What is "megametre"

Megameter \Meg"a*me`ter\, Megametre \Meg"a*me`tre\, n. [Mega- + meter, metre, n., 2.] In the metric system, one million meters, or one thousand kilometers.

n. A distance of 1,000 km. Symbol: Mm

The megametre ( International spelling as used by the International Bureau of Weights and Measures; SI symbol: Mm) or megameter ( American spelling) is a unit of length in the metric system, equal to one million metres, the SI base unit of length, hence to or approximately 621.37 miles.

Megametres (from the Greek words megas = big and metro = count/measure) are rarely seen in practical use, e.g. "" is much more common than "5 Mm". Unless dealing with astronomy, distances on Earth can't be more than ( Equator length), although megametres may be used to measure the extensions of road systems and air travel distances, among other things rather than distances. The symbol (Mm) can also be confused with millimetres (mm). Megametres are also occasionally found in science fiction.

• The Earth's polar circumference is 39.94 Mm. (It was exactly 40 Mm from 1791 to 1875, but the definition of the metre was changed).
• The distance from Amsterdam to Marseille is 1.01 Mm.
• The distance from New York City to Chicago is 1.14 Mm.
• The Earth's equatorial diameter is 12.76 Mm.
• Quebec province's total area is 1.542 Mm.
• The mean distance from the Earth to the Moon is 384.4 Mm.
• Jupiter's equatorial diameter is 143 Mm.
• The speed of light is exactly by the definition of the metre.