Wednesday, September 23, 2015
The Ellensburg WA sky for the week of 9/26/15
Saturday: Did you time the exact length of the day and night on Monday, the first day of autumn? They were not equal in duration. Many people think that the day and night are the same duration on the autumnal equinox. The day is a little longer than the night for two reasons. First, the Sun is an extended object so even when the lower half has set, the upper half is still above the horizon lighting the sky. The second, and more influential, reason is that the atmosphere acts like a lens, bending light from the Sun above the horizon when the Sun is actually below the horizon. Day and night are closest in duration today.
Sunday: Full Moon. Harvest Moon. Super Moon. Blood Moon. These sound like possible names for the children of The Who’s original drummer. In actuality, these describe the moon tonight. Let’s take these names from least to most exciting. Tonight’s moon is full. It is the full moon closest to the Autumnal Equinox (also known as the first day of fall). Before the invention of artificial lighting, farmers used the full moon to illuminate the fields so they could gather crops at night. Since the full moon rises as the Sun is setting, they would get an uninterrupted light source for the harvest. Farmers would have loved tonight’s moon because it is a Super Moon. The Moon is at its closest point to the Earth during the time that it is full. That means the sunlight is reflecting off of an object that appears about 30% larger than when the moon is farthest from the Earth.
But most exciting of all, there is a total lunar eclipse tonight. Total lunar eclipses are not as noticeable as total solar eclipses because light still reaches the Moon even when it is completely blocked by the earth. That is because the Earth’s atmosphere acts like a lens and bends rays of light that would normally miss the moon towards the moon. That doesn’t mean the moon looks the same during a total lunar eclipse as it does during a normal full moon.
Sunlight is white. White light is the sum of all of the colors in the visible spectrum (red, orange, yellow, green, blue, indigo and violet). Our atmosphere scatters the blue component of the Sun’s white light. That is why our sky is blue. (If our atmosphere consisted of different gasses, we would likely have a different colored sky.) When the Sun or moon is near the horizon, the light passes through a lot of the atmosphere meaning a lot of the blue light is scattered and the Sun or moon looks redder than when it is high in the sky. During a total lunar eclipse, sunlight passes through a large slice of the Earth’s atmosphere. The remaining light that reaches the moon is reddish. Some people say the fully eclipsed moon looks Blood Red! These people exaggerate. It arrears a dull reddish color.
From our perspective in central Washington, the moon will be in the partial eclipse stage at sunset. The moon will slowly move into the Earth’s shadow and get dark from left to right. At 7:11 p.m., the moon will be fully eclipsed. The total eclipse lasts until 8:23 p.m. The moon will be moving out of the earth’s darkest shadow or umbra until 9:27 p.m. After that, the moon will look white, just like a normal full moon. Thus, during the entire eclipse, the moon looks white, then black, then red all over. For more information about the eclipse, go to http://goo.gl/rVMevQ.
Monday: Venus, Mars, and Jupiter form a crooked line in the sky at 6 a.m. this morning. Venus is two and a half fists held upright and at arm’s length, Mars is two fists, and Jupiter is just over one fist above the east horizon.
Tuesday: Last week you sailed over Titan’s lakes. Now dream about Enceladus’ ocean. Wow, that’s a lot of liquid in the Saturn moon system. Titan’s lakes are made of methane. But astronomers think there is a liquid water ocean beneath the icy surface of Enceladus. Read all about it at http://goo.gl/6E0sHp. Saturn and its moons are two and a half fists above due southeast at 7:30 p.m.
Wednesday: Aldebaran, the bright orangish star in the constellation Taurus, is one fist above the east horizon at 11 p.m.
Thursday: To the people of Ancient Greece, the stars that are about five and a half fists above the east-northeast horizon at 10 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.
Friday: Vega, the bright bluish star in the constellation Lyra, is five and a half fists above the west horizon at 10 p.m.
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.