Earth-Moon light time
A radio signal is light, so its travel time is a distance divided by one exactly defined constant. NASA's mean Earth-Moon distance is 1.28 s of it, one way.
One constant, exact by definition
Light in vacuum travels 299 792 458 m in one second. That figure is not a measurement with an error bar: the metre is defined by it, so the number is exact by construction and cannot be refined by a better experiment. Every result on this page is that constant divided into a distance, which makes each one our own arithmetic rather than a figure taken from someone else's document. [1]
Two consequences, and they are the whole page. A distance divided by 299 792 458 m/s is a delay. A delay multiplied by it is a distance — which is why one microsecond of clock error is 299.79 m of light path, and whymicroseconds on the Moon are metres on the ground.[1]
Distance into delay
Round steps of distance, plus the two lunar distances our source registry actually carries. Find the range you care about between two rows: the relation is linear, so halving the distance halves the delay. [1]
| Distance | One way | There and back | Source of the distance |
|---|---|---|---|
| 1 km | 3.336 µs | 6.671 µs | [1] |
| 10 km | 33.356 µs | 66.713 µs | [1] |
| 100 km | 333.564 µs | 667.128 µs | [1] |
| 1000 km | 3.336 ms | 6.671 ms | [1] |
| The Moon's radius, about 1740 km | 5.804 ms | 11.608 ms | [2]Official |
| 10 000 km | 33.356 ms | 66.713 ms | [1] |
| 100 000 km | 333.564 ms | 667.128 ms | [1] |
| Mean Earth-Moon distance, 384,400 km | 1.282220 s | 2.564441 s | [2]Official |
| 1 000 000 km | 3.335641 s | 6.671282 s | [1] |
The rows marked as our own calculation are units of length, not claims about where anything is: 1000 km is 1000 km. Two rows describe the Moon, and both come from NASA's Moon Facts page, which puts the Moon an average of 384,400 km away [2]Official and gives its radius as about 1740 km [2]Official. At that average the Moon is 1.282220 s of light path from here — call it 1.28 s one way, and a little over 2.56 s for a question and its answer[1]. Light crosses the Moon itself in 5.804 ms.[1]
The distances we do not print
There is no perigee row and no apogee row above, and the omission is deliberate. What the source gives is an average [2]Official, and an average is not a range: the orbit is elliptical, so the true distance runs above that figure for part of each month and below it for the rest. The nearest and farthest points have published values of their own, but our registry does not carry them, and every quantity on this site is taken from a document we have actually opened. Filling the gap from memory is exactly what this site exists not to do.
So treat 1.28 s as the middle of a band rather than a constant. If you need the delay at a particular moment, take the range from whatever ephemeris you trust and divide it by the constant; the rows above are there to bracket the answer. When our registry gains sourced perigee and apogee figures, the rows will appear here with their citations, and not before.
Our own clock model ignores that variation too, along with the other periodic terms —what we do not model says so in as many words, and lists the varying Earth-Moon distance by name.
Why a delay is a ranging error
A positioning system measures distance by timing a signal and multiplying by the speed of light, so an error in the clock arrives at the user as an error in the position[3]Official. The table above is that sentence in numbers, read in the other direction.
Put the site's own subject into it. A clock at rest on the Moon's selenoid gains 56.02 µs per day on a clock at rest on Earth's geoid[4], and one day of that model drift is 16.794 km of light path[1]. A day of ignoring the difference is a ranging error almost seventeen kilometres wide. The drift calculator extends that to any interval you name, andwhy microseconds matter works the same conversion down to the nanosecond.