The Ellensburg, WA sky for the week of May 31, 2025

Saturday: You know Metis and Thebe and Adrastea and Amalthea. Io and Ganymede and Callisto and Europa. But do you recall? There are 97 Jovian moons in all. Just 60 years ago, Jupiter was thought to have only 12 moons. But astronomers are red nosed with delight that the advent of supersensitive electronic cameras has caused the number of discovered moons to rapidly increase. Jupiter’s 97 moons range in size from Ganymede, the largest in the Solar System with a diameter of 5,262 kilometers, to numerous moons with diameters of only one kilometer. In the past few years, Saturn moved to first place in the moon race with 274 moons. Uranus follows Jupiter with 28. Then comes Neptune with 16, Mars with 2, and Earth with 1. Our moon is the fifth largest in the Solar System, with a diameter of 3,475 kilometers. (One kilometer is 0.62 miles.) Even dwarf planets have moons. Pluto has 5, Eris has 1, Haumea has 2, and Makemake has 1. Eris is an outer solar system object that was discovered in 2005 and named in September of 2006. Because astronomers thought it was larger than Pluto, people called it the tenth planet for a while. (More recent measurements show Eris to be a little smaller in diameter than Pluto.) Haumea, the newest dwarf planet with a moon, was discovered in 2004 and officially named a dwarf planet on September 17, 2008. Go to https://solarsystem.nasa.gov/moons/ for more information about Solar System moons. You’ll have to wait until tonight to see our moon and to see some of the other Solar System planets that have moons.

Sunday: Venus will be as far away from the Sun in the sky as it will get this orbital cycle. This "farthest away" point is known as the planet's greatest elongation. Since Venus is in the morning sky, it is west of the Sun and this occurrence is called the greatest western elongation. This morning, Venus is one fist above due east at 4:30 a.m. Over the next few months, Venus will move toward the Sun in the sky. By late February, it will be visible in the evening sky. While you are up at this time, look to Saturn at two fists above the east-southeastern horizon.

Monday: It looks so peaceful up in the night sky. But the sky is not peaceful for Jupiter. According to a recent study by astronomers, Jupiter gets hit by a 5-20 meter diameter asteroid 10 to 65 times a year. For comparison, the object that exploded over the Russian city of Chelyabinsk in 2013 was 20 meters in diameter. Earth gets hit by a 20-meter asteroid about once every 50 years. Jupiter is a half a fist above the northwestern horizon at 9:30 p.m.

Tuesday: Summer is nearly here. How do I know? Because the days are very long. Because the temperature is rising. Because the school year is ending. Also, because the Summer Triangle is fairly high in the eastern sky at 11:30 p.m. Vega, the third brightest star visible from Ellensburg, is about five fists above the eastern horizon. Deneb, at the tail of Cygnus the swan is about three and a half fists above the east-northeast horizon. The third star in the triangle, Altair in Aquila the eagle, is two fists above the eastern horizon.

If you want to put somebody off, tell them to wait until Deneb sets. At Ellensburg’s latitude of 47 degrees, Deneb is a circumpolar star meaning it never goes below the horizon.

Wednesday: Mars is three fists above the western horizon at 10:00 p.m.

Thursday: Antares is one and a half a fists above the south-southeastern horizon at 11:30 p.m.

Friday: The bright star Capella is a half a fist above the north-northwestern horizon at 11:30 p.m. Interestingly enough, it is only about two fists from the Sun. If you viewed Capella from Ketchikan, the southernmost city in Alaska, Capella would be one and a half fists above the horizon. The Sun would be close enough to the horizon that there would be a twilight glow.

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. All times are Pacific Time unless noted.