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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.

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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


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

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!


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

Sun

~January 3

Aphelion

Perihelion

~July 4

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

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


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


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

Myneni

Lecture 04: Orbiting Sphere

Jan-28-05

(14 of 16)

Another Look at the Seasons


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

Myneni

Lecture 04: Orbiting Sphere

Jan-28-05

(16 of 16)

The Movies

- Seasonal cycle movie

- Sun path movie


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