Fun fact: The speed of light in a vacuum is not constant.
You likely have heard that the speed of light (in a vacuum) is always the same, regardless of your frame of reference (essentially your vantage point). But this is only true if there aren’t any large masses around like the sun. It turns out the gravitational field of the sun affects the speed of light near it, therefore light passing near the sun actually slows down a little bit. That means light from a planet on the other side of the solar system from earth reaches us a tiny bit later than we would otherwise expect.
The first measurement of this time delay was in the late 1960s, when radio signals were bounced off Venus from Earth when the two planets were almost on opposite sides of the sun. The measured delay of the signals’ round trip was about 200 microseconds, just as predicted by general relativity.