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Spring North
equinox Star

Autumnal
23.5° equinox
Celestial
Summer Winter Winter equator
solstice solstice solstice
Summer
Autumnal Ecliptic solstice
equinox
Spring
equinox
FIGURE 16.6 The length of daylight during each season is
determined by the relationship of Earth’s shadow to the tilt of the Direction of the
axis. At the equinoxes, the shadow is perpendicular to the latitudes, Sun's motion
on ecliptic
and day and night are of equal length everywhere. At the summer
solstice, the North Pole points toward the Sun and is completely
out of the shadow for a 24 hour day. At the winter solstice, the FIGURE 16.7 The position of the Sun on the celestial sphere
North Pole is in the shadow for a 24 hour night. The situation is at the solstices and the equinoxes.
reversed for the South Pole.
directly above Earth’s equator. The equinoxes are the points on
the celestial sphere where the ecliptic, the path of the Sun, crosses
the Northern Hemisphere, the shortest daylight periods, and the celestial equator. Note also that the summer solstice occurs
the Sun at its lowest noon height in the sky. when the ecliptic is 23.5° north of the celestial equator, and the
The beginning of a season can be recognized from any one winter solstice occurs when it is 23.5° south of the celestial equator.
of the three related observations: (1) the length of the daylight
period, (2) the altitude of the Sun in the sky at noon, or (3) the EXAMPLE 16.1
length of a shadow from a vertical stick at noon. All of these
On the summer solstice, the latitude of an observer in the Northern
observations vary with changes in the direction of Earth’s axis
Hemisphere can be determined from the noontime altitude of the Sun
of rotation relative to the Sun (Figure 16.6). On about June 22
above the southern horizon. The altitude is measured in degrees with a
and December 22, the Sun reaches its highest and lowest noon sextant, and the latitude is calculated by taking the difference between
altitudes as Earth moves to point the North Pole directly to- zenith and the altitude and adding 23.5°, which is the angle between
ward the Sun (June 21 or 22) and directly away from the Sun the celestial equator and the ecliptic on the solstice. This relation also
( December 22 or 23). Thus, the Sun appears to stop increasing applies on the winter solstice for the Sun’s altitude above the northern
or decreasing its altitude in the sky, stop, then reverse its move- horizon relative to an observer in the Southern Hemisphere. What is
ment twice a year. These times are known as solstices after the the latitude of an observer who measures the Sun’s altitude of 38.5°
Latin meaning “Sun stand still.” The Northern Hemisphere’s above the southern horizon at noon on the summer solstice?
summer solstice occurs on about June 22 and identifies the be-
ginning of the summer season. At the summer solstice, the Sun SOLUTION
at noon has the highest altitude, and the shadow from a vertical
Determine the difference between the zenith and the altitude and add
stick is shorter than on any other day of the year. The Northern
23.5°. The zenith is 90° above the horizon:
Hemisphere’s winter solstice occurs on about December 22 and
identifies the beginning of the winter season. At the winter sol- zenith = 90° latitude = (zenith − altitude) + 23.5°
stice, the Sun at noon has the lowest altitude, and the shadow altitude = 38.5° = (90° − 38.5°) + 23.5°
from a vertical stick is longer than on any other day of the year. latitude = ? = (51.5°) + 23.5°
As Earth moves in its orbit between pointing its North Pole = 75°N
toward the Sun on about June 22 and pointing it away on about
December 22, there are two times when it is halfway . At these
times, Earth’s axis is perpendicular to a line between the center EXAMPLE 16.2
of the Sun and Earth, and daylight and night are of equal length.
On the summer solstice, the latitude of an observer in the Southern
These are called the equinoxes after the Latin meaning “equal
Hemisphere is calculated by taking the difference between zenith and
nights.” The spring equinox (also called the vernal equinox) the altitude above the northern horizon and subtracting 23.5°. This re-
occurs on about March 21 and identifies the beginning of lation also applies on the winter solstice for the Sun’s altitude above the
the spring season. The autumnal equinox occurs on about southern horizon relative to an observer in the Northern Hemisphere.
September 23 and identifies the beginning of the fall season. What is the latitude of an observer who measures the Sun’s altitude
The relationship between the apparent path of the Sun on of 40.0° above the northern horizon at noon on the summer solstice?
the celestial sphere and the seasons is shown in Figure 16.7. (Answer: 26.5°S)
Recall that the celestial equator is a line on the celestial sphere

16-5 CHAPTER 16 Earth in Space 409