Skywatch Line for Friday, March 17, through Sunday, March 19, 2023

This is Dudley Observatory’s Skywatch Line for Friday, March 17, through Sunday, March 19, written by Sam Salem.

On Friday, Sun rises at 7:04am and sets at 7:04pm; Moon rises at 5:04am and sets at 1:56pm. The day and the night are almost equal on Friday (daylength is 11 hours, 59 minutes, and 36 seconds in Schenectady). However, Equinox will not arrive until Monday. Equilux is the word used to describe the day on which day and night are equal. The Equilux happens a few days before the Spring equinox and a few days after the Autumn equinox. The exact date of an equal day and night varies with latitude, much as earliest sunrises and latest sunsets. That’s in contrast to the equinox itself, which is a whole-Earth event, happening at the same instant worldwide.

Venus and Jupiter shine in the west at dusk. Venus is the brightest at magnitude –3.9. Jupiter, at magnitude –2.1, is one fifth as bright. As Jupiter gets lower, it’s increasingly dimmed by atmospheric extinction, and apparent “dimming” by being seen on a brighter sky background. The two planets get farther apart every day. On Friday they’re separated by 15 degrees. Telescopically, Venus is a shimmering little gibbous ball 13 arcseconds in diameter and 83% sunlit. Jupiter is 34 or 33 arcseconds wide, about as small as we ever see it. Jupiter is getting very blurry in a telescope as it sinks to an ever-lower altitude.

Mars is in eastern Taurus, heading east against the stars toward Gemini. Look for it high in the southwestern sky in early evening, lower in the west later. Mars continues to fade. It’s pretty similar to Mars-colored Aldebaran, some 17 degrees below it and Mars-colored Betelgeuse roughly the same distance to Mars’s lower left. They form a big orange Triangle.

Orion stands upright high in the south-southwest as the stars come out. It’s about to start its long spring tilt and departure down toward the west as the hours pass in the evening, and as the weeks pass with the advancing season.

On the traditional divide between the “Winter” and “Spring” sky is the dim constellation Cancer, now very high toward the south. It’s between Gemini to its west and Leo to its east. Cancer holds the Beehive Star Cluster, M44, in its middle. The Beehive shows dimly to the naked eye if you have little or no light pollution. With binoculars it’s easy even under mediocre conditions. Look for it a little less than halfway from Pollux in Gemini to Regulus in Leo.

William Rutter Dawes was born on March 19, 1799. The English amateur astronomer set up a private observatory and made extensive measurements of binary stars. In 1850 he discovered Saturn’s inner Crepe Ring, independently of American William Bond. In 1864, he was the first to make an accurate map of Mars. In 1867, Dawes determined the practical limit on resolving power for a telescope, known as the Dawes limit. Dawes expressed this as the closest that two stars could be together in the sky and still be seen as two stars. The Dawes Limit is 4.56 arcseconds divided by the telescope aperture in inches.

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