Earth’s Motions, Solar Radiation, and the Seasons

Rotation - The spinning of the Earth on its Axis

It takes the Earth 23 hours, 56min, and 4 seconds to rotate.

Earths Rotation Causes sunrise / sunset star trails daily events

The Earth Rotates in a Counterclockwise Direction (CCW)

Revolution - The Earth Revolves around the Sun

It take the Earth 365 days to Revolve around the Sun

Earths Revolution Causes Seasons Annual Events

The Earth Revolves in its orbit in a Counterclockwise Direction (CCW)

A. Earth's Revolution and Rotation

1. The earth's orbital path is an ellipse: (a flattened out circle)

2. Perihelion = point of closest approach to the sun (January) = 9 2. Perihelion = point of closest approach to the sun (January) = 9.1x107 miles

3. Aphelion = point of farthest approach = 9.4x107 miles (July)

4. Earth's rotation = spin about it's axis 4. Earth's rotation = spin about it's axis. Axis points to Polaris (the North Star)

B. Solstices and. Equinoxes Tilt of the earth's axis = 23 B. Solstices and Equinoxes Tilt of the earth's axis = 23.50 always pointed to Polaris

Tilt

1. Summer Solstice = June 21 Northern hemisphere receives the sun's light most directly (insolation) First day of summer. On the summer solstice, the direct sun rays are at =23.50 N latitude = the tropic of Cancer

Summer Solstice Sunlight is most direct at the Tropic of Cancer (23.5 N) on ~ June 21.

SOLSTICE Summer (June 21st-ish) Sun Located at the Tropic of Cancer Winter (December 21st-ish) Sun Located at the Tropic of Capricorn Tropic of Cancer Tropic of Capricorn Equator Prime Meridian

Local Noon Shadow

June Solar Insolation

2. Winter Solstice = December 21 Northern hemisphere receives the sun's light least directly. First day of winter. On the winter solstice, the direct sun rays are at 23.50 S latitude = the tropic of Capricorn

Winter Solstice Sunlight is most direct at the Tropic of Capricorn (23.5 S) on ~ January 21.

Local Noon Shadow

Solar Insolation Energy received by the earth at various latitudes for December 21 (solstice)

December Solar Insolation

3. Equinoxes: days in which the direct sunlight is on the equator 3. Equinoxes: days in which the direct sunlight is on the equator. Equal heating on both hemispheres. Days = nights.

a. Mar 21. the vernal equinox a. Mar 21. the vernal equinox. First day of spring Direct rays of the sun are directly above the Equator.

b. Sept. 21: the autumnal equinox: first day of fall b. Sept. 21: the autumnal equinox: first day of fall. Direct rays of the sun are directly above the Equator.

EQUINOX Equinoxes (Equal Day and Equal Night, 12 hours) Spring (Vernal Equinox) March 21st-ish Fall (Autumnal Equinox) September 21st-ish Sun Located on Celestial Equator (Earth’s Equator) Tropic of Cancer Tropic of Capricorn Equator Prime Meridian

Equinox Shadow Sun rises due East, Sun sets due West

4. Solar Declination: latitude where the noontime sun is directly overhead, on any given day.

5. Factors that affect the amount of insolation we get.

a. Amount of daylight: longer day => more heat shorter day => less heat Artic Circle = +66.50 N lat. no day in winter, no night in summer.  Antarctic Circle = 66.50S lat.

Day Light Hours (Solstice)

Day Light Hours (Equinox)

Day Light Hours

b. Solar angle:The smaller the solar angle, the more the beam is spread out => less intensity => less heat

Changing Aspect Angle Direct Sunlight is more intense and concentrated. Solar Incidence Angle is Latitude and Time Dependent Seasonal and Diurnal

c. Beam Depletion: Some of the sun's light is reflected back into space, and some is absorbed. The rest goes through.

Incident Angle

Earth’s Circle of Illumination 50 % of the earth’s surface is illuminated by the sun, every hour of every day.

Altitude-Azimuth - Angle of the sun above the Horizon Zenith Ecliptic (Path of the Sun) NCP 34 degrees N S W

Zenith – Position Directly Overhead There is much less atmosphere to travel through directly overhead than on the horizon. The sun is more direct and intense.

Horizon – Point where the land meets sky By the time the sun gets on the horizon, it is at a grazing angle and heavily scattered, refracted and absorbed.

Thermal Energy For Any Given Latitude Maximum Daily Solar Insolation Occurs At Hottest Hour of the Day (on average) Maximum Monthly Solar Insolation Occurs Hottest Month of the Year (on average)

Thermal Energy Noon 2 - 4 pm June August For Any Given Latitude Maximum Daily Solar Insolation Occurs At Hottest Hour of the Day (on average) Maximum Monthly Solar Insolation Occurs Hottest Month of the Year (on average) Noon 2 - 4 pm June August

GLOBAL VARIATIONS in INSOLATION Insolation received at the earth’s surface varies with latitude. The higher angle of the sun in the sky at the equator conveys more energy per unit area than at higher latitudes. A given amount of radiation covers a smaller area when overhead than when at a low angle; it is more concentrated Radiation passes through a greater length of atmosphere when at a low angle in the sky than when overhead. Atmospheric gases, dust and vapour absorb more energy before it reaches the earth’s surface. Low sun (as sunset) shows more red due to atmospheric dust and pollution A high sun is more effective than a sun low in the sky

Lengths of day and night vay more between the seasons at higher latitudes. This makes climate more seasonal at the poles than the equator DAY LENGTH Six months daytime (March-Sept), six months night (Sept - March) One day with 24 hours daylight (June 21st); one day with 24 hours darkness (Dec 21st) North Pole 90°N Arctic circle 66.5°N Sun is overhead once a year (June 21st). Day length always at least 10 hours. More seasonal Tropic of Cancer 23.5°N Constant day length - 12 hours day and night all year round Equator 0° Tropic of Capricorn 23.5°N More seasonal Sun is overhead once a year (Dec 21st). Day length always at least 10 hours. Antarctic circle 66.5°S One day with 24 hours daylight (March 21st); one day with 24 hours darkness (June 21st) South Pole 90°N Six months daytime (Sept - March), six months night (March-Sept)

Seasonal Orientations