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Astronomy & optics

Light travel time in a vacuum

Calculate the time light takes to travel a distance and compare one-way with round-trip travel.

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01

Your inputs

The starting values are an example.

Decimals: 12.5 · Thousands: 1,234.56. Follow the field instructions for lists.

02

Your result

Enter your values and select Calculate.

Your calculation will appear here.

Your result will include the relevant values and units.

Understand the calculation

Formula and example

How it is calculated

t = distance/c; c = 299,792,458 m/s

Worked example

299,792.458 km corresponds to exactly one second of light travel time in a vacuum.

Things to check

  • The speed of light in a vacuum is an exactly defined SI constant.
  • Communication also involves processing, transmission paths and movement of the endpoints, beyond the propagation time alone.

Sources and references