Seasons Reasons for the Seasons Tilt of Earths

Seasons

Reasons for the Seasons: • Tilt of Earth’s axis • Axial parallelism • Sphericity

Landmark dates. . . December Solstice • December 21/22 March Equinox • March 20/21 June Solstice • June 20/21 September Equinox, • September 22/23

Landmark latitudes. . . Equator = 0 o Tropic of Cancer = 23. 5 o. N Arctic Circle = 66. 5 o. N Tropic of Capricorn = 23. 5 o. S Antarctic Circle = 66. 5 o. S

On each day, …. . 1. What is subsolar point (SSP)? SSP is latitude where sun is directly overhead; where sun’s rays are vertical.

2. At what latitude will sun’s rays be oblique/tangent?

3. Where is Circle of Illumination? separates light and dark halves

December Solstice 1. SSP? 23. 5 o. S 2. tangent rays ? 66. 5 o. N, 66. 5 o. S 3. C. I. is tangent? at 66. 5 o. N, 66. 5 o. S

March Equinox 1. SSP? 0° 2. tangent rays? 90 o. N, 90 o. S 3. C. I. is tangent? at 90°N, 90°S

June Solstice 1. SSP? 23. 5 o. N 2. tangent rays? 66. 5 o. N, 66. 5 o. S 3. C. I. is tangent? 66. 5 o. N, 66. 5 o. S

September Equinox 1. SSP? 0° 2. tangent rays ? 90 o. N, 90 o. S 3. C. I. is tangent? at 90°N, 90°S

Animated tour of the seasons

Annual March of the Seasons

Angle of noon sun in a location= 90 - difference in degrees between latitude of location and SSP

Latitude?

Solution: Angle of noon sun (March 22) = 50 SSP (March 22) = 0 50 = 90 – (? – 0) ? = 40 for June 21: 73 = 90 – (? -23. 5) ? -23. 5 = 27 ? = 40. 5

What is the angle of the noon sun in Duluth on September Equinox? = 90 – ( 47 N – 0) = 43 degrees

What is the angle of the noon sun in Duluth on December Solstice? = 90 – ( 47 N – 23. 5 S) = 90 – 70. 5 = 19. 5 degrees

What is the angle of the noon sun in Duluth on June Solstice? = 90 – (47 N – 23. 5 N) = 90 – 23. 5 = 66. 5 degrees

What is the angle of the noon sun in Duluth today (January 27)? SSP today is approx. 20˚S. = 90 – (47 N – 20 S) = 90 – 67 = 23 degrees