Lesson 3: Earth’s Tilt and Its Orbit around the Sun

Why is it summer in the United States (North America) when it is winter in Brazil (South America)?

The Sun’s light strikes Earth’s curved surface at different angles in different locations, which causes areas with higher average temperatures (warmer areas) near the equator and areas with lower average temperature (cooler areas) as one moves toward the polar regions. Earth spins on its axis, which passes through the North and South Poles, as it orbits the Sun in a nearly circular path once a year. Earth is tilted on its axis at 23½ degrees from a line perpendicular to its orbital path. As Earth orbits the Sun, its axis always tilts in the same direction—toward the North Star. Earth’s consistent tilt as it orbits the Sun causes the Northern Hemisphere to lean toward the Sun at certain times of year and away from the Sun at other times of the year. When the Northern Hemisphere is tilted toward the Sun, the Southern Hemisphere is tilted away from the Sun. Earth’s Northern Hemisphere experiences summer when it leans toward the Sun while the Southern Hemisphere experiences winter. Conversely, Earth’s Southern Hemisphere experiences summer when it leans toward the Sun while the Northern Hemisphere experiences winter. (Note that the seasonal variations that we call summer and winter do not occur at latitudes close to the equator.) When Earth’s hemispheres lean neither toward nor away from the Sun along Earth’s orbit, we experience spring and fall. As Earth orbits the Sun, the axis always tilts in the same direction, which results in different parts of Earth getting different amounts of sunlight at different times of the year. Consequently, Earth’s consistent tilt produces opposite seasons in the Northern and Southern Hemispheres.

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