As we discussed last week, the Moon will be “new” on Wednesday, December 16th. Look for a very slender crescent Moon on the western horizon after sunset on succeding evenings. As I’ve mentioned before I really enjoy chasing very old waning crescent and young waxing crescent Moons. Try spotting the very old Moon 47 hours before “new” on the morning of the 14th or 23 hours before new on the 15th. The 33 hour young Moon on the evening of the 17th will be a real challenge or the 57 hour young Moon on the 18th will be an easier target.
Some observers enjoy the challenge of seeing opposing crescent Moons, that is an Old crescent Moon and Young crescent Moon with the shortest period between. If you are interested in trying this and reporting your results, positive or negative, message me here.
While you’re out watching the young Moon during evenings this week try spotting the elusive Mercury. Mercury and Venus are “inferior” planets, they orbit the Sun inside Earth’s orbit. They appear relatively near the Sun as “evening stars” or “morning stars” from our vantage point here on the “third stone”.
Mercury will reach maximum eastern elongation, 20.3 degrees from the Sun, on 18 December. For observers in the northern hemisphere it will peak at 16.3 degrees above your western horizon at sunset on Tuesday, 15 December. This elongation does not favor observers in the southern hemisphere as Mercury will peak at 12.5 degrees above your western horizon on 22 December. Mercury is coming around toward us from the far side of the Sun and, early on in this apparition, will appear as a tiny gibbous moon half lit on the date of maximum elongation. As Mercury approaches inferior conjunction with the Sun (on 4 January) it will increase in angular size while it’s illuminated portion shrinks to a thin crescent. Use binoculars or a telescope with a low power eyepiece if you have one to view the tiny planet. See the finder chart with the view looking southwest at 5:30 pm local time on Friday, 18 December. An added treat will be the passing of the waxing crescent Moon which will appear just to the west (lower right) of Mercury on the 17th and just east (above left) of him on the 18th. Click on the chart which was generated using the Stellarium planetarium program to enlarge.
Jupiter, Neptune and the Gallilean moons
You’ll need a telescope for this exercise. This week after Mercury has set and darkness is falling look for the other bright “evening star” to the southwest. This is Jupiter. This week Jupiter and Neptune meet for their third conjunction of 2009. Jove will serve as a locator beacon for Neptune which is 6 times further away and much more difficult to spot. Compare Jupiter at magnitude -2.2 and angular size of 36 arc seconds to Neptune at +7.9 and 2.2″ wide. Observing Neptune is a challenge because of its distance. I have observed the planets of our solar system out to Uranus but never seen Neptune. On Sunday, 20 December, Jupiter will appear lass than half a degree south-southeast of Neptune. I suggest that you try to observe them over several evenings and make a sketch of their changing positions from evening to evening. Neptune will glide north to south to the west of Jupiter against the background of fixed stars. Dress warmly and give this a try.
We’re catching up with and will pass Mars in January. The red planet is rising at about 10:30 pm now and rises out of the atmospheric turbulence around local midnight. Look for Mars at this hour above your eastern horizon east of (below and left of) Cancer near the Sickle of Leo. I think the rusty red disc of this planetary interloper stands out very obviously below and to the left of Orion and Gemini. Can you spot it?
Mars will reach opposition on 29 January, 2010, at a distance of 61.7 million miles or nearly twice the distance of the 2003 opposition. This time around its magnitude will be -1.3 and angular size 14.1″ compared to -2.9 and 25.1″ in 2003. I suggest you try observing Mars at least once a week. We’re close enough now that surface details will be revealed when viewed with telescopes of 4 inch aperature or greater. It is also interesting to observe Mars naked eye and watch it as it grows brighter as opposition approaches.
21 December is the winter solstice, the start of winter in the northern hemisphere, summer in the southern. Remember Earth is tilted 23.4 degrees to the plane of its orbit around the Sun. At the December solstice Earth’s south pole is most tilted toward the Sun. On the 21st the northern hemisphere will have the fewest hours of daylight (”shortest” day) and the southern hemisphere the longest. This tilt of Earth’s axis is the reason we have seasons, it has nothing to do with our distance from the Sun. Earth will be at perihelion (nearest the Sun) on 3 January, 2010 at about 0.983 Astronomical Unit or about 2% nearer than average. The most significant influence on our seasonal temperatures is the angle at which sunlight strikes Earth. Seasons are more extreme in the north because more of the southern hemisphere is covered by oceans. Large bodies of water smooth temperature averages.
Intuitively you may think the days are already getting longer. This is because the earliest sunset for the northern hemisphere occurred on 7 December. Because more people see the sunset than sunrise this is a popular misconception. Although the 21st is the “shortest” day in the north, earliest sunset was 7 December and the latest sunrise will be 4 January, 2010
You can get a sense of why the days are shorter by observing where the Sun rises or sets and comparing this position to a sunrise or sunset at the spring equinox next March or the summer solstice in June. Simply go outside at sunrise or sunset and make note of the position of the Sun along your horizon. Compare this to your observations in Spring and Summer. You can also see how low the Sun is above the southern horizon during the early afternoon hours.
This panoramic image is a composite of three pictures I took looking west at sunsets in November, September and June. See how the position of the Sun changes?
The most significant astronomy, science and spaceflight stories of 2009?
I’ll post my annual poll the last week of the month. If you have a suggestion for this list please email me with it and participate in our poll. 2009 has been a year filled with changes, joy and sadness for me. I was reunited with my daughters after more than 30 years without them and lost 3 of my wonderful feline companions, Shelby age 17, Missy 16 and Dianna age 15. I miss them terribly.
I hope your 2010 will be filled with wonderful days and Clear Skies!
Mark, the StarGeezer