1. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 04: Orbiting Sphere Jan-24-07 (1 of 16) Outline Further Reading: Chapter 03 of the text book - earth-sun geometry - definitions - the seasons - diurnal and seasonal variations

2. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 04: Orbiting Sphere Jan-24-07 (2 of 16) Preludio 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.

3. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 04: Orbiting Sphere Jan-24-07 (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 GE 101 – Spring 2007 Boston University Myneni Lecture 04: Orbiting Sphere Jan-24-07 (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 GE 101 – Spring 2007 Boston University Myneni Lecture 04: Orbiting Sphere Jan-24-07 (5 of 16) Shape of the Orbit AphelionPerihelion Earth Sun Note timing of 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

6. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 04: Orbiting Sphere Jan-24-07 (6 of 16) Axial Tilt Direction of Rotation Axis of Rotation Axis 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

7. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 04: Orbiting Sphere Jan-24-07 (7 of 16) Solar Zenith Angle S N SZA Angle between a line perpendicular to the surface and the incoming ray from the sun

8. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 04: Orbiting Sphere Jan-24-07 (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 GE 101 – Spring 2007 Boston University Myneni Lecture 04: Orbiting Sphere Jan-24-07 (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 GE 101 – Spring 2007 Boston University Myneni Lecture 04: Orbiting Sphere Jan-24-07 (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 GE 101 – Spring 2007 Boston University Myneni Lecture 04: Orbiting Sphere Jan-24-07 (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 GE 101 – Spring 2007 Boston University Myneni Lecture 04: Orbiting Sphere Jan-24-07 (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 GE 101 – Spring 2007 Boston University Myneni Lecture 04: Orbiting Sphere Jan-24-07 (13 of 16) Course of the Sub-Solar Point Varies between 23.5N and 23.5S

14. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 04: Orbiting Sphere Jan-24-07 (14 of 16) Another Look at the Seasons

15. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 04: Orbiting Sphere Jan-24-07 (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 GE 101 – Spring 2007 Boston University Myneni Lecture 04: Orbiting Sphere Jan-24-07 (16 of 16) The Movies - Seasonal cycle movieSeasonal cycle movie - Sun path movieSun path movie