Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 04: Orbiting Sphere
Jan-28-05
(1 of 16)
Further Reading: Chapter 03 of the text book
Outline
- earth-sun geometry
- definitions
- the seasons
- diurnal and seasonal variations
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 04: Orbiting Sphere
Jan-28-05
(2 of 16)
• In last lecture, based on the
Shape and Rotation of the Earth
we were able to devise
Geographical 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.
Preludio
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
An Example
• Latitudinal variation in climate regimes
• Controlled by
- Total incoming radiation
- Seasonality in radiation
• How and Why?
Earth-Sun Astronomical relationship!
Myneni
Lecture 04: Orbiting Sphere
Jan-28-05
(3 of 16)
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 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 Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 04: Orbiting Sphere
Jan-28-05
(5 of 16)
Shape of the Orbit
• 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
Earth
~July 4
Sun
Aphelion
Perihelion
~January 3
Note timing
of seasons!
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 04: Orbiting Sphere
Jan-28-05
(6 of 16)
Axial Tilt
Axis of orbit
Angle of tilt ~23.5 degrees
Axis of Rotation
Direction of Rotation
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 Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 04: Orbiting Sphere
Jan-28-05
(7 of 16)
Solar Zenith Angle
N
SZA
S
Angle between a line perpendicular to the surface and
the incoming ray from the sun
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 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 intensity
Large solar zenith angle -> less intensity
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 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 Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 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 Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 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 Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 04: Orbiting Sphere
Jan-28-05
(12 of 16)
Equinoxes
Sub-solar point located at Equator
Circle of illumination extends between poles
Day-length equal to 12 hours everywhere
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 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 Atmosphere
GG 101 – Spring 2005
Boston University
Another Look at the Seasons
Myneni
Lecture 04: Orbiting Sphere
Jan-28-05
(14 of 16)
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 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 Atmosphere
GG 101 – Spring 2005
Boston University
The Movies
- Seasonal cycle movie
- Sun path movie
Myneni
Lecture 04: Orbiting Sphere
Jan-28-05
(16 of 16)