A couple essential links for landscape photographers

A standby sight of mine for tracking the various stages of twilight is the Naval Meteorology and Oceanography Command sunset moonrise table:

http://aa.usno.navy.mil/data/docs/RS_OneYear.php

You can plug in cities or coordinates of anyplace on the globe, to get a break down of all the phases of Twilight.

Perhaps even better is "The Photographer's Ephemeris"

http://stephentrainor.com/tools

I'm still checking this out, so we'll be evaluating it together.

If you want to listen to a great podcast that describes what happens to light during the day -- particularly during dawn and sunset, Craig Tanner has a great podcast on the subject:

The Ebb & Flow of Natural Light