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in 1772, first suggested that an undiscovered planet could exist between the orbits of Mars and Jupiter

Kepler had already noticed the gap between Mars and Jupiter in 1596.

Giuseppe Piazzi at the Academy of Palermo

Ceres observation interruption

In 1801, Giuseppe Piazzi made a 24th observation of the position of Ceres, the asteroid he discovered between the orbits of Mars and Jupiter, on 1 Jan 1801. It was the first and largest of the dwarf planets now known. After this, it moved into the light of the Sun, and was lost to view for most of the rest of the year. To mathematically relocate Ceres, Carl Gauss, age 24, took up the challenge to calculate its orbital path, based on the limited number of observations available. His method was tedious, requiring 100 hours of calculation. He began with a rough approximation for the unknown orbit, and then used it to produce a refinement, which became the subject of another improvement.. And so on. Astronomers using them found his results in close agreement as they located Ceres again 25 Nov-31 Dec 1801.«

Pluto

In 1930, the planet Pluto was discovered by Clyde Tombaugh, when comparing two photographic plates taken six days apart the previous month. He found a starry speck that changed position between them. The search for Planet X was started three decades earlier (before Tombaugh was born) at Lowell Observatory in Flagstaff, Arizona, by Percival Lowell. Deviations in the positions of Uranus and Neptune were suspected to be due to the gravity of an undiscovered ninth planet. Locating it meant sifting through the millions of star images for one dim dot that moved. Lowell was unsuccessful, but in his will decreed that the hunt should continue. Clyde Tombaugh was a Kansas farmboy when Lowell Observatory director Vesto Slipher hired him in 1929. Pluto was the only planet found by an American