Planet Earth; Tsunamis
In 1932 determination of the parallax displacement of the asteroidEros as it made a close approach to Earth yielded what was at the time a very precise value for the astronomical unit. Astronomers then further refined their knowledge of the dimensions of the solar system and the value of the astronomical unit through a combination of radar ranging of Mercury, Venus, and Mars; laser ranging of the Moon (making use of light reflectors left on the lunar surface by Apollo astronauts); and timing of signals returned from spacecraft as they orbit or make close passes of objects in the solar system.
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