The world can be divided into two hemispheres in several ways, Eastern-Western and Northern-Southern being the most well known. But the next best known is the somewhat hazy Land Hemisphere vs the Water or Sea Hemisphere.
When adjusted just right, the great circle boundary between these hemispheres divides the earth between a hemisphere that has 80% of the earth's land and a hemisphere that has only the remaining 20% of the earth's land.
Of course on a temporary basis there's a daylit hemisphere and a hemisphere that's in night (or at least twilight, with the sun down but near the horizon).
At what time on what day of the year, is the daylit portion of the earth most nearly coincident with the land hemisphere?
There are at least two studies using exacting geographic techniques
to find the center of the Land Hemisphere. These will yield a more
accurate answer. We use the study of S. W. Boggs, 1945 J.
Geography, V 54 No. 9 p. 245. (It is only 3 degrees away from that
found by Berget, A., 1913. and less than 12 degrees in Long. from
my Google Earth guess.) The center is at 47.2 N, 1.5 E (almost on
the prime meridian.)
The premise I will use is this:
if the hemisphere center is of a Lat. > 23.5 deg N, (which it is)
then the best date will be the N Hemisphere Solstice,
June 21, since the sunny half of the globe will have its subsolar point
on every meridian at 23.5 N at some time on that day. Thus the Sun's
northern illumination will best overlap this Northerly Centered
Hemisphere. This occurs at 11:57.5 UT on the 21st of June, for
a long. 1.6 E of the Greenwich Meridian.
Edited on June 7, 2023, 9:02 am
further considerations
