Kevin Simler

Quiz: It’s noon on the spring equinox and you’re standing at the equator; the sun is directly overhead. Suppose that light takes exactly 8 minutes to travel the distance between us and the sun. Where would you have to point a laser in order to hit the sun dead-center?

^^^ a little physics/geometry puzzle I made up as a teenager I suggest trying to answer yourself before checking the replies for spoilers.

The answer (near as I can reason): If we model the sun as stationary and the earth as doing all the movement, then to hit the sun, you just have to aim directly at it. The fact that the sun appears to move in the sky as light takes time to travel is a red herring.

I was thinking the only error should be from Earth's orbital velocity. Avg. appears to be 30 km/s, which the light would inherit, throwing it off by ~14,400 km by the time it gets to the Sun (only 1% of its diameter).

Thanks! This is something I’ve always wondered about, and am still confused by. Can you explain with a few more words?

I'll take a shot at this. Whenever you trigger the laser, it's velocity can be decomposed into a vector of [very nearly c towards the sun, ~14km/s (sic) perpendicular to that]. The magnitude of that vector will be a speed of c, by relativity.