For an observer within the “tropics,” the day-count for a sun calendar is easily determined to the nearest day on four occasions: on the two equinox days, which are the same for all cities, and on the two zenith passages of the sun, which depend upon the observer's latitude. Sunset observations alone, contrary to a tacitly accepted dictum, determine the precise day of equinox, and the average 365¼-day length of the “tropical year,” if the record is continued over several years.