1. Rotation, Revolution, Seasons
EARTH-SUN RELATIONS
Rotation, Revolution, Seasons
4/15/2017
(c) Vicki Drake, SMC

2. EARTH’S ROTATION The Earth rotates on its axis
One complete rotation (3600) takes approximately 24 hours
Rotation is from West to East
Sun appears to ‘rise’ in East and ‘set’ in West
Rotation speed is variable
Fastest at the equator
4/15/2017
(c) Vicki Drake, SMC

3. EARTH’S REVOLUTION ABOUT THE SUN
The Earth revolves about the Sun
One complete revolution takes days
365 days, 5 hours, 48 minutes, 36 seconds
Approximately 365 ¼ Earth days
The Earth’s revolution is slightly elliptical, not circular
Direction of revolution is counter-clockwise from an outer space perspective
4/15/2017
(c) Vicki Drake, SMC

4. AXIS TILT AND REVOLUTION
Earth moves in a constant plane – Plane of the Ecliptic – in its revolution about the Sun
All the planets (and even the sun) are moving in the Plane of the Ecliptic
Earth’s axis is tilted about from perpendicular to Plane of Ecliptic
Earth’s tilt has two characteristics:
Angle of inclination
Parallelism
4/15/2017
(c) Vicki Drake, SMC

5. ANGLE OF INCLINATION AND PARALLELISM
The angle of inclination, the tilt of 23½ degrees, is a constant.
The angle does not change throughout the entire revolution
Parallelism means the axis is always pointed in the same direction
The axis does not point in different directions as the Earth moves in its orbit
4/15/2017
(c) Vicki Drake, SMC

6. EARTH’S ELLIPTICAL REVOLUTION
The Earth, in its elliptical revolution, has an average distance of approximately 93,000,000 miles from the Sun
At two points in the revolution, the distance varies
Perihelion: Earth is closest to Sun, ~91.5 million miles
Aphelion: Earth is farthest from Sun, ~95.5 million miles
4/15/2017
(c) Vicki Drake, SMC

7. PERIHELION AND APHELION
95,500,000 miles
91,500,000 miles
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(c) Vicki Drake, SMC

8. PERHELION AND APHELION - DATES
Perihelion occurs on, or about, January 3
Northern Hemisphere Winter
Aphelion occurs on, or about, July 4
Northern Hemisphere Summer
4/15/2017
(c) Vicki Drake, SMC

9. PERIHELION
At Perihelion, the Earth’s orbit is the closest to the Sun .
The Northern Hemisphere is ‘tilted away’ from the sun, receiving less solar radiation, with shorter daylight hours.
This is the Winter period for the Northern Hemisphere.
4/15/2017
(c) Vicki Drake, SMC

10. PERIHELION
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(c) Vicki Drake, SMC

11. APHELION At Aphelion, the Earth’s orbit is furthest away from the Sun.
The Northern Hemisphere is ‘tilted toward’ the Sun, resulting in more solar radiation, and longer daylight hours.
This is the Northern Hemisphere Summer period.
4/15/2017
(c) Vicki Drake, SMC

12. APHELION
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(c) Vicki Drake, SMC

13. Changes in Axis Orientation, Tilt and Revolution
Orientation of Earth’s axis changes during a 23,000-year cycle called precession
The Earth’s degree of tilt (obliquity) changes through a 41,000-year cycle – ranging between 22.5 and 24 degrees
Earth’s orbit (revolution) about the Sun changes from nearly circular to elliptical and back every 100,000 years – this process is called eccentricity
Milankovitch Theory: these changes can be linked to long-term climate changes based on latitudinal differences in insolation (incoming solar radiation)
4/15/2017
(c) Vicki Drake, SMC

14. CIRCLE OF ILLUMINATION
During rotation, at any given time, half of the Earth is receiving solar radiation – daylight
The other half of the Earth is in darkness – night
The ‘line’ separating day from night is the Circle of Illumination
The image below illustrates the Circle of Illumination without the tilt of the axis
4/15/2017
(c) Vicki Drake, SMC

15. INSOLATION AND LATITUDES
Insolation: solar radiation received by the Earth (incoming solar radiation)
Seasons: Variations of insolation due to spherical surface of Earth
Some latitudes receive more insolation:
Angle of incidence
Duration
4/15/2017
(c) Vicki Drake, SMC

16. INSOLATION AND LATITUDES
Only one latitude, at any time during Earth’s revolution, receives insolation at right angles at noon
The subsolar point on the Earth
Zenith Angle for Sun
Intensity of insolation measured by using Sun’s zenith angle
Sun’s angle above horizon at local noon
The angle at which Sun’s rays strike Earth’s surface determines amount of insolation
More direct angle = greater insolation
Subsolar point
4/15/2017
(c) Vicki Drake, SMC

17. LATITUDES and SUN RELATIONS
The following three latitudes are important because of their significance to seasons on the Earth
On certain days of the year (Equinoxes and Solstices), the Sun’s Zenith Angle, at local noon, will be 900 above one of these latitudes
Equator: 00
an imaginary line on the Earth's surface equidistant from the North Pole and South Pole that divides the Earth into a Northern Hemisphere and a Southern Hemisphere
Two days per year (Autumnal Equinox: September 21,22 and Vernal Equinox: March 20) the Sun’s location, at local noon is directly over the Equator
Tropic of Capricorn: 23½0 South
One day per year (Winter Solstice: December 21, 22) the Sun’s location, at local noon, is in the Capricorn constellation
Tropic of Cancer: 23½0 North
One day per year (Summer Solstice: June 21,22) the sun’s location, at local noon, is in the Cancer constellation
Arctic Circle: 66½0 North
marking the southern limit of the area where the sun does not rise on the Northern Hemisphere winter solstice (December 21) or set on the summer solstice (June 21)
Antarctic Circle: 66½0 South
marks the northern limit of the area where the Sun does not set on the Southern Hemisphere summer solstice (December 21) or rise on the winter solstice (June 21)
4/15/2017
(c) Vicki Drake, SMC

18. ZENITH ANGLE AND LATITUDES – WITHOUT TILT00
23 1/20 S
66 1/20 S
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(c) Vicki Drake, SMC

19. SOLSTICES, EQUINOXES, AND LATITUDES: NORTHERN HEMISPHERE BIAS!
Summer Solstice: Sun’s Zenith Angle of 900, at noon, is located at Tropic of Cancer, (23 ½0) North
On or about June 21, 22
Winter Solstice: Sun’s Zenith Angle of 900, at noon, is located at Tropic of Capricorn, (23 ½0) South
On or about December 21, 22
Vernal Equinox and Autumnal Equinox: Sun’s Zenith Angle of 900, at noon, is located at the Equator, 00
On or about March 20 and September 21, 22 respectively
4/15/2017
(c) Vicki Drake, SMC

20. SUMMER SOLSTICE Summer Solstice, June 21, 22
Northern Hemisphere is tilted towards the Sun
Latitudes higher than North receive 24 hours of sunlight
Latitudes higher than South receive 24 hours of night
Longest period of daylight for one day in year for Northern Hemisphere latitudes
First day of Summer: Northern Hemisphere
Vertical rays of Sun at noon
4/15/2017
(c) Vicki Drake, SMC

21. WINTER SOLSTICE Winter Solstice, December 22
Northern Hemisphere tilted away from the Sun
Latitudes higher than North receive 24 hours of night
Latitudes higher than South receive 24 hours of daylight
Longest period of night for one day for Northern Hemisphere latitudes
First day of Winter: Northern Hemisphere
Vertical rays of sun at noon
4/15/2017
(c) Vicki Drake, SMC

22. EQUINOXES: VERNAL, AUTUMNAL
Sun’s vertical rays at noon
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(c) Vicki Drake, SMC

23. VERNAL (SPRING) EQUINOX
Vernal Equinox, March 20
Zenith Angle of Sun at noon is 900 above Equator
Day and night are of equal length at all locations on the Earth
First day of Spring, Northern Hemisphere
Calendar (including specific dates) and even monuments based on Vernal Equinox
For example: the Council of Nice decreed in 325 A.D. that "Easter was to fall upon the first Sunday after the first full moon on or after the Vernal Equinox”
Julian and Gregorian Calendar
Early Egyptians built the Great Sphinx so that it points directly toward the rising Sun on the day of the Vernal Equinox.
4/15/2017
(c) Vicki Drake, SMC

24. AUTUMNAL (FALL) EQUINOX
Autumnal Equinox, September 22
Zenith Angle of Sun at noon is 900 above Equator
Day and night are of equal length at all locations on the Earth
First day of Fall, Northern Hemisphere
4/15/2017
(c) Vicki Drake, SMC

25. SEASONS AND EARTH’S REVOLUTION
Direct Rays 23 1/20 N
Direct Rays
23 1/20 S
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(c) Vicki Drake, SMC

26. CALENDARS AND SEASONS Julian Calendar: Introduced in 46 BC
A regular year of 365 days divided into 12 months, and a leap day is added to February every four years.
The Julian year is, on average, days long.
4/15/2017
(c) Vicki Drake, SMC

27. CALENDARS AND SEASONS Gregorian Calendar
Decreed in 1582 by Pope Gregory
Equinox and solstices almost two weeks early on Julian Calendar
Pope Gregory dropped 10 days from calendar to put equinoxes and solstices back on track.
October 4 followed by October 15
Changes in Gregorian Calendar
Add extra day to month of February every four years: “Leap Year”
Exception - only century years divisible by 400 become leap years
4/15/2017
(c) Vicki Drake, SMC

28. SEASONS Distance between Earth and Sun NOT a determinant of seasons
Perihelion occurs during Northern Hemisphere winter
Determinant # 1: Angle of Incidence of Sun’s rays striking Earth’s surface
Latitudes receiving more perpendicular rays receive more insolation for heating
Determinant # 2: Length of daylight hours
Longer daylight hours means more insolation
Determinant # 3: Angle of Incidence and length of daylight hours directly affected by tilt of Earth’s axis
4/15/2017
(c) Vicki Drake, SMC

29. ANGLE OF INCIDENCE - INSOLATION
The more vertical the rays of Sun means a more concentrated amount of solar radiation for a location.
4/15/2017
(c) Vicki Drake, SMC

30. ANALEMMA – MAPPING THE SUN’S MOVEMENT
An analemma traces the annual movement of the Sun on the sky.
It illustrates the positions of the Sun at the same time of day (at approximately 24 hour intervals) and from the same location on Earth on successive days through the calendar year.
This apparent shift of Sun’s position is due to the Earth’s orbit about the Sun
An analemma appears as a ‘loopy’ figure eight
the highest point is Summer
the lowest point, Winter
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(c) Vicki Drake, SMC

31. ANALEMMA
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(c) Vicki Drake, SMC

32. ANALEMMA The Analemma has a calendar printed on it
This calendar indicates which latitude (subsolar point) receives the Sun’s direct rays at noon (“Zenith Angle”) on any day of the year.
The most northern latitude is North
The most southern latitude is South
4/15/2017
(c) Vicki Drake, SMC