In the second century BCE the Greek astronomer Hipparchus of Nicaea ranked the stars according to their brightness in six categories called magnitudes (for greatness). The 20 brightest stars were rated first magnitude and the faintest stars were sixth magnitude. This system was retained for two millennia and standardized in the 19th century when much fainter stars were being detected by telescopes and astrophotography. English astronomer Norman Pogson devised a logarithmic system whereby five magnitudes was a difference in star brightness of exactly 100 times. With this system, a magnitude 1 star is about 2.5 times brighter than a magnitude 2 star, and that one is 2.5 times brighter than a star of magnitude 3.
For many of us, the faintest star we can detect with the naked eye in a dark sky is sixth magnitude (commonly called mag 6). Vega, the fifth brightest star, is mag 0, slightly dimmer than Arcturus and slightly brighter than Capella. With the ability to measure the exact brightness of stars, their magnitudes are often recorded to one or two decimal places, and negative values are used for very bright objects. Sirius is mag -1.4, Jupiter is usually around mag -2.4 and Venus around -4.5. The full Moon is mag -12.6, approximately 400,000 times fainter than the Sun at -26.7. A first magnitude star, of which there are 22, is brighter than mag 1.50; a second magnitude star shines from mag 1.50 to 2.49, and so on.
This Week in the Solar System
Saturday’s sunrise in Saint John is at 6:11 and sunset will occur at 8:31, giving 14 hours, 20 minutes of daylight. Next Saturday the Sun will rise at 6:01 and set at 8:40, giving 14 hours, 39 minutes of daylight.
The Moon is near Antares on Sunday morning and it is at third quarter next Saturday. Venus sets around 11 pm this weekend, followed by Jupiter a few hours later. On Friday evening, May 8, telescope users might see a double shadow transit of Jupiter’s moons Ganymede and Europa between 9:45 and 10:26. This weekend Saturn rises an hour before sunrise followed by Mars ten minutes later. Mercury is lost in morning twilight, moving into the evening sky later this month. Early risers might see a few shooting stars from the Eta Aquariid meteor shower on Wednesday.
The Saint John Astronomy Club meets in the Rockwood Park Interpretation Centre this Saturday at 7 pm. Tune in to the Sunday Night Astronomy Show at 8 pm on the YouTube channel and Facebook page of Astronomy by the Bay.
Questions? Contact Curt Nason at nasonc@nbnet.nb.ca.
