Earth-Sun RelationshipsThe Reasons for the Seasons

Solar Radiation

• The earth intercepts less than one two-billionth of the energy given off by the sun.

• However, the radiation is sufficient to provide 99.9% of the energy that heats the earth and drives our climate.

Radiation• Solar radiation is received as

parallel rays of energy (“line of sight, speed of light”)–Earth’s diameter is 12,765

km–Sun’s diameter is

1,390,000 km (over 100 times that of the

Earth)So is the Sun’s radiation felt the

same everywhere on Earth?

The Earth is curved, causing its surface to receive the

Sun’s radiation at different angles.

Intensity of incoming solar radiation (insolation) is related to angle of incidence. Higher angles = higher intensity.

Why is it hot in the tropics and cold at the

poles?• The Earth’s surface is curved

relative to the incoming solar radiation.

• Direct rays (most intense) are always in the tropical latitudes (23.5 º S to 23.5º N)

• As a result, solar radiation is diffused in the polar regions.

What causes the seasons?

• rotation of the earth on its axis?• revolution of the earth around

the sun• 23.5 O tilt of the earth axis from

perpendicular to the plane of the ecliptic

• polarity (axial parallelism)

Rotation of the Earth• Rotate west to east or

‘counter-clockwise’ (when viewed from the North Pole)–sunset and sunrise

• Rotation period is 24 hours per solar day (23 hours 56 minutes with respect to other stars)

Revolution of the earth• Period of revolution is 365.25 solar

days • Elliptical orbit (plane of the ecliptic)

Perihelion - 91.5 million miles

(Jan. 4)

Aphelion - 94.5 million miles

(July 5)93 million miles average distance

Tilt of the earth on its axis • The earth is tilted 23.5º from

perpendicular to the plane of the ecliptic.

• Tilt is currently constantly toward Polaris

SOLSTICESSUMMER, JUNE 21WINTER, DECEMBER 21

Where are the overhead rays of the sun on these days?

Which parts of the earth are in darkness and light? For how long?

Tropic of Cancer at 23.5º N Tropic of Capricorn at 23.5º S Arctic Circle (66.5 º N)Antarctic Circle (66.5º S)

Notice these four important parallels.Where do they occur? Why?

EQUINOXESVERNAL, MARCH 21AUTUMNAL, SEPTEMBER 21

Effects of the Seasons• Changes in solar intensity• Changes in solar altitude• Changes in day length• Changes in temperature

All of these changes are most extreme at high latitudes and minimized at the equator.

Annu

al C

hang

es in

Day

leng

th

E W NN SS

Annual Change in Noontime Solar Altitude

1 of 2

What is the change in height from the solstice to the equinox?

The total annual change?Summer Solstice

Winter Solstice

Equinox

E W NN SS

Annual Change in Noontime Solar Altitude

2 of 2

23.5O

23.5O

Summer Solstice

Winter Solstice

Equinox

Total annual change: 47O

Key Points

• 23.5º N

• Size and Shape of Earth.• Latitude and Longitude• Explain the basic earth-sun

relationships.• Equinoxes, solstices and the

relationship among – seasons– the latitude of the sun’s direct rays,

and – the intensity of solar radiation

received– changes in Daylight and Darkness

Time• Standard Time• The International Date Line• Daylight Savings Time

Year = 365 1/4 days or 365 days 5 hrs. 48 min. 45.685 seconds

Solar Day = 24 hours on average. Changes slightly with the elliptical orbit around the sun.

360o / 24 hr. = 15o standard zones (7.5o E and W from standard meridian)

Standard Time

International Date Line

Cross W to E - gain day; E to W - lose day

International Date Line

A day is 24 hours, but a date

lasts for 48 hours!

At any given time on earth, except

Greenwich noon, there are two dates

on the planet. The new date starts at the IDL when midnight crosses the IDL and moves westward around the planet for 24 hrs. displacing the old date. Then it fades out as the “old” date for 24 hrs.

Daylight Savings Time

• Day starts earlier April 1 - October 31

• Started during war time to save energy (maximize morning sun and increase work day).

• Arizona and Hawaii don’t use it.

• Navajo and Hopi do - causes great confusion.