Ask an ex­pert

Dr Fred Wat­son, astronomer at the Aus­tralian As­tro­nom­i­cal Ob­ser­va­tory

Australian Geographic - - Geobuzz -


Why, dur­ing an eclipse, is the shadow of the moon the same size as the Sun?


In fact, the Moon’s shadow on the Earth’s sur­face dur­ing a to­tal so­lar eclipse varies in size de­pend­ing on the cir­cum­stances of the eclipse and the ob­server’s lo­ca­tion. As it moves across the Earth’s sur­face, the shadow traces out a ‘path of to­tal­ity’, within which any­one watch­ing will see the Sun’s disc blot­ted out by the disc of the Moon. Typ­i­cally (as was the case in the Au­gust 2017 eclipse in the USA), the path of to­tal­ity is about 100km wide.

But I think what this ques­tioner is re­ally ask­ing is why does the Moon ex­actly cover the Sun dur­ing an eclipse – in other words, why are they the same ap­par­ent size in the sky? The Sun is roughly 400 times big­ger than the Moon but, on av­er­age, is 400 times fur­ther away. This ex­tra­or­di­nary co­in­ci­dence means the two bod­ies ap­pear to be the same di­am­e­ter, and has no known cause.

The ‘path of to­tal­ity’ is what we see as the disc of the Moon passes over the disc of the Sun dur­ing a to­tal lu­nar eclipse.

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