Laser Ranging
Measuring enormous distances using the travel time of light.
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c × (t / 2)
Nothing about the distance is seen directly. A pulse leaves, a few photons come back, and the interval between the two events is timed. The speed of light converts that interval into a distance.
A short laser pulse is sent from an observatory toward a retroreflector, and a very small number of photons return. What is measured is the round-trip travel time.
- 01Light travels at a fixed, known speed in vacuum.
- 02Half the round-trip time, multiplied by that speed, gives the distance.
- 03Repeating the measurement over years reveals how the distance changes.
- Observation
Returned photons detected a short interval after the pulse was emitted.
- Measurement
Round-trip travel time, timed to extremely high precision.
- Physical model
Distance equals the speed of light multiplied by half the round-trip time.
- Inference
Instantaneous distance, and its long-term rate of change.
- +Distance to the reflector at the moment of measurement
- +Change in that distance over time
- +Refinements to orbital models
- +Tests of gravitational theory over long baselines
What a method cannot settle matters as much as what it can. Each note below is tagged by how firm the statement is.
- OBSERVATION
It only works where a reflector exists, or where a surface returns enough light. It is not a general-purpose method for distant objects.
- INFERENCE
The raw timing must be corrected for atmospheric delay, station motion and reflector orientation before it becomes a distance.
- OBSERVATION
It measures a distance between two specific points, not the separation of the two bodies' centres; that step requires a model.
Earth–Moon distance
Retroreflector arrays placed on the lunar surface return laser pulses fired from Earth, allowing the Earth–Moon distance to be tracked repeatedly over decades.
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