The Ellensburg, WA sky for the week of September 23, 2023

Saturday: “There’s water in them thar craters”, frozen water, that is. There has been speculation since the 1960s and indirect evidence since the 2000s of water on the Moon. Three years ago, astronomers studied data from four earlier missions. They noticed that the light reflecting off the bottom of craters near the lunar South Pole showed characteristics of light reflecting off pure ice. The water likely came from comet impacts or other solar system objects with trace amounts of water ice. This past summer, the Indian Space Research Organization became the fourth country to successfully land on the moon when Chandrayaan-3 landed near the lunar South Pole to study the water there. Read more about it at https://www.space.com/chandrayaan-3-moon-south-pole-why-nasa-wants-to-go-too.

Sunday: Saturn is nearly three fists held upright and at arm’s length above the south-southeastern horizon at 10:00 pm. Jupiter is about one and a half fists above the eastern horizon.

Monday: Ask someone which day in September has the same duration day and night. Go ahead, ask someone. Why are you still reading this? I can wait. If that person says the first day of autumn, they are wrong. Today, three days after the first day of autumn, is the date in which day and night are closest in duration. There are two main reasons for this. First, the atmosphere acts like a lens, bending light from the Sun above the horizon when the Sun is actually below the horizon. This makes the Sun appear to rise before it actually rises and appear to set after it actually sets. Second, fall starts when the center of the Sun passes through the point called the autumnal equinox. But, the Sun is not a point. The upper edge of the Sun rises about a minute before the center of the Sun and the lower edge sets a minute after the center of the Sun. Thus, even if we didn’t have an atmosphere that bends the sunlight, daytime on the first day of autumn would still be longer than 12 hours.

Tuesday: To the people of Ancient Greece, the stars that are about five and a half fists above the northeastern and eastern horizons, respectively, at 10:00 p.m. were known as Cassiopeia and Andromeda, a mythological queen and her daughter. But not all cultures imagined the same pictures in the sky. To the people of Polynesia, the stars of Cassiopeia and Andromeda represented a dolphin, called Kwu. Cassiopeia formed its tail, the brightest stars of Andromeda formed its fins, and its fainter stars outlined the dolphin’s body.

Wednesday: You discovered Cassiopeia last night. The astronomer Caroline Herschel discovered an open star cluster that looks like a rose over 200 years ago. This cluster, called Caroline’s Rose, is about 6,500 light years away and consists of about 1,000 stars that are one third the age of the Sun. Through binoculars, it looks like a fuzzy patch. At 10:00 p.m., find the star at the top of the sideways “W” known as Cassiopeia. When that star is in the lower left portion of your field of view, Caroline’s Rose is in the center to upper right. For more information about Caroline’s Rose, go to https://stardate.org/radio/program/2020-09-09.

Thursday: Mnemonics are helpful for remembering astronomy facts. (Similarly, “Johnny Mnemonic”, the 1995 cyberpunk film, was helpful in getting Keanu Reeves’ career going.) After all, school children around the world are learning the order of the planets by remembering, “My very excellent mother just served us nine….” Oops, I guess that one needs updating. Well, here’s one that will not need updating for tens of thousands of years: the order of the stars in the Big Dipper. Because the nighttime stars are so far away from us, their actual motion through the sky, called their “proper motion” is not noticeable over even thousands of years. That is why the constellations have remained the same since ancient times. But two stars in the Big Dipper have a proper motion large enough such that in 100,000 years; the stars will no longer make a dipper shape. You can see this simulation at the American Museum of Natural History video found at https://youtu.be/sBfUBtdo8yo. Until then, you can remember the names of the seven dipper stars in order from handle to cup by remembering this helpful advice for teens: “AM, ask mom. PM, dad”. The stars are Alkaid, Mizar, Alioth, Megrez, Phad, Merek, and Duhbe. The Big Dipper is one fist held upright and at arm’s length above the northern horizon at 11:00 p.m. 

Friday: At 6:00 a.m., Venus is nearly three fists and Mercury is about a half a fist above the eastern horizon.

The positional information in this column about stars and planets is typically accurate for the entire week. For up to date information about the night sky, go to https://nightsky.jpl.nasa.gov/planner.cfm.