Seasons Outline for Seasons lecture



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Seasons

Outline for Seasons lecture:


I. Reasons for seasons

II. Solstices and equinoxes

A. Important dates, latitudes

B. For each date:

1. SSP

2. Tangent rays

3. Circle of Illumination

III. Calculating Angle of Noon sun

I. Reasons for the Seasons:

a)Tilt of Earth’s axis

b) Axial parallelism

c) Sphericity

II. Solstices and Equinoxes
A. Important Dates:

December Solstice: December 21/22

March Equinox: March 20/21

June Solstice : June 20/21

September Equinox: September 22/23

Important Latitudes:

Equator = 0o

Tropic of Cancer = 23.5oN

Arctic Circle = 66.5oN

Tropic of Capricorn = 23.5oS

Antarctic Circle = 66.5oS

B. For each date…

1)What is latitude of subsolar point (SSP) or declination? (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 at 66.5oN, 66.5oS

3. C.I. is tangent at 66.5oN, 66.5oS (such that all latitudes above 66.5N are in 24 hour darkness; all latitudes above 66.5 S are in 24 hour daylight)

March Equinox

1.SSP= 0°

2. tangent rays at 90oN, 90oS

3. C.I. is tangent at 90°N, 90°S

June Solstice

1. SSP = 23.5oN

2. tangent rays at 66.5oN, 66.5oS

3. C.I. is tangent at 66.5oN, 66.5oS (such that all latitudes above 66.5 N are in 24 hour daylight and all latitudes above 66.5 S are in 24 hour darkness)

September Equinox

1.SSP= 0°

2. tangent rays at 90oN, 90oS

3. C.I. is tangent at 90°N, 90°S

Annual March of the Seasons

III. Calculating Angle of Noon sun

Angle of noon sun in a location= 90 – (difference in degrees between latitude of location and SSP)

What is Latitude of location in example given in class?

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 – ( 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



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