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FastestThings

Planets & Moons

Neptune

Neptune's mean orbital speed around the Sun is 5.4 km/s on the NASA planetary fact sheet.

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Velocity

Published as 5.4 km/s

Mean orbital speed around the Sun, NASA planetary fact sheet comparison table, one decimal place.

OfficialNASA National Space Science Data Center (opens source in a new tab)

In context

  • Once around Earth in 2 h 4 minConstant · Earth's equator, 40,075 km (WGS 84)
  • London to New York in 17 minConstant · London to New York, 5,570 km great-circle
  • 18% of Earth's speed around the SunSourced · Earth, 29.8 km/s. NASA planetary fact sheet, mean orbital speed

Worked out from the sourced figure above, which stays the ranked number.

This list

Why it's so fast

A planet orbits faster when it is closer to the Sun. Stronger gravity there has to be matched by a higher sideways speed, or the world falls inward. That is the plain form of Kepler's laws.

  1. Closer means faster

    For a near-circular orbit, the speed that stays in orbit is higher when the distance to the Sun is smaller. Mercury is quick because it is in. Neptune is slow because it is far.

  2. A mean, not a moment

    The fact sheet's orbital velocity is an average. Along one ellipse a planet is also fastest at its closest point and slowest at its farthest, which is Kepler's second law.

The fastest is Mercury. The law is Earth's year stretched across the whole table.

This is how the category works. A note written for this entry appears here once it is in the database.

Provenance

Every measurement

Claimed, measured and official figures side by side, each with its source. The ranked figure is marked.

  1. 5.4km/s12,080mphRanked figure

    Mean orbital speed around the Sun, NASA planetary fact sheet comparison table, one decimal place.

    Official
    NASA National Space Science Data Center (opens source in a new tab)

Reference

Specs

Body
Neptune
Frame
Around the Sun
Table
NASA planetary fact sheet
Speed
5.4 km/s

Context

Background

The ranked speed is 5.4 km/s, the orbital-velocity cell for Neptune on NASA's planetary fact sheet. The sheet's notes say orbital velocity is the average speed of the body as it orbits the Sun. HyperPhysics reprints the same NASA column.

HyperPhysics prints 5.43 km/s. The ranking keeps the comparison sheet's 5.4. The Moon's 1.0 km/s is left off this list: the notes say that value is its average speed around Earth, not around the Sun.

Spot a wrong number?

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