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The kilometre (SI symbol: km; / ˈ k ɪ l ə m iː t ər / or / k ɪ ˈ l ɒ m ə t ər /), spelt kilometer in American and Philippine English, is a unit of length in the International System of Units (SI), equal to one thousand metres (kilo-being the SI prefix for 1000).
A length of 100 kilometers (about 62 miles), as a rough amount, is relatively common in measurements on Earth and for some astronomical objects. It is the altitude at which the FAI defines spaceflight to begin. To help compare orders of magnitude, this section lists lengths between 100 and 1,000 kilometers (10 5 and 10 6 meters).
Earth radius (denoted as R 🜨 or R E) is the distance from the center of Earth to a point on or near its surface. Approximating the figure of Earth by an Earth spheroid (an oblate ellipsoid), the radius ranges from a maximum (equatorial radius, denoted a) of nearly 6,378 km (3,963 mi) to a minimum (polar radius, denoted b) of nearly 6,357 km (3,950 mi).
Other SI units are derived from the meter by adding prefixes, as in millimeter or kilometer, thus producing systematic decimal multiples and submultiples of the base unit that span many orders of magnitude. For example, a kilometer is 1000 m.
The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of 1 / 299 792 458 of a second, where the second is defined by a hyperfine transition frequency of caesium.
Using the Greek stadium of 185 to 190 metres, [32] [33] the former translation comes to 754 800 km to 775 200 km, which is far too low, whereas the second translation comes to 148.7 to 152.8 billion metres (accurate within 2%). [34]
The speed of light in vacuum, commonly denoted c, is a universal physical constant that is exactly equal to 299,792,458 metres per second (approximately 300,000 kilometres per second; 186,000 miles per second; 671 million miles per hour).
kilometre (km) or kilometer is a metric unit used, outside the US, to measure the length of a journey; the international statute mile (mi) is used in the US; 1 mi = 1.609344 km; nautical mile is rarely used to derive units of transportation quantity.