![]() You can demonstrate this easily for yourself by holding a finger at arm's length and closing one eye and then the other. The idea of parallax is simple: when objects are observed from two different angles, closer objects appear to shift more than do farther ones. In the middle of the 2nd century BCE, Greek astronomer Hipparchus pioneered the use of a method known as parallax. The first distance to be measured with any accuracy was that of the Moon. But, could we do any better than these vague descriptions? With the invention of geometry, the answer became a resounding yes. The rest of the planets appeared no larger than the stars, yet seemed to move more rapidly they were likely at some intermediate distance. ![]() Solar eclipses revealed that the Moon and Sun were almost exactly the same angular size, but the Sun was so much brighter that perhaps it was larger but farther away (this coincidence regarding the apparent size of the Sun and Moon has been of almost indescribable importance in advancing astronomy). The Moon loomed large in the sky so it was probably pretty close. With nothing but naked-eye observations, a few things could be worked out. Thinkers in Ancient Greece were among the first to try and construct a comprehensive model of the cosmos. Known today as the astronomical unit, the distance serves as our reference within the solar system and the baseline for measuring all distances in the Universe. ![]() Certainly it's a question of nearly unrivaled importance, overshadowed in history perhaps only by the search for the size and mass of the Earth. ![]()
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