al sphere which is directly above you. Now suppose you happen to
be standing at a certain point on the earth and suppose that point was
in 15 deg. N latitude. And suppose at noon the center of the sun was
directly over you, i.e., the center of the sun and your zenith were one
and the same point. Then the declination of the sun at that moment would
be 15 deg. N. In other words, your angular distance from the earth's equator
(which is another way of expressing your latitude) would be precisely
the same as the angular distance of the center of the sun from the
celestial equator. Suppose you were standing directly on the equator
and the center of the sun was directly over you, then the declination of
the sun would be 0 deg.. Now if the axis of the earth were always
perpendicular to the plane of the sun's orbit, then the sun would always
be immediately over the equator and the sun's declination would always
be 0 deg.. But you know that the axis of the earth is inclined to the plane
of the sun's orbit. As the earth, then, revolves around the sun, the
amount of the declination increases and then decreases according to the
location of the earth at any one time with relation to the sun. On March
21st and Sept. 23rd, 1919, the sun is directly over the equator and the
declination is 0 deg.. From March 21st to June 21st the sun is coming North
and the declination is increasing until on June 21st--12 hours--it
reaches its highest declination. From then on the sun starts to travel
South, crosses the equator on Sept. 23d and reaches its highest
declination in South latitude on Dec. 22nd, when it starts to come North
again. This explains easily the length of days. When the sun is in North
latitude, it is nearer our zenith, i.e., higher in the heavens. It can,
therefore, be seen for a longer time during the 24 hours that it takes
the earth to revolve on its axis. Hence, when the sun reaches its
highest declination in North latitude--June 21st--i.e., when it is
farthest North from the equator and nearest our zenith (which is in 40 deg.
N latitude) it can be seen for the longest length of time. In other
words, that day is the longest of the year. For the same reason, Dec.
22nd, when the sun reaches its highest declination in South latitude,
i.e., when it is farthest away to the South, is the shortest day in the
year for us; for on that day, the sun being farthest away from our
zenith and hence lowest down toward the horizon, can be seen for