The Ellensburg WA sky for the week of 6/23/18

Saturday:  Jupiter is about a half a fist to the lower right of the Moon at 10 p.m. tonight. More interesting is that "star" you've noticed below Jupiter for the past few weeks. Now it is about a thumb thickness to the lower left of Jupiter. That star is Zubenelgenubi, the second brightest star in the constellation Libra. The name means "southern claw", a holdover from the time when this part of the sky was associated with the neighboring constellation of Scorpius the scorpion. Notice that the word "star" was in quotes earlier. That's because Zubenelgenubi is a binary star system, easily seen with binoculars as a white and yellow pair. To a person living on a planet orbiting the dimmer of the two stars, the brighter star would be nearly as bright as the full Moon appears from Earth. 

Sunday: “Mom, I can’t sleep. It is too light out!” A poor excuse you say? Good astronomy skills, I say. The latest sunset of the year happens this week. Surprisingly, the earliest sunrise and the latest sunset do not both happen on the longest day of the year, the day of the summer solstice. The earliest sunrise occurs just before the longest day and the latest sunset occurs just after the longest day. This phenomenon relates to the angle of the Sun’s path near rising and setting. In Ellensburg, that angle is about 66 degrees near the first day of summer. Because of the Earth’s orbit, which causes the Sun’s apparent motion, the angles are not symmetric. The asymmetries in orbital angles leads to the asymmetry in rise and set times. By the way, picking a specific night to give you the “can’t sleep because it is too light out” line may just be an excuse because the sunset times change by only a few seconds each day in June. This year, the sun sets between 9:01 and 9:02 p.m. between June 22 and June 29. 

Monday: Venus is one and a half fists and Mercury is a half a fist above the west-northwest horizon at 9:45 p.m. 

Tuesday: Saturn is in opposition tonight. That doesn’t mean that Saturn is in the minority party. Opposition means that Saturn is on the opposite side of the Earth as the Sun. When an object is in opposition, it is at its highest point in the sky during the darkest time of the day. Thus, opposition is typically the best time to observe a planet. Saturn is about two fists above due south at 1 a.m. It is about one fist above the southeast horizon at 10 p.m. Careful readers of this column must struggle through bad jokes and sentences that... that... ah... ain't too good. But they also may recall that Saturn was in opposition on nearly the same date last year: June 14. An outer planet is in opposition when the Earth passes it up as both orbit the Sun. The farther out a planet is, the less it has moved along its orbit, and the closer it is to exactly one year from one Earth passing to the next. For comparison, it is about 18 months between successive oppositions for Mars. 

Wednesday: Don’t wait until a week from today to go to those wimpy firecracker shows. Find the hypergiant star Rho Cassiopeiae. Astronomers think that Rho Cassiopeiae will likely go supernova (explode) in the near future. Of course, for stars, near future might mean today. It might mean 20,000 years from now. Rho Cassiopeiae is in the constellation Cassiopeia the queen. At 11:00 tonight, Cassiopeia looks like the letter “W” about three fists held upright and at arm’s length above the northeast horizon. Rho Cassiopeiae is about a finger’s width to the right of the rightmost star in the “W”. Once you find it you’ll be thinking, “Big deal, I can hardly see it.” Although it is barely visible to the naked eye, it is actually very bright. It is the 20th most luminous star in the sky, a whopping 550,000 times more luminous than the Sun. 

Thursday: Mars is a half a fist above the southeast horizon at midnight. 

Friday: Star light. Star bright. The first star you see tonight might be Arcturus, six fists above the south horizon right after sunset. You’ll be able to see Jupiter and Venus earlier. But those won’t help your wish to come true. 

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.