Definition of Parallax-no printing

This is the difference in appearance of the position of an object ( e.g., planet, star, asteroid, etc. ) as visually viewed from two separate locations. It's usefulness wanes as distances approach the far distant reaches of the cosmos, however it is extremely efficient for near distances, so to speak. And it was successfully employed in calculating Mars distance during the early times of attempting to calculate AU, the astronomical unit of the mean distance between Earth and the Sun. 

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