| Minorplanet: | yes |
| 4001 Ptolemaeus | |
| Background: |
|
| Discovery Ref: |   |
| Discovered: | 2 August 1949 |
| Mpc Name: | (4001) Ptolemaeus |
| Alt Names: | 1949 PV 1987 OE |
| Named After: | Ptolemy  |
| Mp Category: | main-belt  Flora  |
| Orbit Ref: |   |
| Epoch: | 23 March 2018 (JD 2458200.5) |
| Uncertainty: | 0 |
| Observation Arc: | 67.75 yr (24,744 d) |
| Perihelion: | 1.8940 AU |
| Semimajor: | 2.2874 AU |
| Eccentricity: | 0.1720 |
| Period: | 3.46 yr (1,264 d) |
| Mean Motion: | / day |
| Inclination: | 5.4568° |
| Asc Node: | 130.67° |
| Arg Peri: | 204.09° |
| Mean Diameter: | |
| Spectral Type: | SMASS S  |
| Abs Magnitude: | 13.7 |
4001 Ptolemaeus, provisional designation, is a Florian asteroid from the inner regions of the asteroid belt, approximately 5abbr=offNaNabbr=off 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.
Ptolemaeus is a member of the Flora family, a giant asteroid family and the largest family of stony asteroids in the main-belt. 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.
The body's observation arc begins with its observations as at Lowell Observatory on 24 August 1949, or three weeks after its official discovery observation at Heidelberg. On 24 April 1989, Ptolemaeus approached the asteroid 6 Hebe within 5.5 million kilometers at a relative velocity of 3.7 km/s.
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.
According to the survey carried out by the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer, Ptolemaeus measures 4.641 kilometers in diameter and its surface has a high albedo of 0.392. Based on a generic magnitude-to-diameter conversion, assuming a Flora-type typical albedo of 0.24, the asteroid measures 5.0 kilometers for an absolute magnitude of 13.7.
As of 2018, no rotational lightcurve of Ptolemaeus has been obtained from photometric observations. Its rotation period, poles and shape remain unknown.
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. The official naming citation was published by the Minor Planet Center on 21 November 1991 .