[R-sig-Geo] newbie qx on differences between WGS84 and NAD83

[David Nelson]

In my first foray into spatial data, I'm dealing with both NAD83 and WGS84 coordinates.

Now, I understand that (1) they use slightly different reference ellipses and (2) the centers of the two ellipses track different things (apparently, the North American Plate vs. earth's center of mass).

But I don't understand exactly HOW they both define zero degrees longitude. Do they both use the IERS reference median, or does NAD83 use something else, like Airy's prime meridian?

Seems simple, but I can't find the answer. Hints and pointers would be greatly appreciated.

Thanks,

David Nelson