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To do this, they redefined the metre as "the length of the path traveled by light in vacuum during a time interval of 1/ 299 792 458 of a second". [94] As a result of this definition, the value of the speed of light in vacuum is exactly 299 792 458 m/s [164] [165] and has become a defined constant in the SI system of units. [14]
The speed of light in IAU is the defined value c 0 = 299 792 458 m/s of the SI units. In terms of this speed, the old definition of the astronomical unit of length had the accepted value: [ 3 ] 1 au = c 0 τ A = ( 149 597 870 700 ± 3 ) m, where τ A is the transit time of light across the astronomical unit.
≡ 299 792 458 m: light-year: ly ≡ Distance light travels in vacuum in 365.25 days [6] ≡ 9.460 730 472 5808 ... foot per second squared: fps 2: ≡ 1 ft/s 2
The speed of light could then be expressed exactly as c 0 = 299 792 458 m/s, a standard also adopted by the IERS numerical standards. [19] From this definition and the 2009 IAU standard, the time for light to traverse an astronomical unit is found to be τ A = 499.004 783 8061 ± 0.000 000 01 s , which is slightly more than 8 minutes 19 seconds.
c is the speed of light (299 792 458 m⋅s −1 [8]); ε 0 is the electric constant ( 8.854 187 8188 (14) × 10 −12 F⋅m −1 [ 9 ] ). Since the 2019 revision of the SI , the only quantity in this list that does not have an exact value in SI units is the electric constant (vacuum permittivity).
The light-second is a unit of length useful in astronomy, telecommunications and relativistic physics.It is defined as the distance that light travels in free space in one second, and is equal to exactly 299 792 458 m (approximately 983 571 055 ft or 186 282 miles).
Similarly, since 1983 the SI metre has been defined relative to the second by choosing the value of c 0 to be 299 792 458 m/s. Consequently, until the 2019 revision, Consequently, until the 2019 revision,
For example, the speed of light is defined as having the numerical value of 299 792 458 when expressed in the SI unit metres per second, and as having the numerical value of 1 when expressed in the natural units Planck length per Planck time. While its numerical value can be defined at will by the choice of units, the speed of light itself is a ...