Iris said: "Daylength at the Equator, at all altitudes, is 12 hours all year round."
This statement, though frequently repeated and seen in books, etc, is incorrect. Trebly incorrect, in fact. An observer, situated in the open at sea-level on the equator for one year would only experience 2 days where the sun is "above the horizon" for exactly 12 hours. These 2 days are what northern-hemisphere people call the Vernal (Spring) Equinox and the Autumnal Equinox. Only on these 2 days does the sun follow a path directly over the equator, thus giving "12" hours of daylight. In reality, the equatorial observer gets about 12 hours 20 minutes of daylight at each equinox. Since the sun is not a point source, it takes about 20 minutes to rise, and another 20 minutes to set. The 12-hour calculation (above) counts from the time when half the sun has risen to the time when half the sun has set. We thus get an unsolicited 10 minutes extra daylight at both sunrise and sunset. At any other time of year, the equatorial observer get less than 12 hours (or 12:20 hours) of sun-up time since the sun's path wanders to either the north or the south of the equator. Daylength actually varies by about 45 minutes from our longest days (the 2 equinoxes) to our shortest days (the 2 solstices). The above is only true for an observer out in the open, at any altitude. When you go into the mountains you have to consider topographical factors. In a deep valley (the sort of place where many montane orchids prefer to grow), you could quite easily lose 1-2 hours of daylight in either or both the morning and afternoon, thus resulting in considerably shorter daylengths. Peter O'Byrne Nearly on the equator. _______________________________________________ the OrchidGuide Digest (OGD) [EMAIL PROTECTED] http://lists.orchidguide.com/mailman/listinfo/orchids
