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Light travel time calculator

The speed of light is exact by definition, so a light travel time is an exact rational number whenever the distance is.

Light travel time
0:08:19.00
= Exact Light travel time
In minutes
8.3167
In hours
0.138 61
In days
0.005 776
In Julian years
0.000 015 812 507
Distance in light-years
0.000 015 812 507
Round-trip signal delay
0:16:38.01
Distance in metres
149 597 870 700
Show the working
  1. Distance = 1
  2. Distance in metres = 1 * 149597870700 = 149 598 000 000
  3. Light travel time = 149 598 000 000 / c = 499.005
  4. In minutes = 499.005 / 60 = 8.316 75
  5. In hours = 499.005 / 3600 = 0.138 612
  6. In days = 499.005 / 86400 = 0.005 775 52
  7. In Julian years = 499.005 / 31557600 = 0.000 015 812 5
  8. Round-trip signal delay = 499.005 * 2 = 998.01
  9. Distance in light-years = 149 598 000 000 / 9460730472580800 = 0.000 015 812 5

Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.

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    The formula

    • time = distance / c, with c exactly 299 792 458 m/s
    • one light-year = c x 365.25 days = 9 460 730 472 580 800 m exactly
    • one astronomical unit = 149 597 870 700 m exactly

    Both the astronomical unit and the light-year are exact defined lengths, so this conversion involves no measurement at all.

    Sunlight takes 499 seconds to reach Earth — eight minutes and nineteen seconds.

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