Approximate dates when the solar zenith angle reaches a minimum of 0 degrees vs latitude by CMG Lee superimposed on an equirectangular world map from http://commons.wikimedia.org/wiki/File:World_location_map.svg by STyx. The example in blue shows the dates of Lahaina Noon at Honolulu, Hawaii, USA. The red curve is obtained from the Declination of the Sun equation δ=-23.45°*cos[360°/365*(N+10)], where, δ is the Declination of the Sun in ° latitude, N is the day of the year ranging from 1 to 365.
World location map.svg
cmglee, STyx et al