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A light-day 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 day (or 86,400 light-seconds), and is equal to 2.590206837×1013 m. It is just over 1.6×1010 miles and approximately 8.5×1013 feet.

Distance units based on the speed of lightEdit

Unit Definition Distance Example
  m km miles  
light-second   2.997924580×108 m 2.998×105 km 1.863×105 miles average distance from the Earth to the Moon is about 1.282 light-seconds
light-minute 60 light-seconds 1.798754748×1010 m 1.799×107 km 1.118×107 miles average distance from the Earth to the Sun is 8.317 light-minutes
light-hour 60 light-minutes
= 3600 light-seconds
1.079252849×1012 m 1.079×109 km 6.706×108 miles semi-major axis of Pluto's orbit is about 5.473 light-hours
light-day 24 light-hours
= 86400 light-seconds
2.590206837×1013 m 2.590×1010 km 1.609×1010 miles Sedna is currently 0.52 light-days from the Sun
light-week 7 light-days
= 604800 light-seconds
1.813144786×1014 m 1.813×1011 km 1.127×1011 miles The Oort cloud is thought to extend between 41 and 82 light-weeks out from the Sun
light-year 365.25 light-days
= 31557600 light-seconds
9.460730473×1015 m 9.461×1012 km 5.879×1012 miles Proxima Centauri is the nearest star to the Sun, about 4.24 light-years away

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