Thursday, January 1, 2009

The Ellensburg Sky for the week of 1/3/09

Saturday: Tomorrow morning, 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 evening sky, it is east of the Sun. Thus, this evening’s elongation is known as the greatest eastern elongation. (If you care to remember this in general, remember both eastern and evening start with the letter "e".) Tonight and tomorrow will be the best nights to observe Mercury for the next few months. Mercury is about a half a fist held upright and at arm’s length above the southwest horizon at 5:15 p.m. Over the next two weeks, Mercury will toward the Sun in the sky. By late January, it will be visible in the morning sky.

Sunday: If the Sun looks big today, your eyes are not playing tricks on you. The Earth is at perihelion at about 7 a.m. If you dig out your Greek language textbook, you’ll see that peri- means “in close proximity” and helios means “Sun”. So, perihelion is when an object is closest to the Sun in its orbit. Since it is winter in the Northern Hemisphere now, the seasonal temperature changes must not be caused by the Earth getting farther from and closer to the Sun. Otherwise, we’d have summer when the Earth is closest to the Sun. The seasons are caused by the angle of the sunlight hitting the Earth. In the winter, sunlight hits the Earth at a very low angle, an angle far from perpendicular or straight up and down. This means that a given “bundle” of sunlight is spread out over a large area and does not warm the surface as much as the same bundle in the summer.

Monday: Has it been tougher to wake up this past week? It should have been because the sunrise has been getting later as recently as this past weekend. December 22 was the shortest day of the year. But, because the Earth’s orbit around the Sun is elliptical and not circular, the Earth does not travel at a constant speed. It moves faster when it is closer to the Sun and slower when it is farther away. This leads to the latest sunrise occurring in early January and the earliest sunset occurring in early December, not on the first day of winter, the shortest day of the year. On the first day of winter, the interval between sunrise and sunset is the shortest.

Tuesday: This month’s “Hot Topic” for the International Year of Astronomy is Telescopes and Space Probes. Observational astronomy has made significant advances since 1609 when Galileo turned his simple telescope to the sky to discover Jupiter’s four largest moons, the phases of Venus, and the individual stars of the Milky Way. In 2009, astronomers rely on space probes and telescopes to make discoveries that approach the significance of Galileo’s discoveries. Go to for more information about these modern day Starry Messengers.

Wednesday: Venus is two fists above the southwest horizon at 6 p.m. Go to to learn more about Venus.

Thursday: Sirius, the brightest star in the nighttime sky, rises at about 6:40 p.m. By 8:30 p.m., it is about a fist and a half above due southeast.

Friday: Jupiter is just barely hanging out in the evening sky. It is less than a half a fist above the southwest horizon at 5 p.m. Say “good bye” to Jupiter for a few weeks as it gets lost in the glare of the Sun.

The positional information in this column about stars and any planet except Mercury is accurate for the entire week.

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