How Fast Does Each Planet Orbit the Sun?

Which Planet Orbits the Sun the Fastest?

Mercury leads the eight planets with a mean orbital speed of about 47.36 km/s, roughly 1.59 times Earth’s. It is both close to the Sun and on a short orbital path: one circuit takes about 88 Earth days. Its speed varies enough that a single average hides much of the story. That is why the selected-date reading and the focus view show different parts of its orbit.

Which Planet Orbits the Sun the Slowest?

Neptune averages about 5.45 km/s in NASA’s detailed fact sheet. It still travels thousands of kilometers every second, but its vast orbit takes around 165 Earth years to complete. Its much longer period means that a few Earth days cover only a tiny part of its orbit. Choose Neptune in the ellipse demonstration and increase the animation speed to explore its motion.

Does a Planet’s Orbital Speed Stay Constant?

Kepler’s first law describes an ellipse with the Sun at one focus. The second law says that a line joining a planet and the Sun sweeps equal areas in equal times. Close to the Sun, that line is shorter, so the planet must cover a larger angle in the same time. Farther away, the line is longer and the angular motion is slower.

The change is not a sudden acceleration at one special point. Solar gravity continuously changes the velocity vector. On the inward half of an orbit the speed rises, and on the outward half it falls. At perihelion and aphelion the motion is perpendicular to the Sun–planet line.

How Fast Does Earth Orbit the Sun?

Earth’s mean is about 29.78 km/s, equivalent to 107,208 km/h or about 66,615 mph. You do not feel that enormous shared motion as a steady headwind: the ground, atmosphere and everything around you are traveling together. Earth’s orbital speed is slightly higher around its January perihelion and lower around its July aphelion.

That motion is different from Earth’s daily spin. Our planetary day simulator explains axial rotation; the present tool follows travel around the Sun. A longer year does not mean that a planet has a longer day.

How Do Astronomers Calculate Orbital Speed?

Mean speed is the distance traveled around a complete orbit divided by the elapsed time. For an ellipse, its perimeter replaces the circle’s 2πa. The circle approximation is convenient for nearly circular orbits but noticeable for Mercury: 2πa/T is about 47.9 km/s, while the elliptical mean is about 47.4 km/s.

For an ideal two-body ellipse, the vis-viva equation is v = √[μ(2/r − 1/a)]. Here r is the current distance from the Sun and a is the semi-major axis. It shows why speed changes along an ellipse. Precision ephemerides instead provide positions and velocity vectors from models that include the gravitational effects of other bodies.

Our date-dependent tool differentiates the existing approximate position model, keeping its velocities consistent with the positions you see. It does not combine a high-precision speed claim with a lower-accuracy diagram. The projected angular speed in degrees per day is reported separately from the 3D linear speed in km/s.

Kepler’s third law connects period and orbit size: in the solar two-body model, T² is proportional to a³. It explains the large period differences, but period alone is not a measure of linear speed. A planet on a larger ellipse has more distance to cover as well.

Sources and Model Limits

Mean speeds and reference sidereal periods come from the individual NASA planetary fact sheets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. These are fixed reference values, not date-dependent outputs.

Positions reuse the existing JPL approximate-element model for 1800–2050. The fitted elements are lower-accuracy position approximations, rather than precise osculating elements. Frozen-model periods and endpoint speeds can differ slightly from the NASA reference values. No external validation of precise velocity accuracy is implied. The circular calculator uses JPL’s solar GM and AU. See NASA’s Gravity and Mechanics for further explanation.