The best known among these structures is the primary rainbow, which appears in sunlit raindrops as a colored circular segment of radius 42[degrees], centered on one's shadow point (also called the antisolar point
Depending on whether it is day or night, either the antisolar point
or the Sun respectively will be beneath the horizon; at sunset or sunrise, both will be on the horizon.
By day the center of the rainbow is the antisolar point
, the point exactly opposite the Sun and therefore marked by the shadow of your head.
Your shadow marks the antisolar point
, the direction directly opposite the Sun.
This snapshot of a glory, multicolored rings of light around the antisolar point
(directly opposite the Sun), was captured last October 27th from the summit of Mauna Kea in Hawaii.
On February 16th at 23:51 Universal Time, the Moon passes 5.0[degrees] north of the antisolar point
. One lunation later, on March 18th at 10:35 UT, it goes 4.6[degrees] north.
From April through early September each year the antisolar point
is too low in the sky for the gegenschein to be seen well from midnorthern latitudes.
So it is the antisolar point
, the point directly opposite the Sun in our sky, at the time of the March equinox (on March 20th this year).
Since a rainbow encircles the antisolar point
(the shadow of your head) at a radius of 42 [degrees], the Moon seen through a rainbow must always be gibbous, three days from full.
It helps if you know to look for the secondary's red band 51 [degrees] from the antisolar point
, the point marked by the shadow of your head.
The antisolar point
directly opposite the Sun is the center that every rainbow arcs around.
As with a rainbow, the shadow of your head marks the antisolar point
and the bow's center.