Free Astronomy Magazine January-February 2020

JANUARY-FEBRUARY 2020 HISTORY the reference atlases, the two astronomers saw one star too many. Ob- servations at higher mag- nifications showed that the star was not point- like: the eighth planet of the Solar System had been discovered. In the decades following the discovery of Nep- tune, astronomers real- ized that the perturba- tions of Uranus’ orbit could not be entirely ex- plained by the presence of the eighth planet and, therefore, they began to suspect that there was a ninth planet as well, even farther away than Neptune. In the first decade of the 20 th century, a wealthy American astronomer, Percival Lawrence Lowell (already famous for ques- tionable observations of Mars), decided to emulate Adams and Le Verrier, try- ing to calculate the posi- tion of what he called “Planet X”. Assisted by a small circle of valid collab- orators, including Carl Otto Lampland and the brothers Earl Charles Slipher and Vesto Melvin Slipher, Lowell began searching for the planet in the sky from his private observatory in Flagstaff, Arizona. If Planet X had existed, it would have been more likely to be found along what was called “Laplace’s invari- able plane”, a band of sky within 0.5° from the or- bital plane of Jupiter. Earl Slipher assiduously pho- tographed that band with L owell at the eyepiece of the 24-inch Clark refractor, built in 1896 and housed in the great Mars Hill dome. Below, the same instrument as it appears today [Lowell Observa- tory Archives]