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Myneni Lecture 04: Orbiting Sphere Jan-28-05 (1 of 16). Further Reading: Chapter 03 of the text book. Outline. - earth-sun geometry. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University. - definitions. - the seasons. - diurnal and seasonal variations. - PowerPoint PPT Presentation
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Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 04: Orbiting Sphere
Jan-28-05(1 of 16)
Outline
Further Reading: Chapter 03 of the text book
- earth-sun geometry
- definitions
- the seasons
- diurnal and seasonal variations
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 04: Orbiting Sphere
Jan-28-05(2 of 16)
Preludio• In last lecture, based on the Shape and Rotation of the Earth
we were able to deviseGeographical Coordinate Systems and Time.
• In this lecture we will study the Orbit of the earth about the sun
which is the first step in understanding the Energy Balance
for various locations on the earth and at various times of the year.
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 04: Orbiting Sphere
Jan-28-05(3 of 16)
An Example
• Latitudinal variation in climate regimes
• Controlled by - Total incoming radiation - Seasonality in radiation
• How and Why? Earth-Sun Astronomical relationship!
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 04: Orbiting Sphere
Jan-28-05(4 of 16)
Earth-Sun Geometry
• Spin of the earth about the axis (i.e. rotation)
• Inclination of the axis of spin relative to the axis of orbit around the sun
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 04: Orbiting Sphere
Jan-28-05(5 of 16)
Shape of the Orbit
Aphelion Perihelion
EarthSun
Note timingof seasons!
~January 3~July 4
• Orbit is an ellipse• Period: takes ~365.25 days to make one revolution• Direction: orbits counter-clockwise looking down on the north pole• Closest approach - “Perihelion” ~ 147.5 million km• Farthest distance - “Aphelion” ~ 152.5 million km
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 04: Orbiting Sphere
Jan-28-05(6 of 16)
Axial Tilt
Direction of Rotation
Axis of RotationAxis of orbit
Angle of tilt ~23.5 degrees
Axial tilt: The angle at which the axis of the earth’s rotation is tilted with respect to the orbit around the sun
Note: Combination of axial tilt and orbit around the sun causes the SEASONS
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 04: Orbiting Sphere
Jan-28-05(7 of 16)
Solar Zenith Angle
S
N SZA
Angle between a line perpendicular to the surface and the incoming ray from the sun
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 04: Orbiting Sphere
Jan-28-05(8 of 16)
SZA and Radiation Flux
If the same amount of energy is spread over a larger area, the “intensity” of the radiation at a given point is less
Small solar zenith angle -> high intensityLarge solar zenith angle -> less intensity
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 04: Orbiting Sphere
Jan-28-05(9 of 16)
Circle of Illumination & Sub-solar Point (Declination)
Circle of Illumination: The half-sphere which is illuminated by the sun
Sub-solar point: The location on the earth’s surface where the sun is directly overhead
Declination: The latitude of the sub-solar point at a given time of year (varies between 23.5N and 23.5S)
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 04: Orbiting Sphere
Jan-28-05(10 of 16)
The Seasons
• The fixed axial tilt as Earth orbits the sun results in systematic variation in solar geometry, the seasons
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 04: Orbiting Sphere
Jan-28-05(11 of 16)
Solstices
Sub-solar point located at tropics of Cancer and Capricorn (23.5 degrees N and S)
Circle of Illumination extends between 66.5 degrees N and S
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 04: Orbiting Sphere
Jan-28-05(12 of 16)
Equinoxes
Sub-solar point located at EquatorCircle of illumination extends between poles
Day-length equal to 12 hours everywhere
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 04: Orbiting Sphere
Jan-28-05(13 of 16)
Course of the Sub-Solar Point
Varies between 23.5N and 23.5S
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 04: Orbiting Sphere
Jan-28-05(14 of 16)
Another Look at the Seasons
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 04: Orbiting Sphere
Jan-28-05(15 of 16)
Diurnal & Seasonal Variations
Maximum solar zenith angle and Daylength (rotation through circle of illumination) are controlled by
Time of year & Latitude
(e.g., at 40 degrees north – the figure)
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 04: Orbiting Sphere
Jan-28-05(16 of 16)
The Movies
- Seasonal cycle movie
- Sun path movie