Legendre was the first to publish the principle of least squares in 1805, this principle was known to Gauss since 1795. But it was Gauss who introduced the method of least squares. He offered its final justification based on the principle of maximum weight (minimal variance) in 1823 and 1828. I begin with a few words about Legendre and Laplace and continue with describing Gauss’ final justification of least squares. It is extremely complicated, but modern authors removed this difficulty. My own exposition (§ 3) is quite elementary and, I think, methodically necessary.
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