Earth’s Motions, Solar

Radiation, and

the Seasons 

Earth’s Motions, Solar

Radiation, and

the Seasons 

Rotation- The spinning of the Earth on its

Axis

Rotation- The spinning of the Earth on its

Axis

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

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

Earths Rotation Causes

sunrise / sunset star trails

daily events

Earths Rotation Causes

sunrise / sunset star trails

daily events

The Earth Rotates in a

Counterclockwise Direction

(CCW)

The Earth Rotates in a

Counterclockwise Direction

(CCW)

Revolution- The Earth

Revolves around the Sun

Revolution- The Earth

Revolves around the Sun

It take the Earth 365 days to

Revolve around the Sun

It take the Earth 365 days to

Revolve around the Sun

Earths Revolution Causes

SeasonsAnnual Events

Earths Revolution Causes

SeasonsAnnual Events

The Earth Revolves in its

orbit in a Counterclockwis

e Direction (CCW)

The Earth Revolves in its

orbit in a Counterclockwis

e 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) = 9.1x107 miles

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

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

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

TiltTilt

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

21.

SOLSTICESOLSTICESummer (June 21st-ish)

Sun Located at the Tropic of CancerWinter (December 21st-ish)

Sun Located at the Tropic of Capricorn

Tropic of Cancer

Tropic of Capricorn

Equator

Prime Meridian

Local Noon ShadowLocal Noon Shadow

June Solar InsolationJune 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 SolsticeWinter Solstice Sunlight is most direct at the Tropic of Capricorn (23.5 S) on ~

January 21.

Local Noon ShadowLocal Noon Shadow

Solar InsolationSolar Insolation Energy received by the earth at various latitudes for

December 21 (solstice)

December Solar InsolationDecember Solar Insolation

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. First day of spring Direct rays of the sun are directly above the Equator.

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

EQUINOXEQUINOXEquinoxes (Equal Day and Equal Night, 12 hours)

Spring (Vernal Equinox) March 21st-ishFall (Autumnal Equinox) September 21st-ishSun Located on Celestial Equator (Earth’s Equator)

Tropic of Cancer

Tropic of Capricorn

Equator

Prime Meridian

Equinox ShadowEquinox ShadowSun risesdue East,

Sun setsdue 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 heatshorter day => less heatArtic 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 (Solstice)

Day Light Hours (Equinox) Day Light Hours (Equinox)

Day Light HoursDay Light Hours

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

Changing Aspect AngleChanging 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 AngleIncident Angle

Earth’s Circle of IlluminationEarth’s Circle of Illumination

50 % of the earth’ssurface is illuminatedby the sun, every hourof every day.

Altitude-Azimuth - Angle of the sun above the Horizon

Altitude-Azimuth - Angle of the sun above the Horizon

N S

W

NCP

Zenith

Ecliptic (Path of the Sun)

34 degrees

Zenith – Position Directly OverheadZenith – 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 skyHorizon – 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 EnergyThermal EnergyFor 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 EnergyThermal EnergyFor 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)

Low sun (as sunset) shows more red due to atmospheric dust and pollution

GLOBAL VARIATIONS in INSOLATIONGLOBAL 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.

A high sun is more effective than a sun

low in the sky

DAY LENGTHDAY LENGTH

Antarctic circle 66.5°S

Arctic circle 66.5°N

North Pole 90°N

South Pole 90°N

Equator 0°

Tropic of Cancer 23.5°N

Tropic of Capricorn 23.5°N

Six months daytime (March-Sept), six months night (Sept - March)

Six months daytime (Sept - March), six months night (March-Sept)

One day with 24 hours daylight (June 21st); one day with 24 hours

darkness (Dec 21st)

One day with 24 hours daylight (March 21st); one day with 24

hours darkness (June 21st)

Sun is overhead once a year (June 21st). Day length always at least

10 hours.

Sun is overhead once a year(Dec 21st). Day length always at

least 10 hours.

Constant day length - 12 hours day and night all year round

Lengths of day and night vay more between the seasons at higher latitudes. This makes climate more seasonal at the poles than the equator

More seasonal

More seasonal

Seasonal OrientationsSeasonal Orientations