Color Chap 5.Cdr

Chapter 5 - The seasons Updated 10 July 2006

EQUATORIAL CHART +60° +60°

+30° +30° June Solstice

Ecliptic March Equinox September Equinox 0° 0° September Equinox

Direction of the Sun's –30° Annual Motion December Solstice –30° (West to East)

–60° –60° h h h h h 12 6 0 18 12

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.

Zenith ian erid l M ca Lo "12 hour Circle" Solar C Transit el es NCP (Polaris) tia d = 0º l E qu a to r

Sunset West

50º 40º

South North

Ho rizon 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 Q C e an le di s ri d = 0º ti e a M l l E "12 hour Circle" a q c u o a L t o r NCP 64º (Polaris) Sunset West 26º

South North

Ho rizon 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.

Zenith C e l

n e ia id s r t i

e a

M l

l E a

c q

o u L a t o r

90º Sunset "12 hour Circle" West

SCP NCP (Polaris)

Ho rizon 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 Lo cal Me rid ian "12 hour Circle" Solar Transit SCP or at qu d = 0º l E ia st le e C Sunset West

40º 50º

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 NCP (Polaris) n ia d = +23a5 id r e M

l 73a5 a5 a 3 c +2 o L West Sunset C 50º e (NW) 40º le s t ia l E South q North u a to r a5 Ho 3 rizo 2 n + 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 Q n ia Path of the id er Sun for this day M l a c o L 23a5 "12 hour Circle"

NCP (Polaris) 64º 87a5 Sunset (NW) C West e l e 26º s t i a l

E q South u North a t o r Ho ri z o n 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 December Transit June Transit C

d = e d = l

n e ia –23a5 +23a5 id s r t i

e a

M l

l E a

c q

o u L a t o

67a5º r 67a5º December Sunset June Sunset "12 hour Circle" West 90º

SCP NCP (Polaris)

Ho rizon December June Sunrise Sunrise East 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. Zenith an eridi l M ca Lo "12 hour Circle" Q SCP

50º d = +23a5 Suns et (NW) West

40º 26a5

South North + 2 3 a5

Sunrise (NE) Path of the Sun East for this day

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.

Zenith ian erid l M ca Lo "12 hour Circle" Q C el es NCP tia (Polaris) l E q ua to 50º r d = –23a5 W) Sunset (S West

26a5 40º

South North

a5 3 2 – Sunrise (SE) Horizon 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 L ocal Me Q rid Path of the ian Sun for this day C e le s t ia l "12 hour Circle" E q u a to r NCP –23a5 (Polaris) 64º West 26º 40a5

South North

H orizo n 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 Path of the Sun "12 hour Circle" for this day Q SCP L o c a d = –23a5 l M e – r 2 73 5 id 3 a i a5 a n West Sunset 40º (SW) r 50º to a u q E l South a North ti s e – l 2 e 3 C a5 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)

"12 hour Circle" Sky path of Sun on June solstice

September June Equinox Solstice South

23a5 South South r to ua March l Eq estia Equinox = Cel Horizon December South Solstice 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 Wet Grass Pack

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 Sunlight Pole

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.

NCP More than 12 Less than 12 12 -h hours above hours above ou r r the horizon to the horizon c a ir u cl q e E l a ti s + le t 2 e h 3 g a5 C li y a June D December a tr Solstice x 40° 50° Solstice E Horizon Equinox days e NE E im SE tt h ig – N 2 a 3 t a5 xr E 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 Less than 12 More than 12 le hours above hours above rc C ci the horizon e r le the horizon u si o t -h a 2 l 1 E q u a t 5 E o a x r 2 3 t – ra D a June y December l ig Solstice 50° 40° h Solstice Horizon t E xr Equinox days NE t a E SE N ig h t ti 5 m 3 a e + 2 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 23a5 to ecliptic plane (points to NEP) Rotational Axis of Earth (points to NCP) Ecliptic Plane (edge on) Sun Rotational Axis Line, perpendicular of Earth (points to ecliptic plane to NCP) (points to NEP)

December Position 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. To NCP 23a5 21 September

Equator View, Figure 5.25 23a5

To NCP To NCP 23a5 23a5 Earth's axis of rotation.

Equator 23a5 Sun Ecliptic 23a5 Equator Line 21 June

Ecliptic Plane 21 December Earth's axis of Earth's axis of rotation. rotation.

Orbital path of earth. 21 March

View, Figure 5.26 View, Figure 5.24

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.) 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.