What do astronomers refer to as the retrograde motion of the planets?

Close encounters tend to disrupt orbits. So that is retrograde motion. Astronomers use the term to refer to the occasional backwards motion of the planets as seen in Earth's sky. When used in this way, retrograde motion is entirely an illusion caused by the moving Earth passing the outer planets in their orbits.

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explanation:

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the diploid number of chromosomes is sometimes represented by the symbol 2n. for the fruit fly, the diploid number is 8, which can be written as 2n = 8, where n represents twice the number of chromosomes in a sperm or egg cell.