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ay45c1-page9 - First parallaxes date from 1839{1840 Bessel...

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Unformatted text preview: First parallaxes date from 1839{1840: Bessel, von Struve. Technology note: both used lenses made by Fraunhofer (Bavaria). 1.2.6 Luminosities in modern units (watts) We never did get L in watts (or erg/s) because we got sidetracked into \candles." Historically this of course had no meaning until Joule discovered the \mechanical equivalent of heat" (calories per joule) in 1847; before that we could only discuss L in calories/s: Sun 1 m 2 pan of water with black bottom and known depth of water Measure rate of temp. increase when Sun is overhead. Correcting for absorption in atmosphere1, the answer \the solar constant" is about 1400 W/ m2. So: 1 AU Sun 1 L 2 4(1 AU)2 = 1400 W= m ) L = 4  1400  (149  109 m)2 = 3:9  1026W = 3:9  1033 erg= s: How? Try di erent times of day and measure secant e ect, e.g. 9 ...
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