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Changing seasons

The Earth is tilted 23.5 degrees on its axis, which affects the distribution of the sun’s energy across the surface of the planet. As the Earth orbits the sun every 365 ¼ days, the axis is always pointing in the same direction into space, with the North Pole toward Polaris, the North Star. Around June 22, the northern hemisphere is angled towards the sun, and receives the most direct radiation and the most energy. This is the start of astronomical summer in the northern hemisphere and winter in the southern hemisphere. 

Placement of Earth relative to the Sun at the start of each season.
 

Six months later, in December, the Earth has completed half a revolution around the sun. The northern hemisphere is now angled away from the sun and receives less energy than the southern hemisphere; this is the beginning of winter in the northern hemisphere and summer in the southern hemisphere. From north to south, the results of the distribution of solar energy can be seen in changing vegetation, animal behaviors, temperatures, and day length.

Addition of growing days per year as the high-latitude growing season gets longer as a result of climate change.
High-latitude growing season getting longer
Few real-world signs of climate change are easier to read than changes in the growing season of familiar vegetation...

Seasonal changes in precipitation and temperature affect soil moisture, evaporation rates, river flows, lake levels, and snow cover. Leaves fall and plants wither as cold and dry seasons approach. 

These changes in vegetation affect the type and amount of food available for humans and other organisms. Animals in temperate and polar regions must find alternate food sources, move to warmer locations, or hibernate. 

The Gulf of Mexico's dead zone, where oxygen levels drop to dangerously low concentrations, typically forms in the spring and lasts into the summer.
The Gulf of Mexico’s dead zone
The Gulf of Mexico’s dead zone typically begins in the spring and persists into the summer. Springtime floods wash fertilizers from upriver into the gulf.

EDUCATION CONNECTION

What are some ways that animals adapt to seasonal changes in your region? How does this compare to other areas? How do people adjust to the seasonal changesoffsite link in their region? How do the seasons impact the use of energy in your community? The resources in this collection help educators encourage their students connect their own observations of the seasons to concepts in Earth science and biology. 

Green sea turtle. One of several species that may strand on our shores when water temperatures drop.
Cold stun season in the Northeast
Sea turtles are commonly found in waters off the Mid-Atlantic and Northeast United States during the summer and early fall. Typically, they begin to migrate...

Updated February 2019