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Chapter 5 - The seasons
Updated 10 July 2006
EQUATORIAL CHART
+60°
+30°
+60°
+30°
June Solstice
Ecliptic
0°
March Equinox
September Equinox
September Equinox
Direction of the Sun's
Annual Motion
(West to East)
–30°
–60°
12
h
6
h
0
h
December Solstice
18
h
0°
–30°
12
–60°
h
Figure 5.1 These are the locations of the four important solar positions along the ecliptic as found on the Equatorial
Chart. Remember, the chart wraps around at the edges. The points are named with their months because the seasons
are not the same for the Earth’s northern and southern hemispheres.
idian
M er
l
a
c
Lo
Zenith
"12 hour Circle"
Solar
Transit
d = 0º
Ce
les
tia
l
NCP (Polaris)
Eq
ua
to
r
Sunset
West
50º
40º
North
South
H or
izon
Sunrise
East
Figure 5.2 On an equinox day, the Sun’s declination is zero degrees (d = 0°) so the diurnal path of the Sun for these
days matches the celestial equator. It rises due east and sets due west. At 40° N, the Sun transits the local meridian at
90° – 40° + 0° = 50° off the southern horizon. Daylight lasts 12 hours. Compare this figure to figures 5.3, 5.4 and
Zenith
d = 0º
l
tia
ian
id
le s
er
Ce
"12 hour Circle"
ua
Eq
Lo
ca
lM
Q
to r
NCP
(Polaris)
64º
Sunset
West
26º
South
North
H or
izon
Sunrise
East
Figure 5.3 On an equinox day, the Sun’s declination is zero degrees (d = 0°) so the diurnal path of the Sun for these
days matches the celestial equator. It rises due east and sets due west. At 26° N, the Sun transits the local meridian at
90° – 26° + 0° = 64° off the southern horizon. Daylight lasts 12 hours. Compare this figure to figures 5.2, 5.4 and
5.5.
er
Celestial Equator
Lo
ca
lM
Zenith
ian
id
90º
Sunset
West
"12 hour Circle"
NCP
(Polaris)
SCP
H or
izon
Sunrise
East
Figure 5.4 On an equinox day at the Equator, the Sun transits the zenith. It rises straight up, due east, and sets
straight down, due west. Because the 12-hour circle matches the horizon at this latitude (0°), the Sun is always
above the horizon for 12 hours. Compare this figure to figures 5.2, 5.3 and 5.5.
Zenith
Loca
lM
eri
"12 hour Circle"
C
e
SCP
le
ia
st
lE
r
at o
qu
dia
n
Solar
Transit
d = 0º
Sunset
West
50º
40º
South
North
on
Horiz
Sunrise
East
Figure 5.5 This is the apparent motion of the Sun on the equinox days at 40° S. On the equinox days the Sun transits
at the same altitude as the celestial equator. The celestial equator represents the diurnal path of the Sun on this day.
The Sun rises due east and sets due west with daylight lasting for 12 hours. Compare this figure to figures 5.2, 5.3
and 5.4.
Zenith
Solar Transit
Path of the Sun
for this day
"12 hour Circle"
Q
eri
dia
n
NCP
(Polaris)
al M
d = +23a5
73a5
Loc
3
+2
a5
West
Sunset
(NW)
les
Ce
50º
40º
tia
or
at
qu
lE
South
Horiz
North
on
+2
3a
5
Sunrise (NE)
East
Figure 5.6 The apparent motion of the Sun on the June solstice at 40° N. In the northern hemisphere, the June
solstice is the Summer Solstice. The Sun rises in the northeast, transits high in the southern sky at 90° – 40° + 23°.5
= 73°.5, and sets in the northwest. Its path for the day is parallel to the celestial equator, with the Sun at a constant
declination of +23°.5. Daylight lasts for more than 12 hours. Compare this figure to figures 5.7, 5.8 and 5.9.
Zenith
Lo
ca
l
Q
M
er
a
idi
n
Path of the
Sun for this day
"12 hour Circle"
23a5
64º
NCP
(Polaris)
87a5
al E
e s ti
C el
26º
North
t
qua
South
Sunset (NW)
West
or
Hori
zon
Sunrise (NE)
East
Figure 5.7 The apparent motion of the Sun on the June solstice at 26° N. In the northern hemisphere, the June
solstice is the Summer Solstice. The Sun rises in the northeast, transits high in the southern sky at 90° – 26° + 23°.5
= 87°.5, and sets in the northwest. Daylight lasts for more than 12 hours. Compare this figure to figures 5.6, 5.8 and
5.9. Note especially, as one gets closer to the equator, the variation in the length of daylight during the year
decreases. Sunrise and sunset occur closer to the 12-hour circle.
Zenith
er
June Transit
d=
–23a5
ian
id
67a5º
Celestial Equator
Lo
ca
lM
December Transit
December
Sunset
d=
+23a5
67a5º
June
Sunset
"12 hour Circle"
West
90º
NCP
(Polaris)
SCP
H or
izon
December
Sunrise
East
June Sunrise
Figure 5.8 The apparent motion of the Sun on the solstice days at the Equator. On the solstice days at the Equator,
the Sun transits at 67°.5. It rises straight up, and sets straight down. Because the 12-hour circle matches the horizon,
the Sun is above the horizon for 12 hours – every day – for observers at the Equator. Compare this figure with the
other solstice figures: 5.6, 5.7, 5.9, 5.10, 5.11 and 5.12.
L
ridian
l Me
oca
Zenith
"12 hour Circle"
Q
SCP
50º
d = +23a5
West
Sunset (N
W)
40º
26a5
South
+2
3
North
a5
Sunrise (NE)
Path of the Sun
for this day
East
Figure 5.9 The apparent motion of the Sun on the June solstice at 40° S. For the southern hemisphere, the June
solstice is the Winter Solstice. The Sun rises in the northeast, transits the local meridian low in the northern sky and
sets in the northwest. The Sun crosses the 12-hour circle before it rises in the northeast and after it sets in the
northwest. Daylight lasts less than 12 hours. Compare this figure with figures 5.6, 5.7 and 5.8.
M
cal
Lo
n
eridia
Zenith
"12 hour Circle"
Q
Ce
l es
tia
l
50º
NCP
(Polaris)
Eq
ua
to
r
d = –23a5
W)
Sunset (S
West
40º
26a5
South
North
–2
5
3a
Horizon
Sunrise (SE)
Path of the Sun
for this day
East
Figure 5.10 The apparent motion of the Sun on the December solstice at 40° N. In the northern hemisphere, the
December solstice is the Winter Solstice. The Sun rises in the southeast, transits low in the southern sky at 90° – 40°
– 23°.5 = 26°.5, and sets in the southwest. Its path for the day is parallel to the celestial equator, with the Sun at a
constant declination of –23°.5. Daylight lasts for less than 12 hours. Compare this figure to figures 5.8, 5.11 and
5.12.
Zenith
Loca
l
Q
Me
rid
ian
Ce
Path of the
Sun for this
day
les
l
tia
"12 hour Circle"
u
Eq
r
ato
–23a5
NCP
(Polaris)
64º
West
26º
40a5
South
North
H or
izon
Sunrise (SE)
East
Figure 5.11 The apparent motion of the Sun on the December solstice at 26° N. Here again, the December solstice is
the Winter Solstice. The Sun transits low in the southern sky. It rises in the southeast, sets in the southwest and it is
above the horizon for less than 12 hours. Compare this figure with figures 5.8, 5.10 and 5.12. Note especially in
comparison to figure 5.10 that at this latitude (Miami, Florida), the Sun remains high enough during winter that
snow is not a concern.
Zenith
Solar Transit
"12 hour Circle"
Path of the Sun
for this day
Q
SCP
l
ca
Lo
d = –23a5
73a5
5
an
West
or
Sunset
(SW)
50º
Eq
ua
t
40º
3a
ridi
Me
–2
North
–2
Ce
le s
tia
l
South
3a
5
on
Horiz
Sunrise (SE)
East
Figure 5.12 The apparent motion of the Sun on the December solstice at 40° S. For the southern hemisphere, the
December solstice is the Summer Solstice. The Sun rises in the southeast, transits the local meridian high in the
northern sky and sets in the southwest. The Sun follows a path parallel to the celestial equator, bringing it above the
horizon before it gets to the 12-hour circle. Daylight lasts longer than 12 hours. Compare this figure with figures 5.8,
NCP (Zenith)
Sky path of Sun
on June solstice
"12 hour Circle"
June
Solstice
September
Equinox
South
23a5
South
South
March
Equinox
December
Solstice
Horizon
l
estia
= Cel
E
r
to
qua
South
Ecliptic
Sky path of Sun
on December solstice
Figure 5.13 The apparent motion of the Sun at the North Pole for the four important seasonal dates. Like everything
else, the Sun’s daily path must be parallel to the celestial equator. For the June solstice the Sun is 23°.5 above the
horizon all day. On the equinox days there is 24 hours of sunrise or sunset. On the December solstice the Sun never
rises – 24 hours of night. For the South Pole, reverse the results for June and December and reverse the direction of
the motion arrows along their respective paths.
Figure 5.14 The shadow cast by a metal plate gives an indication of how well the sunlight is heating the plate. In the
left drawing the plate is casting a maximum shadow and the plate heating rate is maximum. In the center drawing
the plate is angled, casting a smaller shadow, blocking a smaller amount of sunlight and thus the plate is not heated
as efficiently. In the right drawing the plate casts a minimum shadow. The sunlight passes over the plate, not heating
it at all.
Summer sun rays
Winter sun rays
Unit of Area
Figure 5.15 When the Sun is high in the sky during the summer, the amount of sunlight landing on a unit of area is
larger than the amount landing on the same unit of area during the winter. The sun rays drawn here are all the same
distance apart, but because they hit the earth at a lower angle during the winter, they land much farther apart than
during the summer. Thus, the unit area of ground does not receive as much heat energy from the Sun during the
winter as during the summer. This is exactly the same effect as tilting the metal plate, shown in figure 5.14. The
dashed lines mark an equal amount of area on each surface and also show the normal to each surface.
Sunlight
Snow
Pack
Snow
Pack
Wet Grass
Mud
Figure 5.16 The efficiency of solar heating is determined by the angle at which the sunlight strikes the ground.
Maximum heating occurs when the sunlight strikes the ground at high angles (or angles near the surface normal),
such as on the Sun-facing slope. Minimum heating occurs when the sunlight passes over a surface rather than
landing on it, such as the opposite-facing slope. The snow remains on this slope because the sunlight “skims over”
the surface of the snow pack. Compare what’s happening here with the views presented in figures 5.14 and 5.15.
North
Pole
Sunlight
Equator
South
Pole
Figure 5.17 Solar heating of terrestrial regions. The Sun heats the equatorial regions with greater efficiency than the
polar regions because of the angle at which the sunlight strikes the surface of the Earth in these regions.
gh
Equinox days
at
or
December
Solstice
50°
E
Horizon
SE
3a
5
Ex
tr a
–2
Ni
gh
NE
40°
tti
m
e
Ex
tra
June
Solstice
a5
Da
yl i
+2
3
le
st
ia
lE
qu
ci r
cle
Less than 12
hours above
the horizon
Ce
ur
More than 12
hours above
the horizon
t
NCP
12
-h
o
SCP
Figure 5.18 Sunrise as seen by an observer at 40° N from “inside the bowl” of figures 5.2, 5.6 and 5.10. Curvature in
the lines that would occur from projecting a sphere onto the page is not taken into account in this diagram. However,
you should see a better understanding of why the Sun’s rising point moves along the horizon and why the amount of
time the Sun spends above the horizon changes with the seasons. Notice the relationship between the angle of the
12-hour circle and the horizon, the celestial equator and the horizon, and the observation latitude. Compare this
figure to figure 5.19.
SCP
More than 12
hours above
the horizon
Ce
Less than 12
hours above
the horizon
le
a
sti
-h
12
lE
r
to
3a
5
r
tra
Ex
a
qu
Da
50°
40°
December
Solstice
gh
June
Solstice
yl i
t
Horizon
–2
ou
cl e
ci r
tr a
Ex
NE
E
Equinox days
SE
g
Ni
e
tim
ht
+2
3a
5
NCP
Figure 5.19 Sunrise as seen by an observer at 40° S from “inside the bowl” of figures 5.5, 5.9 and 5.12. Curvature in
the lines that would occur from projecting a sphere onto the page is not taken into account in this diagram. However,
you should see a better understanding of why the Sun’s rising point moves along the horizon and why the amount of
time the Sun spends above the horizon changes with the seasons. Notice the relationship between the angle of the
12-hour circle and the horizon, the celestial equator and the horizon, and the observation latitude. Compare this
figure to figure 5.18.
Direction of Angular Momentum
Ecliptic Vertical
Direction of Rotation
Ecliptic Plane
Equator
Figure 5.20 If the Earth’s axis were not tilted, the axis would be perpendicular to the ecliptic plane. The ecliptic
plane would pass through the Earth at the Earth’s equator, causing the celestial equator and the ecliptic line to be the
same. In this case, there would be no variation in the Sun’s declination and thus, no seasons. There would perhaps be
climate zones with frozen polar regions, temperate latitudes and tropical equatorial zones.
Direction of Angular Momentum
Ecliptic Vertical
23a5
Direction of Rotation
Ecliptic Plane
Equator
Figure 5.21 The Earth’s rotational axis is tilted by 23°.5 to the perpendicular to the ecliptic plane. The ecliptic line
(as seen on the Equatorial Chart) is created by the intersection of the ecliptic plane with the celestial sphere. The
ecliptic line (and thus the Sun’s) declination varies from the celestial equator by the amount of the Earth’s axial tilt.
The variation in the Sun’s declination causes the effects which bring about the seasons on Earth.
23a5
Line, perpendicular
to ecliptic plane
(points to NEP)
Ecliptic Plane (edge on)
Rotational Axis
of Earth (points
to NCP)
December Position
23a5
Rotational Axis
of Earth (points
to NCP)
Sun
Line, perpendicular
to ecliptic plane
(points to NEP)
June Position
Figure 5.22 No matter where the Earth is located in its orbit about the Sun, the axis of rotation always points to the
north celestial pole (Polaris, ignoring the precessional motion covered in section 4.6). The short line shown across
the diameter of the Earth is the equator. Notice the equator is also tilted by 23°.5. The two positions shown here are
six months apart. The hemisphere having winter weather is “tilted away from the Sun,” causing the sun rays to strike
the ground at a lower angle. Ultimately, the seasons are caused by the tilt of the Earth’s rotational axis.
23a5
Ecliptic
Line
23a5
Orbital path of earth.
Earth's axis of
rotation.
Ecliptic Plane
21 March
Sun
23a5
Earth's axis of
rotation.
Equator
21 September
23a5
To NCP
Earth's axis of
rotation.
21 June
Equator
View, Figure 5.24
23a5
View,
Figure 5.25
Figure 5. 23 The seasons are caused by the tilt of the Earth’s rotational axis with respect to the ecliptic plane. For each position shown, the darker line is the
celestial equator and the lighter line is the ecliptic, as they would be seen on the celestial sphere. In June, the northern hemisphere is “tilted toward the Sun”
and experiences summer while the southern hemisphere is having winter weather. In December, the southern hemisphere is “tilted toward the Sun” and is in
its summer season while the northern hemisphere is in winter. Use this figure in combination with figures 5.24 through 5.26 to understand how the sunlight
hits the earth during each of the seasons. Imagine looking at the ecliptic plane edgewise in the respective direction for each point of view shown in these
figures. The Earth must keep its axis of rotation pointed near Polaris all year because of conservation of angular momentum. (See appendix A, page 244.)
View, Figure 5.26
21 December
Equator
To NCP
23a5
To NCP
To NCP (Polaris)
Rays of Sunlight
24 hours of daylight
Tropic of Cancer
Arctic Circle
Equator
Tropic of Capricorn
Antarctic Circle
24 hours of darkness
Rotation Axis
Figure 5.24 At the June solstice sunlight hits the Earth’s surface directly on the Tropic of Cancer (23°.5 N). Anyone
standing at this latitude sees the Sun directly overhead at high noon. Anyone below the Antarctic Circle has 24 hours
of darkness. Anyone above the Arctic Circle has 24 hours of daylight. This is summertime for the northern
hemisphere.
To NCP (Polaris)
Rays of Sunlight
Arctic Circle
Tropic of Cancer
Equator
Tropic of Capricorn
South Pole
Antarctic Circle
Rotation Axis
Figure 5.25 At the March equinox sunlight hits the Earth’s surface directly on the Equator. Anyone standing on the
Equator sees the Sun directly overhead at high noon. Everyone on Earth sees 12 hours of daylight and 12 hours of
night, except at the poles, where they sees 24 hours of sunrise/sunset.
To NCP (Polaris)
Rays of Sunlight
24 hours of darkness
Arctic Circle
Tropic of Cancer
Equator
Antarctic Circle
Tropic of Capricorn
24 hours of daylight
Rotation Axis
Figure 5.26 At the December solstice sunlight hits the Earth’s surface directly on the Tropic of Capricorn (23°.5 S).
Anyone standing at this latitude sees the Sun directly overhead at high noon. Anyone below the Antarctic Circle has
24 hours of daylight. Anyone above the Arctic Circle has 24 hours of darkness. This is summertime for the southern
hemisphere.