Saturday: You know Metis and Thebe and Adrastea and Amalthea. Io and Ganymede and Callisto and Europa. But do you recall? There are 79 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 79 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. Recently, Saturn moved into first in the moon race with 82. Uranus is next with 27. Then comes Neptune with 14, 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.
Tonight, Mars is about a finger-width to the left of our own Moon, three fists held upright and at arm’s length above due west at 9:00 p.m.
Sunday: Mercury 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 Mercury is in the evening sky, it is east of the Sun so this occurrence is called the greatest eastern elongation. This evening will be the best evening to observe Mercury for the next few weeks. Mercury is nearly a fist and a half above the west-northwestern horizon at 9:00 p.m. Over the next few weeks, Mercury will move toward the Sun in the sky. By July, it will be barely visible in the morning sky. Venus is to the lower right of Mercury… and much brighter.
Monday: Are you thirsty? I'll wait while you get some water. I will NOT wait while Corvus the crow gets you some water. The Greco-Roman god Apollo made this mistake. He sent Corvus the crow to get some water in the cup known as Crater. Some figs distracted Corvus and he waited for them to ripen so he could eat them. When Corvus got back late, Apollo put Corvus and Crater in the sky with the gently tipping cup just out of the reach of the perpetually thirsty crow. Corvus is a trapezoid-shaped constellation about two fists above due south at 10:00 p.m. Crater is just to the right of Corvus.
Tuesday: You can set your watch tonight by carefully observing Navi, the center star in the W-shaped Cassiopeia. It will be due north at exactly 10:12 p.m. However, another star in Cassiopeia is causing astronomers to doubt whether or not they can use neutron stars as the most precise known clocks in the universe. Neutron stars have such a precise spin rate that they are used to set super accurate clocks on Earth. In 2013, astronomers using NASA’s Swift x-ray telescope noticed that the neutron star called 1E 2259+586 exhibited a spin glitch that had never been seen before. The spin rate of about eight times a minute decreased by 2.2 millionths of a second. Read more about this at http://goo.gl/C4V8R1. In 2016, astronomers using NASA’s Swift x-ray telescope observed the slowest rotating neutron star, once every 6.5 hours. What is it with using a telescope named Swift to make discoveries about something slowing? Maybe that slow developing question will be answered at https://tinyurl.com/y6ag6g7c.
Wednesday: Are you thirsty when you get up in the morning? I know you are not waiting for Corvus. That’s okay because the Big Dipper is positioned to hold water in the morning sky. Look three fists above the northwest horizon at 4:30 a.m. You’ll see three stars that make a bent handle and four stars that make a cup.
Thursday: At 4:30 a.m bright Jupiter is two fists above due southeast. Saturn is about two fists to the upper right of Jupiter.
Friday: Are you up at 2:23 a.m., looking due north and thinking you see a UFO coming to take you away? That's no UFO. It's the bright star Capella, a circumpolar star that never goes below the horizon as viewed from Ellensburg.
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.
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