This Week’s Sky at a Glance, March 17 – 24 ~by Curt Nason
With this weekend being party time for many O’Revelers, is there anything green that we can see in the sky? Yes, but rarely. We can see stars that are red, orange, yellow, blue or white, but not green. The colours are representative of their outer temperature, with red being coolest and blue the hottest. Any star with an outer temperature corresponding to green, which is in the middle wavelengths of the visible spectrum, emits approximately equal but lesser amounts of red and blue light. This combination gives us white light, and our Sun is such a star.
Some stargazers have claimed to see green stars that are part of a binary pair with a red giant star. Green is the complementary colour of red, and it is thought that if you observe a white star after staring at a red one, the complementary after-image can make the white star look green. I tested this by looking at a dim red light in a darkened room for a minute and then I switched on the incandescent light. It had a green tinge. It is said that Zubeneshamali, the brightest star in Libra and the one with the longest common name, is green. It might have been the power of suggestion, but I did see it as a very pale green in an 8-inch telescope.
Some people have seen the Sun (aka Sol, the shortest name for a star) flash green just before setting, and usually over water under steady atmospheric conditions. The most common reason for green in the sky, although still fairly rare in New Brunswick, is the northern lights. Energetic electrons from the Sun can make oxygen atoms in our upper atmosphere emit green light in a manner similar to that of a neon light. Northern lights are seen more frequently around the equinoxes, and if electrons have escaped the Sun through holes in its magnetic field lines we could get lucky this weekend. If not, then take a break from the partying to look up at the constellation O’Ryan.
This Week in the Solar System
Saturday’s sunrise in Moncton is at 7:28 am and sunset will occur at 7:27 pm, giving 11 hours, 59 minutes of daylight (7:33 am and 7:32 pm in Saint John). Next Saturday the Sun will rise at 7:14 am and set at 7:37 pm, giving 12 hours, 23 minutes of daylight (7:19 am and 7:41 pm in Saint John). The Sun crosses the equator heading north on Tuesday at 1:15 pm, beginning our spring season. Notice that March 17 is the day when we are closest to having 12 hours of daylight, rather than on the equinox as many people believe.
The Moon is at third quarter on March 17, making a great weekend for the Messier Marathon. On Sunday in evening twilight the slim crescent Moon anchors a line-up above the western horizon, with Venus a few degrees to its upper right and Mercury an equal distance beyond. Next Saturday we get to observe the first quarter Moon during Earth Hour. Start your Monday morning with binocular observing before twilight. Mars lies between the Lagoon Nebula (M8) and the Trifid Nebula (M20), and Saturn is just north of the splendid globular cluster M22 to the left of the Sagittarius Teapot lid. Jupiter is rising before midnight again by midweek.
The provincial astronomy club, RASC NB, meets this Saturday at 1 pm in the Moncton High School. All are welcome.
Questions? Contact Curt Nason.