Geography Reference
In-Depth Information
called solstices, the vertical rays strike one of the Tropics. The significance of these dates is
that they mark the beginnings of the four seasons of the year. The following sections show the
annual cycle as it relates to the Northern Hemisphere.
Keep in mind that seasons are relative. Summer never happens everywhere at once. Ditto fall,
winter, and spring. Instead, summer occurs in one hemisphere while winter is happens in the
other, and vice versa. Likewise, spring occurs in one hemisphere while autumn falls in the
other, and vice versa. Thus, when the “Summer Olympics” were held in Sydney, Australia
(Southern Hemisphere), it wasn't summer as far as the locals were concerned, but instead late
winter.
Spring
Sometime around March 21, the vertical rays strike the equator, marking the spring equinox. This is
the first day of spring in the Northern Hemisphere and the period of daylight and darkness are the
same. Everyday for about the next three months, the vertical rays strike the Earth at progressively
more northerly latitudes. The daylight hours get longer while night gets shorter.
Summer
On or about June 21, the vertical rays strike the Tropic of Cancer, marking the summer solstice. This
is the first day of summer as well as the date that has the longest period of daylight and the shortest
nighttime. From this point, the vertical rays then “turn south.” Daylight hours lessen while nighttime
hours increase.
Fall
On or about September 21, the vertical rays again strike the equator, marking the fall, or autumnal,
equinox. This is the first day of fall and again the period of daylight and darkness are equal. The ver-
tical rays then move into the Southern Hemisphere, striking ever more southerly latitudes each day
for about the next three months. In the Northern Hemisphere, nighttime now exceeds daytime by a
margin that increases each day.
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