Earth-Sun Relationships The 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 / drives our climate.

Radiation Receipt 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 receipt of radiation the same everywhere on Earth?

Why is it hot in the tropics and cold at the poles?

The Earth is curved and this cause differing angles of incidence.

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 diffuse 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

SOLSTICES SUMMER, JUNE 21 WINTER, 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?

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

EQUINOXES VERNAL, MARCH 21 AUTUMNAL, 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.

Annual Changes in Daylength

Annual Change in Noontime Solar Altitude 1 of 2 Annual Change in Noontime Solar Altitude What is the change in height from the solstice to the equinox? The total annual change? Summer Solstice Equinox Winter Solstice N E S S W N

Annual Change in Noontime Solar Altitude 2 of 2 Annual Change in Noontime Solar Altitude Total annual change: 47O Summer Solstice 23.5O Equinox 23.5O Winter Solstice N E S S W N

Key Points 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 23.5º N

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. Use analemma to calculate.

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

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. For more help on this see: http://www.vistech.net/users/rsturge/dateline.html

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.