Friday, June 19, 2015
The Ellensburg WA sky for the week of 6/20/15
Saturday: Jupiter, Venus, and the Moon crowd the western sky just after sunset. Venus, the brightest point of light in the night sky is to the right of the Moon. Jupiter, the second brightest point of light is between the two.
Sunday: Today is the first day of summer, the day that the Sun reaches its highest declination (the official name for sky latitude) of 23.5 degrees above the celestial equator. The celestial equator is the line that divides the northern sky from the southern sky. In Ellensburg, the Sun is about seven fists held upright and at arm’s length above the south horizon at 1:00 p.m. (noon standard time). Contrary to popular belief, the Sun is never straight overhead in Ellensburg or anywhere else in the 48 contiguous states. The northernmost portion of the world where the Sun can be directly overhead is 23.5 degrees north latitude. In ancient times, the Sun was in the constellation Cancer the crab on the first day of summer. Hence, 23.5 degrees north latitude has the nickname "Tropic of Cancer". Because the Earth wobbles like a spinning top, the Sun's apparent path through the sky changes slightly over time. Now, the Sun is in the constellation Taurus the bull on the first day of summer. However, citing the high cost of revising all of the science books, geographers are not changing the name of 23.5 degrees north latitude to "Tropic of Taurus". The first day of summer is often called the summer solstice. However, astronomers refer to the summer solstice as the point in the sky in which the Sun is at its highest point above the celestial equator. Thus, summer starts when the Sun is at the summer solstice point. This year, that happens at 9:38 a.m.
Monday: Constellation light, constellation bright. The first constellation I see tonight. I wish I may, I wish I might, recognize these stars in ten on nights. The orientations of the stars provide an effective backdrop to help us remember stories because these orientations are relatively unchanging. Stars are so far away that their positions do not change over thousands or, in some cases, millions of years. But a few bright stars are close enough or do move fast enough to change the shapes. For example, in about 30,000 years, the Little Dipper will no longer hold water. (Luckily, it comes with a lifetime warranty.) For more information about the future of some of your favorite constellations, go to http://goo.gl/qP2BR3.
Tuesday: “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 late 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 21 and July 3.
Wednesday: Tonight's first quarter Moon is in the constellation Virgo the goddess of the harvest.
Thursday: Don’t wait until the 4th of July to go to those wimpy firecracker shows. Find the hypergiant star Rho . Astronomers think that Rho 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 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 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.
Friday: Mizar is a well-known binary star in the constellation Major. You can find it at the bend in the Big Dipper handle, nearly straight overhead at 10 p.m. tonight. Its name is Arabic for waistband. Mizar has an optical double called , which is less than a pinky width away and can easily be seen with the naked eye. Optical doubles are stars that are close together in the sky but do not orbit a common center of mass as true binary stars. Not wanting to deceive sky gazers who call Mizar a binary star, two stars that DO orbit a common center of mass, Mizar actually is a binary. It was the first binary star system discovered by telescope. Mizar A and Mizar B are about 400 astronomical units apart from each other and about 80 light years from Earth. 400 astronomical units is about 10 times the distance between the Sun and Pluto.
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 http://nightsky.jpl.nasa.gov/planner.cfm.