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Seasons on Earth

  • Direct sunlight

  • How the sun illuminates the Earth

  • … at different latitudes and times of year

  • The cause of seasons

Intro

Dr. Gabriela Gonzalez, who played a leading role in the first detection of gravitational waves, did her undergraduate degree in Córdoba, Argentina. She then moved to Syracuse, NY, USA for her doctorate.

Map of the Americas with labeled markers for Syracuse, NY, USA (green) in the northeastern United States and Córdoba, Argentina (purple) in central Argentina. A curved black arrow points from Córdoba northward to Syracuse. An oval portrait of a woman appears over North America near the Syracuse label.

Córdoba is hottest in January and coldest in July, but Syracuse is hottest in July and coldest in January.

Line graph of monthly average high and low temperatures (°F) for Córdoba (purple) and Syracuse (green). Córdoba’s curves are highest in January and lowest in July, while Syracuse’s curves are lowest in January and highest in July.
Source: © WeatherSpark.com

Why did Dr. Gonzalez experience different seasonal patterns in each location?

Sunlight and the Earth's Orbit

  • Earth’s axis is tilted by 23.5 degrees w/r/t the ecliptic plane

  • The Northern Hemisphere is tilted toward the Sun in June

  • The Southern Hemisphere is tilted toward the sun in December

Diagram of Earth at four points in its orbit (March equinox, June solstice, September equinox, December solstice). In all four positions, Earth’s axis is tilted about 23.5° relative to a line perpendicular to the orbital plane and remains parallel to itself; in the June position the North Pole leans toward the Sun, in the December position it leans away, and at the equinox positions it tilts sideways relative to the Sun. Source: timeanddate.com

Sunlight carries energy

The angle of sunlight changes how much the Sun heats the Earth.

When a surface is tilted, it absorbs less energy from the Sun. Sunlight illustration with parallel yellow arrows pointing downward across the image. In the center, a horizontal green bar casts evenly spaced vertical shadow lines below it. When the bar is tilted diagonally relative to the vertical arrows, it shadows fewer arrows.

Direct sunlight

  • More direct sunlight when the sun is high: energy is concentrated
    Illustration of a person standing on a flat surface under a dome-shaped sky. Dashed arcs show different Sun paths across the sky.The Sun on the path highest toward the zenith, casting a yellow beam at a steep angle toward the person, illuminating a small spot on the ground.
  • Less direct sunlight when the sun is low: energy is spread out
    Illustration of a person standing on a flat surface under a dome-shaped sky. Dashed arcs show different Sun paths across the sky.The Sun on the path lowest near the right horizon, casting a yellow beam at a shallow angle toward the person, illuminating a larger spot on the gorund.

The changing angle of sunlight

From geosynchronous orbit, the Meteosat satellite recorded infrared images of the Earth every day at the same local time, from September 2010 to September 2011.

Geosynchronous: always above the same position on Earth's surface.

Because it is the same local time, the Sun is always to the left of the camera when the photo was taken.

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Which of these pictures shows a physically realistic diagram of the Sun’s rays hitting the Earth?

Three images of sun rays and half-illuminated earths. The left side of each Earth is bright and the right side is dark. (A) - Rays come from lower left. (B) - Rays come from upper left. (C) - Rays come from direct left.

Sunlight on the Earth

The way the sun lights the earth determines the length of days and the amount of direct sunlight at each latitude.

Earth spins on its axis, with the north pole tilted to the right in this image. Sunlight comes in from the left and the left half is illuminated. (A) marks a latitude line near the top of the sphere, less than half in light. (B) marks a latitude line near the bottom of the sphere, more than half in light. (C) marks a latitude near Antarctica, almost all in light.

In this animation, (A) and (B) are each the same distance from the equator, but (A) is in the northern hemisphere and (B) is in the southern hemisphere. (C) is close to Antarctica.

Build your understanding

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In the animation above, which latitude has more hours of daylight?

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In the animation above, which latitude has more direct sunlight?

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Which is more important for seasonal temperature: direct sunlight or hours of daylight?

Hint: Compare location B to location C in the animation above.

Changing sunlight on Earth

The Sun is at the zenith for different regions on Earth, depending on the ecliptic:

Map of earth showing the region around the Equator. The Tropic of Cancer (green) is a bit above the equator, just below Florida and the Tropic of Capricorn (blue) is a bit below, through Australia.

  • The sun is directly above the Equator at noon on the equinoxes
  • The sun is directly above the Tropic of Cancer in the Northern Hemisphere at noon on the June solstice (our Summer Solstice)
  • The sun is directly above the Tropic of Capricorn in the Southern Hemisphere at noon on the December solstice (our Winter Solstice)

Sun over Equator

Diagram of Earth half illuminated and half in darkness, with sunlight coming from the left. Earth’s axis is straight up and down. Latitude lines labeled Arctic Circle, Tropic of Cancer, Equator, Tropic of Capricorn, and Antarctic Circle appear on the right. A figure above the Tropic of Cancer and a figure below the Tropic of Capricorn are shown. They make the same angle with the sun rays.
Base image

This image shows the angle of the sunlight when the sun is directly overhead on the equator.

  • This is an Equinox.

  • Northern and southern hemispheres experience the same sunlight.

Sun over Tropic of Cancer

Diagram of Earth half illuminated and half in darkness, with sunlight coming from the left. Earth’s axis is shown tilted, with the North Pole angled away from the Sun. Latitude lines labeled Arctic Circle, Tropic of Cancer, Equator, Tropic of Capricorn, and Antarctic Circle appear on the right. A figure above the Tropic of Cancer is angled more toward the sun. A figure below the Tropic of Capricorn is angled more away from sun and wears a hat.

This image shows the angle of the sunlight when the sun is directly overhead on the Tropic of Cancer.

  • This is the June solstice.

  • Summer in Northern Hemisphere

  • Winter in Southern Hemisphere

Sun over Tropic of Capricorn

Diagram of Earth half illuminated and half in darkness, with sunlight coming from the left. Earth’s axis is shown tilted, with the North Pole angled away from the Sun. Latitude lines labeled Arctic Circle, Tropic of Cancer, Equator, Tropic of Capricorn, and Antarctic Circle appear on the right. A figure above the Tropic of Cancer is angled more away from sun and wears a hat. A figure below the Tropic of Capricorn is angled more toward the sun.

This image shows the angle of the sunlight when the sun is directly overhead on the Tropic of Capricorn.

  • This is the December solstice.

  • Winter in Northern Hemisphere

  • Summer in Southern Hemisphere

The Cause of the Seasons

The tilt of Earth’s rotational axis and Earth’s movement in its orbit change how much direct sunlight shines on the northern and southern hemispheres.

Seasonal Lag

Average Temperatures at LAX Weather Station:

Line graph of monthly temperatures from January to December with three lines labeled High (yellow), Average (blue), and Low (purple). Highs increase from about 65°F in January to about 75°F in August–September, averages from about 57°F to 69°F, and lows from about 50°F to 65°F, then all decrease toward December.

Sunlight is most direct here on the Summer Solstice. Why isn’t June the hottest month?

Partly, it takes time to warm up and cool down. The Pacific Ocean acts as a heat reservoir, and wind patterns from the ocean or from the inland desert also change our weather.

Check your understanding

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Which of the following experiences the smallest change in sunlight over the course of a year?

Two Earths are shown, one on each side of the sun. The North pole is tilted to the left for each. On the left side, A is near the North pole, B is on the Equator, and C is below the equator, while sunlight comes in from the right. On the right side, D is near the North pole, E is on the Equator, and F is near the South pole, while sunlight comes from the left.

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In the image above, which letter represents a location on Earth that is experiencing winter in the Northern Hemisphere?

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In the image above, which letter represents a location on Earth that is experiencing summer in the Southern Hemisphere?