Understand the relationship
The reasoning behind the result
Distance divided by vacuum light speed gives one-way time
toneway=d/c
The model uses c=299,792,458 metres per second, the exact defined vacuum speed of light. A path length d divided by c has units of seconds. An astronomical unit is exactly 149,597,870,700 m; it is a fixed length unit, not a live measurement of any planet's present separation.
A light-second is also a distance unit: exactly 299,792,458 m. Distances entered in metres, kilometres, au or light-seconds are converted consistently before the result is displayed. The arithmetic does not infer current planetary positions or an ephemeris.
A reply has two travel legs and a local delay
texchange=toutbound+treply+treturn=2d/c+treply
For this fixed equal-path model, the outgoing and returning signals each cover the same distance. After the outgoing signal arrives, an entered processing or waiting delay occurs before the reply departs. The total exchange time includes all three intervals.
A round-trip time is therefore not generally twice the displayed one-way time unless the reply delay is zero. The timeline preserves the arrival and reply-departure events so the delay cannot accidentally be counted as propagation on both legs.
Inverse ranging removes the known delay first
d=(c/2)(telapsed−treply)
If elapsed exchange time and the delay at the remote end are independently known, subtract the delay to obtain the total propagation time, then divide it equally between the two assumed identical paths. A measured time shorter than the specified delay is inconsistent and is rejected. Equal values imply zero path length in this simplified model.
Unknown delays cannot be distinguished from extra distance by this one measurement. Motion during flight, unequal outward and return paths, atmospheric or plasma delays, clock calibration and relativistic corrections are excluded. The result is an entered static vacuum model, not precision navigation or spacecraft travel at light speed.
Follow the numbers
An exchange with a one-second processing delay
- A reply is received 3 s after transmission. The independently known remote reply delay is 1 s.
- Propagation takes 3−1=2 s in total. Under the equal-path assumption each leg takes 1 s.
- The inferred one-way distance is 299,792,458×1=299,792,458 m=299,792.458 km=1 light-second. The remote end receives at 1 s, replies at 2 s and the reply arrives back at 3 s.
Using all three seconds as propagation would overestimate the path length by 50%.
Quick guide
How to use this calculator
- Choose entered distance or measured round-trip elapsed time.
- Select units and add named scenarios. A light-second is a distance unit equal to the path light covers in one second in vacuum.
- Enter a known reply delay in seconds; leave it at zero only when that is the intended assumption.
- Inspect the signal timeline and both propagation legs. Actual moving-body ranging needs additional position, motion, medium and clock models.
Calculation method
Calculation and interpretation
Distinguish light propagation from processing delay and from the travel time of a spacecraft.
toneway=d/c; troundtrip=2d/c+treply; d=c(troundtrip−treply)/2; c=299,792,458 m/s exactly.
Worked example
An exchange with a one-second processing delay
Using all three seconds as propagation would overestimate the path length by 50%.
toneway=d/c; troundtrip=2d/c+treply; d=c(troundtrip−treply)/2; c=299,792,458 m/s exactly.
Supported inputs
Precision and limits
Static, equal vacuum paths
The endpoints remain fixed and both legs have the same path length. This tool does not fetch planetary positions or model medium delays, moving receivers, relativistic ranging or cosmological lookback time.
Signals and spacecraft are different
This is electromagnetic signal propagation in vacuum. It does not calculate an achievable spacecraft trajectory, acceleration program, fuel requirement or travel at light speed.
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