Sky at a Glance June 3 – 10

This Week’s Sky at a Glance, June 3 – June 10      ~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 biggest 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 and currently near Jupiter (now at mag -2.2), is mag 0.98, almost 1.0. A mag 2 star is Alphard in the constellation Hydra. It will look dimmer this time of year because it is low in the sky when darkness settles in, shining through a thicker layer of our atmosphere which will absorb more of the starlight. This effect is called extinction. A mag 3 star is Pherkad, the dimmer of the two stars at the base of the Little Dipper. By the way, that star we see in daytime is mag -26.75 at midday.

This Week in the Solar System

Saturday’s sunrise in Moncton is at 5:30 am and sunset will occur at 9:04 pm, giving 15 hours, 34 minutes of daylight (5:38 am and 9:06 pm in Saint John). Next Saturday the Sun will rise at 5:27 am and set at 9:09 pm, giving 15 hours, 42 minutes of daylight (5:36 am and 9:11 pm in Saint John).

The Moon is full and at apogee on Friday, June 9, making this the smallest full Moon of the year, the annual Puny Moon. Watch it rise around the time of sunset: Does it really look small? Jupiter is in great position for viewing all evening, while Mars is getting lost in twilight. The shadows of Jupiter’s moons Io and Ganymede might be seen on the planet’s cloud tops through a telescope at high magnification this Saturday beginning at 11:21 pm. Saturn rises soon after sunset and will be at opposition on June 15. Venus dominates the morning sky and reaches its greatest elongation from the Sun on Sunday.

Those attending the Nature NB Festival of Nature at Kouchibouguac National Park this weekend should also check out the RASC NB star party at the park for solar observing, talks and evening observing. See the park website for a schedule. The Saint John Astronomy Club meets at the Rockwood Park Interpretation Centre on June 3 at 7 pm. All are welcome.

Questions? Contact Curt Nason.

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