|
« Return to Thread: day 3000 of 3rd
Re: Vernal Equinox Re: day 3000 of 3rd
by Irv Bromberg
::
Rate this Message:
On 2009.03.20, at 11:21 , Victor Engel wrote:
On 2009.03.19, at 19:56 , Brij Bhushan Vij wrote:However, Vernal Equinox is the point in time when SUN is directly over head and causing EQUAL 12-hours in TWO hemispheres of Earth.
On 3/20/09, Irv Bromberg <irv.bromberg@...> wrote:
The only way that you'll reckon a 12-hour day and 12-hour night on the day of the vernal equinox is if you use an equatorial sundial to reckon the time!
Victor wrote: How do you figure? Atmospheric refraction certainly impacts sundials.
Irv replies: Yes, it does, but would you not be hard-pressed to read the moment of sunrise / sunset on a sundial so tight enough precision that it would make a difference?
Victor continued: I suspect the ancients may have used another method for determining
when the equinox occurred: the angle of the path of the sun above the
horizon varies with the seasons. The maximum and minimum occur at the
solstices. Exactly half way (not in terms of time, but in terms of
angle) in between marks the equinoxes.
Irv replies: Indeed, there is an example of that method being described in the Talmud Eruvin page 56a.
However the observation of the solstice sunrise directions are also subject to similar refraction errors, greater at higher latitudes because Sun skims along the horizon at a more acute angle, and the true maximum north-east and south-east directions of sunrise can only be seen if the moment of the north solstice and south solstice respectively both occur at the moment of sunrise at the observer's locale, and there are periodic variations of solstices and equinoxes of about ±15 minutes due mainly to the lunar cycle (with non-negligible contributions from Venus and Jupiter). (Similarly for the sunset direction.) If the positions were marked and recorded over a number of years then there is a better chance that the correct maxima will be found. These are limitations of observational techniques.
« Return to Thread: day 3000 of 3rd
| Free embeddable forum powered by Nabble | Forum Help |
