Geoscience Reference
In-Depth Information
Path of Sun
Path of Sun
W
Southern
horizon
Earth's rotation
Earth's rotation
S
N
E
Horizon
Mar. 21
Sept. 21
June 21
Dec. 21
June
December
(a)
(b)
Figure 3.14 The celestial dome.
(a) The Sun's arc at the Winter Solstice, Equinoxes, and Summer Solstice when viewed from 45˚ N
latitude. (b) The position of the Sun at solar noon varies dramatically between the winter and summer seasons. It is much higher in the sky
in summer and lower in the sky in winter. This difference occurs due to the Earth's axial tilt.
ed with respect to a person standing at 45° N latitude. The surface
of Earth is presented as a two-dimensional feature that extends
from pole to pole, with the arc and position of the Sun shown at
three periods: Summer Solstice, Winter Solstice, and Equinox. In
other words, this diagram illustrates how you would see the Sun
as the day progresses at three different times of year.
One of the most obvious ways in which we notice the sea-
sonal change in Sun angle is with respect to the number of day-
light hours, or the length of day. Look again at the celestial
dome in Figure 3.14 or the one in the
Celestial Dome
animation
and notice how the curvature of Earth influences when the Sun
emerges from the horizon in the east or disappears beneath it in
the west. You should be able to see that the Sun emerges from
the horizon earlier and disappears later in the summer months
than during winter. It should make sense, too, that this seasonal
variation is more pronounced at higher latitudes than at lower
ones. Table 3.1 illustrates this concept by showing the num-
ber of daylight hours at five different latitudes in the Northern
Hemisphere at the equinoxes and solstices. Notice how the day
length varies among the five sites.
KEY CONCEPTS TO REMEMBER ABOUT
HOW WE SEE EARTH-SUN GEOMETRY
1.
We experience day and night because Earth rotates on
its axis.
2.
Due to the Earth's rotation, the Sun arcs from east to
west across the sky.
3.
As the Sun arcs across the sky, it is at its highest point
at solar noon.
4.
Sun angle refers to the angle at which solar radiation
strikes Earth. Sun angle is generally highest at the low
latitudes and progressively lessens toward the poles.
Variations in Day Length on Equinoxes and Solstices across LatitudeT
T A B L E 3 . 1
Day Length on Equinox or Solstice
Location
Latitude (N)
March 20
June 21
September 22
December 21
Bogotá, Colombia
4°329
12 h, 06 min
12 h, 24 min
12 h, 07 min
11 h, 51 min
Miami, Florida
25°469
12 h, 08 min
13 h, 45 min
12 h, 09 min
10 h, 32 min
Topeka, Kansas
39°079
12 h, 07 min
14 h, 55 min
12 h, 13 min
9 h, 26 min
Calgary, Alberta
45°019
12 h, 12 min
16 h, 33 min
12 h, 15 min
7 h, 56 min
Barrow, Alaska
71°039
12 h, 23 min
24 h, 00 min
12 h, 22 min
00 h, 00 min
