December 28, 1612 – Galileo Galilei becomes the first astronomer to observe the planet Neptune, although he mistakenly catalogued it as a fixed star.
Galileo was an Italian astronomer, physicist, engineer, philosopher, and mathematician who played a major role in the scientific revolution during the Renaissance. Galileo has been called the “father of observational astronomy”, the “father of modern physics”, and the “father of science”. His contributions to observational astronomy include the telescopic confirmation of the phases of Venus, the discovery of the four largest satellites of Jupiter (named the Galileian moons in his honour), and the observation and analysis of sunspots.
Neptune is not visible to the unaided eye and is the only planet found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently observed with a telescope on September 23, 1846 by Johann Galle within a degree of the position predicted by Urbain Le Verrier. Its largest moon, Triton, was discovered shortly thereafter, though none of the planet’s remaining 14 moons were located telescopically until the 20th century. The planet’s distance from Earth gives it a very small apparent size, making it challenging to study with Earth-based telescopes. Neptune was visited by Voyager 2, when it flew by the planet on August 25, 1989. The advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has allowed for more-detailed observations.
The United States Postal Service issued a 29-cent stamp picturing the planet Neptune and Voyager 2 (Scott No. 2576) on October 1, 1991 as part of a thematic booklet of 20 stamps entitled “Space Exploration.”
Italy issued a 0,70 euro stamp in 2014 honoring the 450th anniversary of the birth of Galileo.