Tuesday, June 7, 2022

Discovering Neptune

Neptune is the outermost planet of the Solar System: it moves on a orbit with a semi-major axis of 30 AU.

Being so far away, its apparent magnitude varies between 7.7 and 8, invisible to the naked eye but within the reach of most telescopes.

Neptune was discovered on September 23, 1846 by Galle from the Berlin observatory. The discovery was not accidental: the astronomer based his observations on the calculations of Adams and Le Verrier. These two mathematicians studying the perturbations of the orbit of Uranus predicted with great precision the position of an eighth planet of the Solar System, which was later found by Galle exactly where it should be.

Before 1843 Neptune was observed by several other astronomers. In particular, the first known observation dates back to 1612 and was made by Galileo Galilei! In fact, the Tuscan astronomer used to keep drawings on what he observed during the night. Scrolling through his notes, historians discovered that on December 27, 1612 Galileo had observed a stellar field inside which Neptune was present! The astronomer had reported the position of the planet in his drawings, but mistaking it for a fixed star. At that moment Neptune had just started its retrograde motion and its movement in the sky was extremely slow. This, in addition to the primitive instrumentation, did not allow Galileo to recognize the true nature of the object and to discover a new planet in the Solar System.

Credit: NASA, JPL.

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