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Skywatch for the week of June 22 , 2020

Mon June 22, 2020        PLUTO AND ITS MOONS

On June 21st, 1978, Pluto's moon Charon was discovered by the American astronomer James Christy. In mythology, Pluto was god of the underworld. Charon was his ferryman, who transported souls across the river Styx to the other side. Styx is another, more recently discovered moon, along with three more – Hydra, Nix and Kerberos. Charon is the biggest one though, it’s about half the size of Pluto. So when it orbits this distant world, Charon's mass has a substantial effect on Pluto, pulling it first one way, and then the other. The two are often referred to as a double planet, because their common center of gravity lies between them. In the summer of 2015, a space probe flew past Pluto and Charon, and sent back incredible pictures and information – ice mountains two miles high, vast nitrogen ice plains, and mysterious dark patches on Pluto’s farside. If you visit the website NASA dot gov, and enter the word “Pluto” in the search box, you can see these pictures for yourself.



Tue June 23, 2020         FALSE DAWN

Sometimes, when the night has worn on toward its end, you may see a faint glow in the east which suggests the beginning of the new day. Yet it is still over an hour until sunrise. The night sky is providing us with one final treat – the zodiacal light, sometimes called, “false dawn.”

Fine dust particles that accompany our planet as it orbits the sun align themselves with the ecliptic, the line that traces out earth’s orbital path. This also aligns with the part of the sky that contains the constellations of the zodiac, and this dust catches the sun’s light long before it sunrise, resulting in a large, triangular patch of faint light – the zodiacal light, the false dawn. The zodiacal light appears strongest near earth’s equator, but if sky conditions are right it can also be seen in higher latitudes north or south of the tropics. As the zodiacal light slowly fades, it is soon replaced by another light in the east as we turn once more toward the sun.


Wed June 24, 2020        SEASONAL CONSTELLATIONS

We are now a few days into the new season, and summer is definitely sizzling. The change of seasons has also brought a change in the sky and its constellations. The sun has moved from Taurus the Bull into Gemini the Twins, rendering that part of the sky difficult to see. The great constellations of winter, such as Orion the Hunter and Taurus the Bull, can now only be glimpsed just before sunrise, near the eastern horizon. Springtime star patterns such as Leo the Lion or the Big Dipper, which were once at the top of our northern evening sky, have now slipped over into the west, supplanted by Boötes the Shepherd, Hercules the Hero and Virgo the Maiden. And new star groups appear in the east – Libra and Scorpius, and the three bright stars of the Summer Triangle. The sky wheels about us, and the summertime constellations take their places in the heavens above.


Thu June 25, 2020         NAME THAT CONSTELLATION – JUNE

Can you identify the twelfth largest constellation in the heavens? It is bordered on the north by Ursa Major and Leo Minor; on the south by Hydra, Sextans, Crater the Cup and Virgo; on the west by Cancer the Crab; and on the east by Virgo again and Coma Berenices. Roughly a dozen of its stars are known to have planets orbiting them. This part of space is also the source of the Leonid meteor shower which peaks in mid-November, and every 33 years, the shower becomes a meteor storm, displaying dozens of “shooting stars” each hour. Many beautiful galaxies are found within its borders, one of which is a favorite of mine – the Hamburger galaxy. In mythology, this creature was the first labor of Hercules, which was defeated after a month-long battle. Tonight the waxing crescent moon can be found to the east of its brightest star, Regulus, sometimes called “the King star.” Can you name this constellation, the fifth sign of the zodiac? The answer is Leo the Lion.



On this day, in the year 240 BC, the Greek astronomer Eratosthenes calculated the circumference of the earth. He did it by using the changing angle of sunlight at different latitudes in Egypt to make the measurement. Eratosthenes made two assumptions: 1. the earth is round; 2. the sun is far enough away that its rays fall parallel across the whole earth. At his latitude in Alexandria, the sun was about 83 degrees, or 7.2 degrees off the zenith) at noon on the first day of summer. He knew of a town south of Alexandria, called Syene, where the sun’s image could be seen reflecting off the water at the bottom of a deep well at noon on the same day. That meant that the sun was at 90 degrees altitude, directly overhead. The Alexandria - Syene distance must therefore be 7.2/360th of the earth’s circumference. So Eratosthenes measured the distance to Syene (almost 500 miles), then multiplied that by 50 (360 divided by 7.2), and got 25,000 miles for an answer. He was off by a hundred miles – “pretty good work for 2,260 years ago!”*

*Carl Sagan quote from “Cosmos”