s later than it rose on June twentieth and fifty-three
seconds later than it rose yesterday. Instead it rose at 4:20, sixteen
minutes _earlier_ than it did on June twentieth and fifty-three seconds
earlier than yesterday."

"I don't understand what is causing it, Doctor. I have tried to follow
your published explanations, but they are a little too deep for me."

* * * * *

"As to the real underlying cause, I am in grave doubts, Carnes, although
I can make a pretty shrewd guess. As to the reason for the unnatural
lengthening of the day, the explanation is simplicity itself. As you
doubtless know, the earth revolves daily on its axis. At the same time,
it is moving in a great ellipse about the sun, an ellipse which it takes
it a year to cover. If the axis of rotation of the earth were at right
angles to the plane of its orbit; in other words, if the earth's equator
lay in the plane of the earth's movement about the sun, each day would
be of the same length and there would be no seasons. Instead of this
being the case, the axis of rotation of the earth is tipped so that the
angle between the equator and the elliptic is 23-1/2 deg."

"I seem to remember something of the sort from my school days."

"This angle of tilt may be assumed to be constant, for I won't bother
with the precessions, nutations and other minor movements considered in
accurate computations. As the earth moves around the sun, this tilt
gives rise to what we call the sun's declination. You can readily see
that at one time in the year, the north pole will be at its nearest
point to the sun, speaking in terms of tilt and not in miles, while at
another point on the elliptic, it will be farthest from the sun and the
south pole nearest. There are two midway points when the two poles are
practically equidistant."

"Then the days and nights should be of equal length."

"They are. These are the periods of the equinoxes. The point at which
the sun is nearest to the south pole we call the winter solstice, and
the opposite point, the summer solstice. The summer solstice is on June
twenty-first. At that time the declination of the sun is 23-1/2 deg.
north of the equatorial line. It starts to decrease until, six months
later, it reaches a minus declination of 23-1/2 deg. and is that far
south of the line. The longest day in the northern hemisphere is
naturally June twenty-first."

"And the shortest day when the sun has the greatest