Skywatch for the week of June 21, 2021
Mon June 21, 2021 SUMMER BEGINS
Summer began last night, June 20th, at 11:32 pm Eastern Daylight Time. It was at that precise moment that the sun was shining directly overhead at local noon, not here in Florida obviously, but as seen from a point on the Tropic of Cancer at twenty-three and a half degrees North latitude, over in southeast Asia. When it’s local noon here, the sun will be as high in the sky as possible for our latitude. Along this part of the Treasure Coast, we’re at 27½ degrees North latitude, so at midday today the sun will be about 4 degrees south of our zenith. This is the summer solstice, as the sun stops its northerly progression, due to the inclined tilt of the earth’s axis as it revolves about the sun; sol/stice – sun stop. It also marks the longest period of daylight and the shortest period of night in the year, at least in Earth’s northern hemisphere. In the southern hemisphere, winter has begun. But for those of us in the US, for the next six months the sun’s altitude at noon will drop and then we’ll be at the winter solstice.
Tue June 22, 2021 WAXING GIBBOUS MOON
The waxing new gibbous moon is well up in the southeast after sunset today. When the moon seems to grow larger night by night, we say that it is waxing. This is an old colonial term. Anyone who's ever tried to make candles knows that when you "wax the candle", you dip the candle wick into the molten wax; with repeated dippings, the waxed candle grows larger. To colonials who made these candles, it seemed as though the moon too was being dipped in wax, growing larger with each night. Today the moon is egg-shaped, or gibbous. Gibbous is a great crossword puzzle word. Its Latin origin, “gebbosus,” means “humpbacked,” and so the gibbous phase of the moon runs from just past the half moon up to full. In order to find the new gibbous moon today, you can begin by looking for it in the afternoon, starting after 4 o'clock, over in the eastern sky. As day gives way to sunset and twilight, the moon will stand out in greater contrast with the darkening sky. In two more days the moon will be full.
Wed June 23, 2021 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 year 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.
Thu June 24, 2021 JUNE FULL MOON
The moon is full and in the constellation Sagittarius the Archer, and tonight it marks a point in the heavens that is where the center of our galaxy lies. Now the moon’s only a couple of hundred thousand miles away, and the center of the Milky Way is roughly 30,000 light years, or nearly 200,000 trillion miles beyond the moon. Still, it’s nice to have a perspective on these things. The names for the June full moon are many: according to the Ponca Indians, this is the Hot Weather begins Moon – no argument there. Back in Europe, this was the Rose Moon, so named for the pink color of this full moon, which rides low in the southern sky. The Omaha Indians call this the Moon When Buffalo Bulls Hunt the Cows; to the Tewa Pueblo it’s the Moon When the Leaves are Dark Green. The Winnebago call this the Corn Tasseling Moon, while the Sioux regard it as the Moon of Making Fat. But to the Objiwa Indians, this is the Lovers' Moon, named for En-a-ban'dang the dreamer and A-nou-gons', or Little Star, who first met when the full moon rose.
Fri June 25, 2021 ERATOSTHENES MEASURES THE EARTH
In late June, in the year 240 BC, the 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”