Arthur Louis Day (October 30, 1869 – March 2, 1960) was an American geophysicist and volcanologist.

[2] Day received an honorary doctorate from the University of Groningen (The Netherlands) on July 1, 1914.

[3][4] In 1894 and 1895 he worked with German physicist Friedrich Kohlrausch studying the conductive properties of electrolytes.

[6] He worked with the U.S. Geological Survey from 1900 to 1907 studying the properties of rocks and minerals at very high and low temperatures.

[7] Day served as the director of the Geophysical Laboratory of the Carnegie Institution for Science from 1907 until his retirement in 1936.