Thanks -- it's so cool! (We care about this difference-in-the-direction-of-down because we need to lock one of our turntables to "local vertical" with nanoradi

Thanks -- it's so cool! (We care about this difference-in-the-direction-of-down because we need to lock one of our turntables to "local vertical" with nanoradian stability. It was initially a surprise, before my time, when it was realized that it was impossible to do so simultaneously for two sensors on the same turntable but at different locations on the turntable axis.) Also, if you think that's cool, depending on your latitude, the "down" you experience doesn't point in the direction of the center of the earth. The earth is rotating, so there is a substantial centrifugal contribution to the acceleration you experience. Off the top of my head, I believe the effect here in Seattle is about 14 milliradians. It is zero at the pole and equator (the apparent centrifugal acceleration makes you effectively lighter!), though :).