4001 Ptolemaeus, provisional designation 1949 PV, is a Florian asteroid from the inner regions of the asteroid belt, approximately 5 kilometers (3.1 miles) in diameter. It was discovered on 2 August 1949, by German astronomer Karl Reinmuth at the Heidelberg-Königstuhl State Observatory in Heidelberg, Germany. In 1991, the International Astronomical Union named the S-type asteroid after Greco-Roman astronomer Ptolemy.^[1]

Ptolemaeus is a member of the Flora family (402),^[3] a giant asteroid family and the largest family of stony asteroids in the main-belt.^[6] It orbits the Sun in the inner main-belt at a distance of 1.9–2.7 AU once every 3 years and 6 months (1,264 days; semi-major axis of 2.29 AU). Its orbit has an eccentricity of 0.17 and an inclination of 5° with respect to the ecliptic.^[2]

The body's observation arc begins with its observations as 1949 QD₁ at Lowell Observatory on 24 August 1949, or three weeks after its official discovery observation at Heidelberg.^[1] On 24 April 1989, Ptolemaeus approached the asteroid 6 Hebe within 5.5 million kilometers at a relative velocity of 3.7 km/s.^[2]

In the SMASS classification, Ptolemaeus is a common, stony S-type asteroid, which is in agreement with the overall spectral type for members of the Flora family.^[6]: 23

This minor planet was named after 2nd-century Greco-Roman astronomer Ptolemy (Latin: "Ptolemaeus") by IAU's Minor Planet Names Committee. He is best known for his influential Almagest, a mathematical and astronomical treatise on the apparent motions of the stars and planetary paths. Its ideas dominated astronomy for 1200 years until Copernicus in the early Renaissance.^[1] The official naming citation was published by the Minor Planet Center on 21 November 1991 (M.P.C. 19335).^[7]