[OGD] Daylength at the equator.
Dave wrote: >The daylength at the equator *regardless* of your height (altitude) is precisely 12 hours every single day of the >year. Now this holds only directly on the equator. This is because the light always hits the earth exactly >perpindicularly (90 degrees) at the equator. Actually, the sun is only directly over the equator (incident at 90 degrees) at the time of either equinox. When it is summer in the northern hemisphere, the sun is almost 90 degrees incident on the tropic of Cancer (23.5 degrees north latitude), and when it is winter up north the sun is almost directly over the tropic of Capricorn(23.5 degrees south latitude). This is because of the tilt of the Earth (about 23.45 degrees). So daylength does vary at the equator, being longest at the time of an equinox, and shortest at the time of a solstice. Helga Gray Wildcatt Database Co. ___ the OrchidGuide Digest (OGD) [EMAIL PROTECTED] http://lists.orchidguide.com/mailman/listinfo/orchids
[OGD] Daylength at the equator
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
[OGD] Daylength at the equator
I see my message bounced yesterday for some unknown reason and that Iris already answered, however I've got a little more input ___ The daylength at the equator *regardless* of your height (altitude) is precisely 12 hours every single day of the year. Now this holds only directly on the equator. This is because the light always hits the earth exactly perpindicularly (90 degrees) at the equator. As you start going north or south of the equator the divergence from 12 hours gets more and more pronounced as you get further and further away. Furthermore at the equator the sun rises and sets always in due west and east. For the rest of the world all these conditions are fufilled only at the fall and spring equinox. Final consequence of this all is there is no "red" shift of the light as fall approaches. Sunlight remains at the same temperature all year round. -Dave (Graduate student in optics/biophysics) ___ the OrchidGuide Digest (OGD) [EMAIL PROTECTED] http://lists.orchidguide.com/mailman/listinfo/orchids
