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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. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University. - definitions. - the seasons. - diurnal and seasonal variations.

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Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University

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  1. 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

  2. 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

  3. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 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!

  4. 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

  5. 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 Sun ~January 3 Aphelion Perihelion ~July 4 Note timing of seasons!

  6. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 04: Orbiting Sphere Jan-28-05 (6 of 16) Axial Tilt Angle of tilt ~23.5 degrees Axis of orbit 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

  7. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 04: Orbiting Sphere Jan-28-05 (7 of 16) Solar Zenith Angle SZA N S Angle between a line perpendicular to the surface and the incoming ray from the sun

  8. 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

  9. 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)

  10. 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

  11. 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

  12. 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

  13. 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

  14. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 04: Orbiting Sphere Jan-28-05 (14 of 16) Another Look at the Seasons

  15. 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)

  16. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 04: Orbiting Sphere Jan-28-05 (16 of 16) The Movies - Seasonal cycle movie - Sun path movie

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