I am hoping to recover some of the knowledge of physics I gained in high school that has vanished into the mists of time. Currently, I am being abused on Facebook by a self-proclaimed authority who ridicules my contention that gravitational acceleration is a constant. I get the idea that gravity varies slightly with altitude, but the sources I have consulted refer to gravitational acceleration as a constant. I suspect that the twit is playing word games, splitting hairs, but I simply don't know enough. Can a genuine authority help me?