# LLR & Orbital Motion

As I understand it, light that is emitted from a source is not imparted with the motion of the source and so always follows a "straight line". If this is correct, I am having a difficult time conceiving how the Lunar Laser Ranger experiments can detect photons.

In this experiment, a pulse of light is aimed towards a retro-reflector and it is reflected back. It takes about 2.5 seconds round trip and due to diffraction, the returning pulse covers a circle of approximately 20kms in diameter.

The Earth's orbital velocity is approximately 30km/s and in the time the pulse takes to make the round trip, the detector would be 75kms further along the Earth's orbital path. If my first paragraph is accurate, and the light pulse is not imparted with the Earth's orbital velocity, then how is detection of the returning light pulse achieved?

I hope this makes sense and thank you.

• You have discovered one of the inconsistencies that you need relativistic mechanics to understand. A good frame of reference to choose in this situation is the one where the Earth is stationary. That the whole system has a velocity relative to the Sun is irrelevant. If the Earth has a velocity or not is actually your choice. Commented May 8, 2016 at 20:33
• Thanks for the reply. So then, from the Earth the light pulse goes to the target and returns in an apparently straight line, but from Venus, for example, the light path might appear not straight and perhaps with some Doppler shifting? Commented May 8, 2016 at 20:43
• Sure, from Venus it may seem like it has another wavelength, takes more or less time to return and the like. It just appears different from different frames of reference. Commented May 8, 2016 at 20:46