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A gigametre (American spelling: gigameter, symbol Gm) is a unit of length in the metric system, equal to one billion metres, the SI base unit of length, hence to 1,000,000 km or approximately 621,370 miles.

Gigametres (from the Greek words gigas = giant and metro = count/measure) are rarely seen in practical use, because they are too big for any terrestrial tasks. However, gigametres are sometimes used in astronomy to measure the distance of companion objects such as planets from their stars including our Sun.^{[citation needed]}

• One astronomical unit (AU) or Earth's average distance from the Sun is 149.6 Gm.
• Jupiter's average distance from the Sun is 778.5 Gm.
• The GJ 1214 b's average distance from the star GJ 1214 is 2.14 Gm, which is the smallest orbit of any known exoplanets.
• The mean diameter of the Sun is 1.391 Gm.
• The mean diameter of red supergiant star Betelgeuse is 1302 Gm.

See also[link]

• 1 E+9 m
• SI
• SI prefix
• Metric system
• Orders of magnitude (length)
• Conversion of units, for comparison with other units of length
• Light year
• Parsec

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