, a precurser to it going supernova. So watch the skies, and keep your eye on Betelgeuse. It’s likely to explode sometime in the next four hundred thousand years!


Wed Jan 8, 2020                              GALILEO'S MOONS

Here is an entry from the observational logbook of the Italian astronomer Galileo, written over four hundred years ago: "On the seventh day of January in this present year 1610, 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." From these observations, Galileo concluded that the four starlike objects were moons going around Jupiter. With a small telescope or a good pair of binoculars, you can see what Galileo saw – but not tonight: Jupiter is on the other side of the solar system, and the sun is blocking our view for the moment. But as we approach the end of this month, Jupiter will reappear in the southeastern sky before sunrise, a bright morning star in the constellation Sagittarius, and you should be able to discover its four largest moons for yourself!


Thu Jan 9, 2020                               ISAAC NEWTON

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. Besides carrying out investigations into the nature of light, Newton built the first reflecting telescope, called the Newtonian reflector after him.



The moon is full today. The Passamaquoddy Indians would call this the Wolf Moon, a time of year when wolves that normally avoided humans, would be forced by winter famine to scavenge from the villages. Wolves were seen more often, especially at night when the moon was full and bright. To the Sioux, this is the Moon of Strong Cold; the Zuni know it as the Moon When the Limbs of Trees are Broken by Snow. The Tewa Pueblo call it the Ice Moon, the Cherokee the Windy Moon. And to the Omaha Indians, it is the Moon When the Snow Drifts into the Tepees. There’s also a lunar eclipse today, but since it’s happening in the afternoon, before the moon rises here, we won’t see it. But you can see the planet Venus this tonight, and I’m giving a talk this evening about our sister planet at 6 and 7:30 pm at Indian River State College’s Hallstrom Planetarium. There’ll be shows tomorrow afternoon as well, and you can purchase tickets at the Planetarium.