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Jupiter's Orbit & Rotation

It was Claudius Ptolemy, the Greco-Roman astronomer that lived the 2nd century AD, who first calculated Jupiter's orbital period. In his astronomical and mathematical treatise Almagest, Ptolemy determined that Jupiter took 11.86 years (or 4,332.38 days) to orbit around the Earth. Ptolemy was a proponent of the geocentric model in astronomy, which held the view that the Earth was the center of the universe and all celestial bodies orbited around it. Aryabhata, an Indian mathematician and astronomer, also calculated Jupiter's orbital period at 11.86 years (or 4,332.27 days) in 499 AD using the geocentric model. It was not until the 16th century when the geocentric model was challenged by Nicolaus Copernicus, who presented a mathematical model of a heliocentric system in which the Sun was the center of the Solar System and the Earth and other planets orbited it. Regardless of this change in view, Ptolemy's calculation of Jupiter's orbital period were later proven accurate, although this time it would be the Sun that Jupiter orbited.

Jupiter Compared to Earth

Jupiter orbits the sun at an average velocity of 47,002 km/h (29,205 mi/h). This makes it less than half (0.438 times) the Earth's orbital velocity. So the Earth actually overtakes Jupiter every 399 days. Jupiter's orbit size around the Sun is a little over 778 million kilometers wide (483 million miles), which makes it 5.2 times wider than the Earth's. Its orbital circumference is 4.88 billion kilometers (3.03 billion miles) long, 5.2 times longer than the Earth's. Along this orbital path, the closest that Jupiter comes to the Sun (perihelion) is at a distance of 740, 679, 835 km (460, 236, 112 mi), while its farthest distance (aphelion) is 816,001,807 km (507,040,015 miles).

Jupiter's rotational axis tilts at an angle of 3.13°. This is relatively small compared to the Earth's 23.44°, which is the reason why there are no significant changes in Jupiter's seasons compared to the seasonal changes experienced by Earth or even Mars.

Jupiter completes one rotation on its axis in 9.925 h or 9h 55m 29.71s, the fastest of all planets in the Solar System and despite its large size. This value is based on a calculation of Jupiter's interior rotation, which is based on the rotation of its magnetic field. Its atmospheric cloud, on the other hand, rotates faster than the interior. Because Jupiter is a gaseous planet, its atmosphere exhibits differential rotation, that is, the rate of rotation varies with latitude. Toward the poles, the planet completes one rotation in 9h 55m 40.6s, whereas along the equator the rate of rotation is 9h 50m 30s. These differing rotations are designated as Systems I, II and III. System I applies to the equatorial rotation at latitudes from 10°N to 10°S; System II applies to latitudes outside of System I including at the poles; and System III applies to the interior rotation (9h 55m 29.71s), which is designated as the official rotation of Jupiter.

As a result of its very rapid rotation, Jupiter's shape is noticeably flat at the poles and bulging at the equator resulting in an oblate spheroidal planet. This can be seen even with a simple telescope. Jupiter's diameter at the equator of 142,984 km (88,846 mi) is 6% larger than the polar diameter (diameter from pole to pole).

Differential rotation does not occur on terrestrial planets such as the Earth, but it is normal in gaseous or fluid objects like Jupiter. Jupiter's atmosphere is comprised mostly of hydrogen and some helium. However, very little is known about the planet's composition beneath the atmosphere. Jupiter's equatorial bulge reveals one important clue: if the planet's interior were composed of only hydrogen and helium (gases), then the planet would have a greater bulge along the equator and more flattened poles. Because this is not the case, then it is probable that Jupiter has a dense and rocky core that could be 10 to 20 times the mass of the Earth.