The Sky This Week, 2012 April 10 - 17
Saturn, showing the "Seeliger Opposition Effect"
The Moon wanes as she hugs the southern reaches of the ecliptic in the morning sky this week. Last Quarter occurs on the 13th at 6:50 am Eastern Daylight Time. Luna begins the week above the teapot-shaped asterism formed by the brightest stars in the constellation of Sagittarius. She spends the rest of the week drifting though the star-poor reaches of the rising autumnal constellations of Capricornus and Aquarius. She charts a lonely course this week, passing no objects of any substantial brightness.
April 11th through the 20th are the nights set aside for the final campaign in 2012’s "Globe At Night" citizen-science program to encourage people to make simple observations of their local skies and report their results for scientific evaluation. The idea is quite simple: locate and observe the constellation of Leo, which is on the meridian at around 10:30 pm. Ignoring the bright planet Mars, located in Leo at this time, count the number of stars you see in the vicinity of the constellation and report your findings on the program’s website. In the previous two campaigns in February and March nearly 10,000 respondents sent in observations. The program hopes to hit a milestone of 15,000 observations by the end of the current campaign. If you observe from different locations report your findings for each one. Your results will help astronomers measure the brightness of the sky from many diverse locations, giving us a measure of the impact of human activity on the nighttime sky. Globe At Night is just one of many activities ongoing in April, which has been named as Global Astronomy Month. This international program will wrap up with activities planned for Astronomy Day on the 28th. More details of local activities for Astronomy Day will be given here on the 24th.
The early evening sky still hosts the two brightest planets, but one of them is now fading fast. Giant Jupiter hangs low in the west after sunset, and by the 16th he sets at the end of evening astronomical twilight. Old Jove has been a fixture in the sky since the late fall, and he will soon appear to pass behind the Sun in mid-May, emerging into morning twilight by mid-summer. We will once again enjoy his cheery glow in the evening sky as wintertime approaches.
Brilliant Venus continues to dominate the evening sky, and this week she is at her absolute best. She sets at her latest time for most of the week, remaining above the horizon until nearly midnight. She is also shining at her most brilliant level now, bright enough to cast shadows at dark-sky locations. In another two weeks she will begin a precipitous fall from the evening sky, leading up to an extremely rare transit across the Sun’s disc on June 5th.
Mars reaches the second stationary point in this year’s apparition on the 15th. At this time he stops his apparent westward motion toward the bright star Regulus in Leo and begins to slowly accelerate in an eastward direction. He closes to just over four degrees from the star on the 15th, and by the end of the month tacks over a degree of separation from that closest approach. The planet’s apparent diameter is beginning to shrink rapidly as well, much to the consternation of small telescope owners. His disc has lost over two arcseconds in size and his brightness has faded by almost a full magnitude from his opposition in early March.
However, where Mars fades Saturn emerges. The ringed planet reaches opposition on the 15th, rising at sunset and setting at sunrise. He’s easy to spot in the southeastern sky as twilight fades, forming a wide pair of first-magnitude objects with the bright star Spica in the constellation of Virgo. If you have a small telescope, try to observe Saturn on the nights before, during, and after opposition. You may notice a curious phenomenon known as the Seeliger Opposition Effect where the rings become noticeably brighter than the disc of the planet and the fainter moons become a bit easier to see. This brightening is caused by a very low phase angle of sunlight falling on the rough surfaces of these bodies. At these extreme angles there are no projected shadows along out line of sight, so the objects are as fully illuminated as they can get. My experience has been that these effects are very fleeting, but they are quite eye-catching.