April 10, 2020

AstroEvent: A Challenging Dawn Conjunction.

Saturn & Mercury on closest approach. (Created by the Author with Starry Night).

Saturn & Mercury on closest approach. (Created by the Author with Starry Night).


   Set your alarm clocks; one of the closest but most challenging planetary pairings of the year happens this week in the early dawn skies. Mercury and Saturn will be within 1° degree of arc separation the morning of October 8th. Saturn is fresh from superior conjunction behind the Sun, and Mercury is currently undergoing a dawn apparition. Both will fit well in a binocular field of view or a low power eyepiece. The pairing will rise about 45 minutes prior to local sunrise, which for middle northern latitudes will occur around 7:45 AM local. The waning crescent Moon will pass the closing pair early in the week, and will provide a good guide post before it reaches New on the 7th. Start following the grouping early in the week to note any local markers on the horizon to aid you in your search for this interesting planetary pairing.

The Astro-term for this week is Pogson’s Ratio. First proposed by English astronomer Norman R. Pogson in 1856, this ratio codified and standardized the stellar magnitude scale first laid out by the Greek astronomer Hipparchus. Under the system, five magnitudes difference would represent 100-fold change in brightness. This means that a variation of one magnitude would be 2.512 times as bright or dim, or the fifth root of 100. Thus, Mercury currently at magnitude -1.3 is over two magnitudes brighter than Saturn at magnitude +0.9, or 2.5122= or more than 6.31 times as bright. This is because although Mercury is much tinier, it is also much closer to us and that all-illuminating source, the Sun. The lower the quoted magnitude number, the brighter the object. Venus is the brightest planet at about a maximum magnitude -4.5, a handful of stars have negative magnitudes, and the faintest a person with outstanding eyesight can discern under pristine dark skies is maybe magnitude +7.0.  This cleaned up magnitude scale as defined by Pogson’s Ratio is still in use today. Pogson also discovered eight asteroids. 21 variable stars, and got a lunar crater and asteroid (1830 Pogson) named after him.

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