NavList:

This question also highlights the issue of sunrise and sunset definition. If you start up a bit of software, or consult a published table, you can look up the times of sunrise and sunset for a place like Mexico City or Denver, both of which are high up in the mountains. What do those numbers mean? And why are they not affected by temperature and pressure? And what does it mean to say that daylight on the equinox is not twelve hours there? We could go back to one of the older definitions and say that sunrise and sunset occur when the center of the geocentric Sun reaches 0° true altitude. Then there would be no ambiguity, and as a side-benefit the equinox days would be more nearly twelve hours in length. I'm not suggesting we should --I'm just pointing out that this issue arises from arbitrary definitions.

-FER