Seasons | The Earth-sun-moon system | Middle school Earth and space science | Khan Academy

Why Earth Has Seasons

Earth’s seasons are not primarily caused by changes in its distance from the Sun. The Northern and Southern Hemispheres have opposite seasons, which a change in Earth’s overall distance from the Sun would not explain. Earth is farthest from the Sun in July, during Northern Hemisphere summer, and closest in January, during Northern Hemisphere winter.

Seasons result from Earth’s axial tilt. Earth’s axis is tilted about 23.5 degrees relative to the plane of its orbit around the Sun. As Earth orbits the Sun, each hemisphere alternately tilts toward or away from it:

• When the Northern Hemisphere tilts toward the Sun, it receives sunlight more directly and has summer.
• When it tilts away, sunlight arrives at a lower angle, spreading the same incoming energy over a larger area. This means less energy per unit of surface and contributes to winter.
• The Southern Hemisphere experiences the opposite seasons.

The tilt also affects daylight hours. During Northern Hemisphere summer, the North Pole has continuous daylight while the South Pole has continuous nighttime. This pattern reverses when the Northern Hemisphere is tilted away from the Sun.

In spring and autumn, neither hemisphere is tilted strongly toward or away from the Sun, so comparable locations receive sunlight at more similar angles.

The diagram discussed is not to scale: the Sun is much larger than Earth, and the actual distance between them is far greater than the illustration suggests.

Speaker and Source

• Speaker: One unnamed narrator explaining the topic
• Source: Khan Academy