Tuesday, September 14, 2010

The Ellensburg sky for the week of 9/18/10

Saturday: Last Sunday, I gave you a very brief overview of how to use the Big Dipper as a clock. But, my explanation was helpful only for a late evening in the autumn or spring. Some of you go out other times of the year and need a way to tell time then. First, find the two stars at the far end of the Big Dipper cup, the stars that do not touch the handle. Draw an imaginary line segment starting at the North Star and passing through the two Big Dipper cup stars. Now, draw a big circle around the North Star. Your circle is a 24-hour clock. Number the circle from 0 hours at the top, counterclockwise to 12 hours at the bottom of the circle, and back up to 24 hours at the top. (O hours and 24 hours are the same on this clock because the day is 24 hours long.) The hour number on the big circle closest to where your imaginary line intersects this circle is called your raw time. Due to the location of the Big Dipper compared to the rest of the stars, the time nearest the intersection (the raw time) is correct for March 6. For any other night, subtract two times the number of months the current date is after March 6 from the raw time. For example, let’s say the imaginary line between the North Star and the Dipper stars is pointed to the right. That means the raw time is 18 hours or 6 p.m. If you made this observation on October 6, which is seven months after March 6, you would subtract two times seven or 14 hours from the raw time. Thus, the time for November 6 is 18 hours minus 14 hours or 4 hours. In other words, 4 a.m. Don’t forget to convert for daylight savings time if needed. For a more complete set of instructions, go to http://prdupl02.ynet.co.il/ForumFiles_2/24505461.pdf. There is a simple “star clock” template and instructions at http://www.skyandtelescope.com/letsgo/familyfun/Make_a_Star_Clock.html. Use this paper star clock whenever you watch is broken.

Sunday: Let me tell you the story of the ghostly white figure that rises early in the morning around Halloween. It appears to be a huge dim glow of white light that rises up from the east in the pre-dawn sky. No, I’m not writing about the ROTC student who has her first early morning physical training. I’m describing an effect called the zodiacal light. This light comes from sunlight reflecting off dust grains in our solar system. The effect is the most visible when the band of constellations called the zodiac makes a steep angle with the horizon. You need a clear sky with no haze or light pollution to see the zodiacal light which will be visible for the next week or so. At its brightest, the zodiacal light rivals the light of the central Milky Way.

Monday: Mercury is about a half a fist above the east horizon at 6 a.m.

Tuesday: Jupiter and Uranus are both in opposition tonight. No, that doesn’t mean they will disagree with everything you say. (You: “My, the sky is a pretty blue today.” Jupiter: “No, it’s a pretty yellow.” Uranus: “No, it’s a pretty red.”) Opposition means that a planet is on the opposite side of the Earth as the Sun. An object is in opposition when it is due south 12 hours after the Sun. When an object is in opposition, it is at its highest point in the sky during the darkest time of the night. A planet in opposition shines brighter and appears larger in a telescope than any other night of its orbital cycle. Jupiter and Uranus are four fists above due south at 1 a.m. daylight savings time which is midnight standard time. If you’d rather not stay up so late,

Wednesday: At precisely 8:13 p.m. the center of the Sun crosses the celestial equator and passes into the southern sky. The celestial equator is an imaginary line that divides the sky into a northern and southern half. When the Sun is in the southern half of the sky, it appears to take a shorter path from rising to setting. It also does not get as high in the sky at noon. This leads to shorter days and longer nights. Since the Sun crosses the celestial equator today, there is an instant when it is equally in the northern and southern sky, called the north and south celestial hemispheres. This so-called “equal night” is given by the Latin word equinox. Thus, today is known as the Autumnal Equinox. However, the day and night are not of equal duration today. The sun rises at 6:45 a.m. and sets at 7:03 p.m. Day and night are of equal duration next Monday.

Thursday: If Venus was not the brightest planet, you’d have no hope of seeing it with the naked eye for a few weeks. Look for it this week just above the southwest horizon within 45 minutes after sunset. By next week, it will be lost in the glare of the Sun.

Friday: Christmas Eve is three months away which means it is time to start making your wish list of gifts and recipients. Perhaps someone on your list (maybe you?) wants a telescope. The Sky & Telescope website has a good article on choosing your first telescope. Thanks to improved materials, you can buy a high quality starter scope for under $250. People never outgrow their first small telescope if it is well made. Even people who own giant telescopes or have their own observatory sometimes only want to spend five minutes setting up a telescope to show the neighbor kids Jupiter’s moons. Go to http://www.skyandtelescope.com/equipment/basics/12511616.html for more information.

The positional information in this column about stars and planets is typically accurate for the entire week.

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