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

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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

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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