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Light Travel Time & Distance in Julian Years

Convert travel time to distance using the vacuum speed of light or a lower positive propagation speed, with astronomical units.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Private by default

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

Light travels 9,460,730,472,580,800 m in one Julian year of 365.25 days.

Distance = propagation speed × elapsed time

Worked example

Enter these known values and leave the other values blank.

Time
1 yrs
Distance
9460730472580800 m

Assumptions and limitations

  • The default speed is exactly 299,792,458 m/s. A smaller entered speed models constant propagation in another setting; it is not then a vacuum light-year conversion.
  • One year is 365.25 days, or 31,557,600 seconds. Month means one twelfth of that year, not a calendar month.
  • Astronomical distance conversions use the exact astronomical unit and Julian light-year; parsec uses 648,000/π astronomical units. The retained universe-age time choice is a fixed illustrative 13.799-billion-year value.
  • Time and distance are nonnegative. The model ignores cosmic expansion, moving endpoints and relativistic clock comparisons.

Common questions

What should I check before using this result?

The default speed is exactly 299,792,458 m/s. A smaller entered speed models constant propagation in another setting; it is not then a vacuum light-year conversion.

References

A million light-years uses exactly one million Julian light-years, consistently with the reviewed single light-year unit. A megaparsec uses exactly one million IAU-defined parsecs, consistently with the reviewed single parsec unit. The light-year uses exactly 299792458 m/s times a 31557600-second Julian year, replacing the rounded reference factor. The parsec uses the IAU definition 648000/π astronomical units, with one astronomical unit exactly 149597870700 meters, instead of a rounded decimal factor.

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