Natural Environments: The AtmosphereGG 101 – Spring 2005

Boston University

MyneniLecture 04: Orbiting Sphere

Jan-28-05(1 of 16)

Outline

Further Reading: Chapter 03 of the text book

- earth-sun geometry

- definitions

- the seasons

- diurnal and seasonal variations

Natural Environments: The AtmosphereGG 101 – Spring 2005

Boston University

MyneniLecture 04: Orbiting Sphere

Jan-28-05(2 of 16)

Preludio• In last lecture, based on the Shape and Rotation of the Earth

we were able to deviseGeographical 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.

Natural Environments: The AtmosphereGG 101 – Spring 2005

Boston University

MyneniLecture 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 AtmosphereGG 101 – Spring 2005

Boston University

MyneniLecture 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 AtmosphereGG 101 – Spring 2005

Boston University

MyneniLecture 04: Orbiting Sphere

Jan-28-05(5 of 16)

Shape of the Orbit

Aphelion Perihelion

EarthSun

Note timingof 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

Natural Environments: The AtmosphereGG 101 – Spring 2005

Boston University

MyneniLecture 04: Orbiting Sphere

Jan-28-05(6 of 16)

Axial Tilt

Direction of Rotation

Axis of RotationAxis 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

Natural Environments: The AtmosphereGG 101 – Spring 2005

Boston University

MyneniLecture 04: Orbiting Sphere

Jan-28-05(7 of 16)

Solar Zenith Angle

S

N SZA

Angle between a line perpendicular to the surface and the incoming ray from the sun

Natural Environments: The AtmosphereGG 101 – Spring 2005

Boston University

MyneniLecture 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 intensityLarge solar zenith angle -> less intensity

Natural Environments: The AtmosphereGG 101 – Spring 2005

Boston University

MyneniLecture 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 AtmosphereGG 101 – Spring 2005

Boston University

MyneniLecture 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 AtmosphereGG 101 – Spring 2005

Boston University

MyneniLecture 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 AtmosphereGG 101 – Spring 2005

Boston University

MyneniLecture 04: Orbiting Sphere

Jan-28-05(12 of 16)

Equinoxes

Sub-solar point located at EquatorCircle of illumination extends between poles

Day-length equal to 12 hours everywhere

Natural Environments: The AtmosphereGG 101 – Spring 2005

Boston University

MyneniLecture 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 AtmosphereGG 101 – Spring 2005

Boston University

MyneniLecture 04: Orbiting Sphere

Jan-28-05(14 of 16)

Another Look at the Seasons

Natural Environments: The AtmosphereGG 101 – Spring 2005

Boston University

MyneniLecture 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 AtmosphereGG 101 – Spring 2005

Boston University

MyneniLecture 04: Orbiting Sphere

Jan-28-05(16 of 16)

The Movies

- Seasonal cycle movie

- Sun path movie