Radiation Basics and Earth's Energy Budget

Earth's shape

Earth is an oblate spheroid, meaning it is approximately spherical but wider around its horizontal axis than its vertical axis.

Oblate spheroid

A sphere-like object that is wider around its equator than from pole to pole.

Why is Earth an oblate spheroid?

Earth's rapid rotation causes the planet to bulge outward at the Equator.

Earth's rotational velocity

Earth's spinning speed is fastest at the Equator, approximately 1,674.4 km/h (1,040.4 mph).

Earth's rotation

The spinning of Earth around its rotational axis.

Direction of Earth's rotation

Earth rotates from west to east.

Rotational axis

The imaginary line running through Earth's North and South Poles around which Earth rotates.

Length of one Earth rotation

A complete rotation takes approximately 24 hours.

Equator

The midpoint between Earth's North and South Poles, located at 0° latitude.

Why is rotational velocity greatest at the Equator?

The Equator is farthest from Earth's rotational axis, so locations there travel the greatest distance during each rotation.

Latitude

A geographic coordinate measured as an angle north or south of the Equator, ranging from 0° at the Equator to 90° at the poles.

0° latitude

The Equator.

90° latitude

The North and South Poles.

Boreal

Relating to the Northern Hemisphere.

Austral

Relating to the Southern Hemisphere.

Earth-Sun orbital mechanics

The motions and geometric relationships between Earth and the Sun, including Earth's rotation, revolution, axial tilt, and orbital position.

Earth's revolution

The movement of Earth around the Sun.

Length of Earth's revolution

One complete revolution around the Sun takes approximately 365¼ days.

Orbital plane

The flat plane in which Earth travels around the Sun.

Plane of the ecliptic

Another name for Earth's orbital plane.

Earth's orbit

Earth's orbit is approximately elliptical rather than a perfect circle.

Ellipse

A closed, oval-shaped curve used to approximate Earth's orbit around the Sun.

Orbital eccentricity

The degree to which Earth's orbit differs from a perfect circle.

Perihelion

The point in Earth's orbit when Earth is closest to the Sun.

Perihelion date

The lecture identifies approximately January 4 as perihelion.

Earth-Sun distance at perihelion

Approximately 147 million km.

Aphelion

The point in Earth's orbit when Earth is farthest from the Sun.

Aphelion date

The lecture identifies approximately July 4 as aphelion.

Earth-Sun distance at aphelion

Approximately 152 million km.

Energy difference between perihelion and aphelion

Earth receives approximately 7% more solar energy at perihelion than at aphelion.

What causes the seasons?

Earth's approximately 23.5° axial tilt relative to its orbital plane causes the seasons.

What would happen if Earth's rotational axis were perpendicular to its orbital plane?

There would be no seasons caused by axial tilt.

Axial tilt

The angle at which Earth's rotational axis is tilted relative to the plane of its orbit; approximately 23.5°.

Earth's axial inclination

Approximately 23.5°.

Boreal summer

Summer in the Northern Hemisphere, occurring when the Northern Hemisphere is tilted toward the Sun.

Austral winter

Winter in the Southern Hemisphere occurring at the same time as boreal summer.

Boreal winter

Winter in the Northern Hemisphere, occurring when the Northern Hemisphere is tilted away from the Sun.

Austral summer

Summer in the Southern Hemisphere occurring at the same time as boreal winter.

Opposite seasons between hemispheres

When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere, and vice versa.

Axial parallelism

Earth's axis maintains approximately the same orientation in space as Earth moves around the Sun.

Does Earth's axial tilt change as Earth orbits the Sun?

The lecture emphasizes that Earth's axial tilt remains approximately 23.5° and remains parallel in orientation regardless of orbital location.

Why does axial tilt produce seasons?

The tilt changes the angle at which sunlight reaches different parts of Earth and changes the length of daylight through the year.

Solar calendar

The annual cycle defined by Earth's changing orientation toward the Sun and marked by solstices and equinoxes.

Corners of the solar calendar

The solstices and equinoxes, which mark important points in Earth's seasonal cycle.

Solstice

An endpoint of the seasonal calendar associated with the longest day or longest night of the year.

Summer solstice

The solstice associated with the longest daylight period and shortest night in a hemisphere.

Winter solstice

The solstice associated with the shortest daylight period and longest night in a hemisphere.

Equinox

A point in the seasonal calendar when day and night are approximately equal in duration.

Equinoxes

The two points in Earth's annual cycle when day and night are approximately equal in duration.

March equinox

The equinox associated with the transition toward boreal spring and austral autumn.

September equinox

The equinox associated with the transition toward boreal autumn and austral spring.

Boreal seasons

The seasonal cycle of the Northern Hemisphere.

Austral seasons

The seasonal cycle of the Southern Hemisphere.

Seasonal calendar

The annual cycle of seasons resulting from Earth's axial tilt and orbital position.

Earth rotation vs. revolution

Rotation is Earth's approximately 24-hour spin on its axis, while revolution is Earth's approximately 365¼-day orbit around the Sun.

Milankovitch cycles

Subtle variations in Earth's orbital mechanics that can produce major climate changes over millennial timescales.

Why are Milankovitch cycles important?

Small changes in Earth's orbital mechanics can alter the distribution of solar energy received by Earth and influence climate over very long timescales.

Sun as an energy source

The Sun is the principal source of energy for life on Earth.

Solar energy

The energy received from the Sun in the form of electromagnetic radiation.

Insolation

Incoming solar radiation.

Total solar energy and latitude

The amount and intensity of solar energy vary with latitude.

Solar energy concentration

The concentration of solar energy depends on the angle at which sunlight strikes Earth's surface and the area over which that energy is distributed.

Solar energy intensity

The intensity of incoming solar energy depends on the angle of incoming sunlight and the area over which the energy is spread.

Why does solar intensity vary with latitude?

The curvature of Earth causes sunlight to strike different latitudes at different angles, changing the area over which solar energy is distributed.

Direct solar rays

Solar rays that strike a surface at a relatively high angle and therefore concentrate energy over a smaller area.

Oblique solar rays

Solar rays that strike a surface at a lower angle and spread the same energy over a larger area.

Subsolar point

The point on Earth's surface where the Sun's rays strike perpendicular to the surface and solar energy is at maximum intensity.

Maximum insolation

The greatest concentration of incoming solar energy occurs at the subsolar point.

Subsolar point and latitude

The intensity of solar energy decreases with increasing distance in latitude from the subsolar point.

Why is the subsolar point important?

It identifies where incoming solar radiation is most concentrated at a given time.

Solar radiation angle

The angle at which solar rays strike Earth's surface, which affects the concentration and intensity of incoming energy.

Insolation intensity and latitude

Insolation intensity generally decreases with increasing latitude because solar rays strike the surface at increasingly oblique angles.

Tropics

The region of Earth in which the subsolar point can occur during the year.

Seasonal movement of the subsolar point

The subsolar point migrates north and south during Earth's annual orbit around the Sun.

Why does the subsolar point move?

Earth's 23.5° axial tilt causes the location receiving perpendicular solar radiation to shift north and south throughout the year.

Tropic of Cancer

The northernmost latitude reached by the subsolar point during the year, approximately 23.5° N.

Tropic of Capricorn

The southernmost latitude reached by the subsolar point during the year, approximately 23.5° S.

Why are the Tropics located at approximately 23.5° N and 23.5° S?

They correspond to Earth's approximately 23.5° axial tilt and mark the seasonal limits of the subsolar point.

Solar energy and season

Solar energy varies with both latitude and season.

Total insolation at the top of Earth's atmosphere

The amount of incoming solar radiation reaching the top of Earth's atmosphere, which varies according to Earth's position and the distribution of solar energy.

Seasonal variation in total insolation

Total insolation changes throughout the year as the subsolar point migrates north and south.

Subsolar migration

The seasonal movement of the location where the Sun is directly overhead at solar noon.

Latitude and season relationship

Solar energy received at a location depends on both its latitude and the season.

Why does a location receive different solar energy during the year?

Earth's axial tilt changes both the angle of incoming solar radiation and the length of daylight at a location throughout the year.

Photoperiod

The length of daylight experienced by an organism or location.

Day length

The number of hours of daylight in a given day.

Orbital mechanics and day length

Earth's axial tilt and orbital position cause the amount of daylight received at different latitudes to change throughout the year.

Photoperiodism

The physiological reaction of organisms to the length of day or night.

Photoperiodism and environmental cues

Organisms can use changes in day length as a seasonal environmental signal.

Day length and organisms

Changes in day length can influence physiological and behavioral processes in organisms.

Photoperiod and temperature

Together, photoperiod and temperature changes can trigger seasonal physiological and behavioral changes.

Photoperiod effects on animals

Changes in photoperiod can influence fur and feather coloration, migration, hibernation, sexual behavior, and the size of sexual organs.

Photoperiod and migration

Seasonal changes in daylight can provide a signal that influences when some animals migrate.

Photoperiod and hibernation

Changes in day length can help trigger entry into or preparation for hibernation.

Photoperiod and sexual behavior

Changes in daylight duration can influence seasonal sexual behavior.

Photoperiod and fur/feather color

Seasonal changes in day length can contribute to changes in the color of fur and feathers.

Canary example

Some birds, such as canaries, sing more or less frequently in response to changes in photoperiod.

Long-day plants

Plants that flower when day length increases beyond a critical threshold.

Long-day plant flowering trigger

An increase in day length beyond a critical threshold promotes flowering.

Bigleaf maple

Bigleaf maple (Acer macrophyllum) is presented as an example of a plant that produces raceme flowers as day length increases in spring.