Sky at a Glance 2021 May 22 – 29

Photo showing locations of Vega, Arcturus and Spica for brightness comparison purposes.

This Week’s Sky at a Glance, 2021 May 22 – 29 ~by Curt Nason

The basis for ranking stars by brightness dates back to the Greek astronomer Hipparchus in the second century BC. He grouped several hundred stars by their apparent size, with the brightest being in the first magnitude group and the faintest to the naked eye being sixth magnitude. Magnitude in this sense means size, and even now many people refer to bright stars as big. The telescope and astrophotography allowed us to detect stars much fainter, and in the 19th century Norman Pogson adapted the old system to a standard. A five magnitude difference was defined as a difference in brightness of exactly 100. Therefore, a first magnitude star is a tad more than 2.5 times brighter than a second magnitude star, about 16 times brighter than a fourth magnitude star, and 100 times brighter than one of sixth magnitude. The scale extends into negative numbers for very bright objects, including planets and a few stars.

Check out a cloudless sky this week when it is dark. The bright star Vega is often regarded as the benchmark, being very close to mag 0 (astronomers usually shorten magnitude to mag). Arcturus is slightly brighter, edging into the negative decimals at mag -0.05. Spica, the brightest star in Virgo, is very close to mag 1 at 0.98. A mag 2 star is Polaris, the North Star, at the end of the Little Dipper’s handle. Obviously, it is not the brightest star as some people believe; it barely makes the top 50. A mag 3 star is Pherkad, the dimmer of the two stars at the base of the Little Dipper. Venus is currently at mag -3.8 but it will brighten to -4.6 by December. By the way, that star we see in daytime is mag -26.7 at midday.

This Week in the Solar System

Saturday’s sunrise in Moncton is at 5:39 am and sunset will occur at 8:53 pm, giving 15 hours, 14 minutes of daylight (5:46 am and 8:55 pm in Saint John). Next Saturday the Sun will rise at 5:33 am and set at 9:00 pm, giving 15 hours, 27 minutes of daylight (5:41 am and 9:02 pm in Saint John).

The Moon is full on Wednesday morning, just nine hours after perigee, resulting in some of the most extreme tides for the year at the end of the week. There is also a lunar eclipse that morning but it begins shortly after the Moon sets for New Brunswick. Mercury’s sunward movement brings it within half a degree to the left of Venus on Friday, May 28. A scope might be required to see Mercury. Mars makes an ever-changing triangle in fading twilight with Pollux and Castor, the Twins of Gemini. Saturn is stationary on Sunday, beginning its westward retrograde motion relative to the stars. Jupiter rises around 2:00 am late in the week, 45 minutes after Saturn.

With astronomy meetings and outreach activities on hold, you can watch the local Sunday Night Astronomy Show at 8 pm and view archived shows.

Questions? Contact Curt Nason.

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