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 = 0o

Tropic of Cancer = 23.5oN

Arctic Circle = 66.5oN

Tropic of Capricorn = 23.5oS

Antarctic Circle = 66.5oS

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?

23.5oS

2. tangent rays ?

66.5oN, 66.5oS

3. C.I. is tangent?

at 66.5oN, 66.5oS

March Equinox

1.SSP?

0°

2. tangent rays?

90oN, 90oS

3. C.I. is tangent?

at 90°N, 90°S

June Solstice

1. SSP?

23.5oN

2. tangent rays?

66.5oN, 66.5oS

3. C.I. is tangent?

66.5oN, 66.5oS

September Equinox

1.SSP?

0°

2. tangent rays ?

90oN, 90oS

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) = 50SSP (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 – ( 47N – 0)

= 43 degrees

What is the angle of the noon sun in Duluth on December Solstice?

= 90 – ( 47N – 23.5 S)

= 90 – 70.5

= 19.5 degrees

What is the angle of the noon sun in Duluth on June Solstice?

= 90 – (47N – 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 – (47N – 20 S)= 90 – 67= 23 degrees