A megametre (American spelling: megameter, symbol Mm) is a unit of length in the metric system, equal to one million metres, the SI base unit of length, hence to 1,000 km or approximately 621.37 miles.

Megametres (from the Greek words megas = big and metro = count/measure) are rarely seen in practical use, because they are too big for most terrestrial tasks. Their symbol (Mm) can also be confused with millimetres (mm). Megametres are also occasionally found in science fiction.^{[citation needed]}

• The Earth's polar circumference is 39.94 Mm. (It was exactly 40 Mm from 1791 to 1875; see metre).
• The distance from Amsterdam to Marseille is approximately 1 Mm.
• The Earth's equatorial diameter is 12.76 Mm.
• The mean distance from the Earth to the Moon is 384.4 Mm.
• Jupiter's equatorial diameter is 143 Mm.
• Quebec province's total area is 1.542 Mm².

[edit] See also

• 1 E+6 m
• SI
• SI prefix
• Metric system
• Orders of magnitude (length)
• Conversion of units, for comparison with other units of length
• light year
• parsec

A megameter is also a type of astrometrical instrument used for determining longitude by observation of the stars.