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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

slide2
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

slide3
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!
slide4
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
slide5
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!

slide6
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

slide7
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

slide8
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
slide9
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)
slide10
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
slide11
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

slide12
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

slide13
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

slide14
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

slide15
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)
slide16
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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