Higher Education

Unit 05

EARTH-SUN RELATIONSHIPS

UNIT OVERVIEW

This unit focuses on changes in the Earth's receipt of the solar energy associated with its revolution around the Sun, its rotation on its axis, and the tilt of its axis with respect to the plane of the eliptic. The main sections pertain to:

Earth's planetary motions
Seasonality
Insolation and its variation

The Earth's orbital path around the Sun is nearly circular. As it is revolving, the Earth is also rotating on its axis. The tilt of the Earth's axis causes the four seasons. At any given moment in time, the Earth is divided into a sunlit half and dark half. This division is known as the circle of illumination.

The intensity and duration of sunlight greatly affects the amount of solar energy received at the Earth's surface. Intensity and duration vary over space and time¿both the time of day and the time of year.

Teaching Objectives

To examine the Earth's motions relative to the Sun
To demonstrate the consequences of the Earth's axis tilt for the annual march of the seasons
To introduce the time and spatial variations in solar radiation received at surface locations

Glossary of Key Terms

Antarctic circle	The latitude marking the northern boundary of the Southern Hemisphere portion of the Earth's surface that receives a 24-hour period of sunlight at least once a year.
Aphelion	The point in the Earth's orbit, which occurs every July 4th, where the distance to the Sun is maximized.
Arctic Circle	The latitude marking the southern boundary of the Northern Hemisphere portion of the Earth's surface that receives a 24-hour period of sunlight at least once each year.
Axis	The imaginary line that extends from the North Pole to the South Pole through the center of the Earth; the planet's rotation occurs with respect to this axis.
Circle of illumination	At any given moment on our constantly rotating planet, the boundary between the halves of the Earth that are in sunlight and darkness.
Daylight-saving time	By law, all clocks in a time zone are set one hour forward from standard time for at least part of the year.
Equinox	One of the two days (March 21 and Sept. 21) in the year when the Sun's noontime rays strike the Earth vertically at the Equator; in Northern Hemisphere terminology, the March 21 event is called the spring (vernal) equinox and the September 23 event is called the fall (autumnal) equinox.
Fall (autumnal) equinox	The equinox that occurs when the Sun's noontime rays strike the Equator vertically, on approximately September 23.
Insolation	Incoming solar radiation.
International date line	For the most part is antipodal to the Prime Meridian and follows the 180th meridian; crossing the line toward the west involves skipping a day, while crossing the line toward the east means repeating a day.
Perihelion	The point in the Earth's orbit, which occurs every January 3, where the distance to the Sun is minimized.
Plane of the ecliptic	The plane formed by the Sun and the Earth's orbital path.
Revolution	One complete circling of the Sun by a planet; it takes the Earth precisely one year to complete such an orbit.
Rotation	The spinning of a planet on its axis, the imaginary line passing through its center and both poles; it takes the Earth one calendar day to complete one full rotation.
Solar elevation	The number of degrees above the horizon of the noontime Sun, the position at which the solar rays strike the surface at their highest daily angle. Also called the angle of incidence.
Solstice	(see summer solstice; winter solstice).
Spring (vernal) equinox	In Northern Hemisphere terminology, the equinox that occurs when the Sun's noontime rays strike the Equator vertically on approximately March 21.
Summer solstice	The day each year of the poleward extreme in the latitude where the Sun's noontime rays strike the Earth's surface vertically. In the Northern Hemisphere, that latitude is 23 degrees North (the Tropic of Cancer) and the date is June 22; in the Southern Hemisphere, that latitude is 23 degrees South (the Tropic of Capricorn) and the date is December 22.
Tropic of Cancer	The most northerly latitude (23 degrees North) where the Sun's noontime rays strike the Equator vertically (June 22).
Tropic of Capricorn	The most southerly latitude (23 degrees South) where the Sun's noontime rays strike the Earth's surface vertically (December 22 ).
Winter solstice	The day each year of the poleward extreme in latitude in the opposite hemisphere where the Sun's noontime rays strike the Earth's surface vertically. In the Northern Hemisphere, that latitude is 23 degrees South and the date is December 22; in the Southern Hemisphere, that latitude is 23 degrees North and the date is June 22.
Zenith	The point in the sky directly overhead, 90° above the horizon

Unit Outline

Earth's planetary motions
- Revolution is movement of a planet around the Sun
  - perihelion
  - aphelion
- Rotation is the spinning motion of a planet on its own axis
  - Coriolis force is a product of rotation
Seasonality
- Axis tilt
  - plane of the ecliptic
  - Tropic of Cancer
  - Arctic circle
  - Tropic of Capricorn
  - Antarctic circle
- Solstices and equinoxes
  - summer solstice (June 22)
  - winter solstice (December 22)
  - earth receives 12 hours light and 12 hours darkness at equinoxes
  - vernal equinox (March 21)
  - autumnal equinox (September 23)
- The four seasons
  - spring (March 21-June 22)
  - summer (June 22-September 23)
  - autumn (September 23- December 22)
  - winter (December 22-March 21)
Insolation and its variation
- Circle of illumination
- Insolation factors
  - solar elevation (angle of incidence)
  - duration of daily sunlight
- Global profiles for insolation

Review Questions

Trace the path of the Earth as it travels on its yearly revolution around the Sun. What are the dates of the solstices and the equinoxes?
Describe the Coriolis force.