WHAT CAUSES THE SEASONS AND ASSOCIATED CHANGES IN INSOLATION?

September 21 December 22 June 21 SUN March 21

S U N WHAT DO WE KNOW SO FAR? March & September 21 Old School Geography N Equator 0° Antarctic Circle 66.5°S Tropic of Cancer 23.5°N Arctic Circle 66.5°N Capricorn 23.5 °S S Sun on horizon z = 90° Noon z = Lat S U N Sun overhead z = 0° Sun on horizon z = 90° Gainesville, 29.5°N Southampton, 52°N Perth, W.A., 32°S Zenith Angle at noon on these two days is exactly equal to a location’s Latitude.

S U N WHAT IS THE SUB-SOLAR POINT? March & September 21 Arctic C. S U N Cancer Equator Sun overhead z = 0° Capricorn Antarctic C. S Sub-solar point is the latitude at which the noon day Sun appears directly overhead (z=0) at noon.

WHAT IS THE ZENITH ANGLE AT NOON ANYWHERE IN THE WORLD?

WHAT IS THE ZENITH ANGLE AT NOON ANYWHERE IN THE WORLD? Z = LATITUDE OF LOCATION – LATITUDE OF SUB- SOLAR POINT Thus on March and September 21: Z at Noon = Latitude of location - 0

WHAT IS THE SUB-SOLAR POINT FOR ANY DAY OF THE YEAR?

WHAT THE *%$* ?

WHAT THE *%$* ? ANALEMMA

WHAT THE *%$* ? ANALEMMA Latitude of Sub-solar point 23.5°N Equator

September 21 December 22 June 21 N SUN S March 21

ANALEMMA 23.5°N June 21

S S U N U N s June 21 N Sun on horizon z = 90° Sun overhead z = 0° Cancer Equator U Sun overhead z = 0° N Antarctic s Sun on horizon z = 90°

S S U N U N s June 21 N Equator: z = 23.5° Sun on horizon z = 90° Cancer Equator U Sun overhead z = 0° N Equator: z = 23.5° Antarctic s Sun on horizon z = 90°

S S U N U N s WHERE IS THE SUN OVERHEAD (z=0) AT NOON? June 21 N Sun on horizon z = 90° S U N S Cancer Equator U 23.5°N Sun overhead z = 0° N Antarctic s Sun on horizon z = 90°

S S U N U N s WHAT IS NOONTIME Z IN GAINESVILLE? June 21 N Sun on horizon z = 90° S U N S Cancer Z = <29.5° = 29.5 – 23.5 = 6° Equator U Sun overhead z = 0° Lat = 29.5° N Antarctic s Sun on horizon z = 90° Small Zenith Angle: Sun Close To Overhead – Higher Proportion of Solar Constant – More Energy – HOT!

S S U N U N s WHAT IS NOONTIME Z IN SOUTHAMPTON? June 21 N Z = <52° Sun on horizon z = 90° S U N S Cancer Z = <52° = 52 – 23.5 = 28.5° Equator U Sun overhead z = 0° Lat = 52°N N Antarctic s Sun on horizon z = 90° Small Zenith Angle: Sun Closer To Overhead – Higher Proportion of Solar Constant – More Energy – WARM!

S S U N U N s WHAT IS NOONTIME Z IN PERTH, W.A.? June 21 N Lat = 32°S Sun on horizon z = 90° S U N S Cancer Equator U Sun overhead z = 0° Lat = 32°S Z = >32° = 32 + 23.5 = 8.5° N Antarctic s Sun on horizon z = 90° Large Zenith Angle: Sun Further From Overhead – Lower Proportion of Solar Constant – Less Energy – COOLER!

September 21 December 22 June 21 N SUN S March 21

ANALEMMA 23.5°S December 22

S S U N U N December 22 N s WHERE IS THE SUN OVERHEAD AT NOON? Sun on horizon z = 90° S U N S Arctic Z = >0° = 0 + 23.5 = 23.5° Sun overhead z = 0° U Equator Capricorn N 23.5°S Sun on horizon z = 90° s

S S U N U N December 22 N s WHAT IS ZENITH ANGLE AT GAINESVILLE? Sun on horizon z = 90° Z = >29.5° = 29.5 + 23.5 = 53° S U N S Arctic Sun overhead z = 0° U Equator Capricorn N Sun on horizon z = 90° s

S S U N U N December 22 N s WHAT IS ZENITH ANGLE AT SOUTHAMPTON? Sun on horizon z = 90° S U N Z = >52° = 52 + 23.5 = 75.5° S Arctic Sun overhead z = 0° U Equator Capricorn N Sun on horizon z = 90° s

S S U N U N December 22 N s WHAT IS ZENITH ANGLE AT PERTH, W.A.? Sun on horizon z = 90° S U N S Arctic Sun overhead z = 0° U Equator Z = < 32° = 32 - 23.5 = 23.5° Capricorn N Sun on horizon z = 90° s

WHAT IS THE SUB-SOLAR POINT TODAY? Equator Gainesville latitude ~ 29.5°N Z = 29.5 + 21.5 = 51° Latitude of Sub-solar point 21.5°S 23.5°S

BOTTOM LINE

BOTTOM LINE March and September: Z at noon – Lat.

BOTTOM LINE March and September: Z at noon = Lat. Summer (Jun. in N, Dec. in S): Z at noon = Lat -23.5°

BOTTOM LINE March and September: Z at noon = Lat. Summer (Jun. in N, Dec. in S): Z at noon = Lat -23.5° Winter (Dec. in N, Jun. in S): Z at noon = Lat +23.5°