Astronomy
Distance from light travel time
The definition of the light year. Speed of light in vacuum multiplied by one year of seconds gives 9.4607304725808 trillion kilometres.
The Formula
d = c t
The definition of the light year. Speed of light in vacuum multiplied by one year of seconds gives 9.4607304725808 trillion kilometres.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| c | Speed of light | kilometres per second | A defined constant, 299792.458 kilometres per second. |
| t | Time | seconds | One Julian year of 31557600 seconds for the light year itself. |
| d | Distance | kilometres | The gap light covers in that time. |
What Each Symbol Means
ckilometres per second
Speed of light
A defined constant, 299792.458 kilometres per second.
tseconds
Time
One Julian year of 31557600 seconds for the light year itself.
dkilometres
Distance
The gap light covers in that time.
See It Worked With Real Numbers
A formula on its own does not tell you which number goes where. The how to calculate light years page applies this equation to a concrete case, shows every intermediate step, and names the mistakes that produce a wrong answer even when the algebra is right.
Where This Formula Appears
This equation is the working core of How To Calculate Light Years, filed under Science. Every number the guide produces comes from substituting into this one line.