Ever-brilliant Venus at its brightest Friday evening

2013-12-05T00:00:00Z Ever-brilliant Venus at its brightest Friday eveningBy Tim Hunter Special to the Arizona Daily Star Arizona Daily Star

I call Venus ever‑brilliant because it is always truly brilliant whenever it is in our sky, whether in the evening after sunset or in the morning before sunrise.

Venus is exceeded in brightness only by the sun and the moon. It is so bright that it can be seen in the daytime if you know exactly where to look.

Enjoy Venus tonight after sunset. Look toward the southwest at 6 p.m. to see the 3-day-old crescent moon just above and to the right (north) of Venus.

Friday night Venus will be at its “greatest illuminated extent” — meaning it will be as bright as it gets at magnitude -4.9. The brightness of stars and planets is referred to as their “magnitude,” which means bigness, since brighter stars looked bigger than fainter stars to ancient observers.

The magnitude system is attributed to the Greek astronomer Hipparchus (circa 190-120 B.C.) of Nicaea. The ancients classified the brightest stars as those of first magnitude and the faintest stars visible to the naked eye as sixth magnitude. So the higher the magnitude number, the fainter the star.

In 1856 Norman R. Pogson (1829-1891) of Oxford proposed the modern system of magnitudes in which five magnitude steps correspond to an exact difference in brightness of a factor of 100. Each magnitude is 2.512 times dimmer than the preceding magnitude. Very bright objects, such as the sun (-26.8), the full moon (-12.6), and Sirius (-1.5) actually have negative magnitudes. The faintest stars visible from Earth-based telescopes are around 25th magnitude.

The magnitude system at first glance is somewhat counterintuitive, but it works and has been well accepted by astronomers.

Now go enjoy Venus at its brightest.

Contact Tim Hunter at skyspy@azstarnet.com

Copyright 2014 Arizona Daily Star. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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