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Transits of Venus across the face of the Sun were, for a long time, the best method of measuring the astronomical unit, despite the difficulties (here
Transits of Venus across the face of the Sun were, for a long time, the best method of measuring the astronomical unit, despite the difficulties (here, the so-called "black drop effect") and the rarity of observations.
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A ruler, depicting two customary units of length, the centimeter and the inch
A ruler, depicting two customary units of length, the centimeter and the inch