Thursday, December 21, 2017
The Ellensburg WA sky for the week of 12/30/17
Saturday: The sky will be giving us a late holiday present in the form of a comet. Comet Heinze (C/2017 T1) will pass closest to Earth on January 4. At its brightest, it will be visible with small telescopes and 50-mm or larger binoculars. Go to https://goo.gl/2iL224 for a finder chart.
Sunday: The next few mornings provide a great example of how the planets move with respect to each other and with respect to the background stars. Every morning at 7 a.m., look about two and a half fists held upright and at arm’s length above the south-southeast horizon. This morning, Jupiter and Mars are a little less than a half a fist from each other with the dimmer Mars to the upper right of Jupiter. As the days go by, they move closer together with Mars “passing” Jupiter next Saturday.
Monday: Today is the day we celebrate the anniversary of something new – a new classification of celestial objects. Giuseppe Piazzi discovered Ceres [pronounced sear’-ease], the first of what are now called “asteroids”, on January 1, 1801. Ceres is the largest asteroid in the belt between Mars and Jupiter. At first, Piazzi thought it was a star that didn’t show up on his charts. But, he noted its position changed with respect to the background stars from night to night. This indicated to him that it had to be orbiting the Sun. The International Astronomical Union promoted Ceres to the status of “dwarf planet” in August of 2006.
Tonight’s Full Moon is a supermoon, the biggest one of the year. That’s because the Moon is full at nearly the same time as it is at its closest point to Earth, called perigee. Read more about the supermoon at http://earthsky.org/?p=270121.
Tuesday: If the Sun looks big today, your eyes are not playing tricks on you. The Earth is at perihelion at about 9 p.m., Pacific Standard Time. 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, about 1.5 million miles closer than its average distance of 93 million miles. 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. For the Northern Hemisphere, that very low angle occurs in December, January and February.
Wednesday: Late tonight and early morning’s weather forecast: showers. Meteor showers, that is. The Quadrantid meteor shower peaks late tonight and early tomorrow morning between midnight and dawn. Meteor showers are named after the constellation from which the meteors appear to originate. That makes this shower mysterious because there isn’t any constellation with this name now. The shower was named after Quadrans Muralis, an obsolete constellation found in some early 19th century star atlases. These meteors appear to come from a point in the modern constellation Draco the dragon. This point is about three fists above the northeast horizon at 1 a.m. This year, the nearly full Moon will obscure the dimmer meteors. Meteors are tiny rocks that hit the Earth and burn up in the atmosphere. Most meteors are associated with the path of a comet. This shower consists of the debris from an asteroid discovered in 2003. Keeping with the comet-origin paradigm, astronomers think the asteroid is actually an “extinct” comet, a comet that lost all of its ice as it passed by the Sun during its many orbits. For more information about the Quadrantid meteor shower, go to http://earthsky.org/?p=155137.
Thursday: Has it been tough to wake up this past week? It should have been because the sunrise has been getting a little later since summer started. I know. I know. December 21 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, however, the interval between sunrise and sunset is the shortest. For more information, go to http://goo.gl/SJC5r.
Friday: You’ve heard the term “a pinch to grow an inch.” Well, Jupiter’s extremely strong gravitational field “pinches” Jupiter so much that it causes Jupiter to shrink by about an inch a year. Look for the svelte Jupiter two and a half fists above the south-southeast horizon at 7 a.m. The diminutive Mars snuggles within a pinky-width to the right of Jupiter.
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.