A common belief is that summer happens when Earth is closest to the Sun. In fact, Earth reaches its closest point, perihelion, in early January, during the Northern Hemisphere’s winter. The seasons are caused by something else: Earth’s tilt.
Key Takeaways
- Earth’s axis is tilted about 23.4 degrees relative to its orbit.
- When your hemisphere tilts toward the Sun, sunlight arrives more directly and days are longer, producing summer.
- The Northern and Southern hemispheres have opposite seasons.
- Solstices mark the extremes of tilt; equinoxes are when day and night are roughly equal.
The tilt
Earth spins on an axis tilted about 23.4 degrees. As Earth orbits the Sun, the axis keeps pointing in the same direction in space, toward Polaris, the North Star. For half the year, the Northern Hemisphere tilts toward the Sun; for the other half, away.
Why tilt matters
Two effects combine:
- Angle of sunlight: When the Sun is high in the sky, its energy is concentrated on a smaller area. When it is low, the same energy spreads over a larger area and passes through more atmosphere.
- Day length: Tilting toward the Sun means longer days and more hours of heating.
Distance makes a small difference
Earth’s orbit is slightly elliptical. At perihelion in early January, Earth is about 147 million kilometres from the Sun; at aphelion in early July, about 152 million kilometres. The difference changes sunlight intensity by about 7%, but the effect of tilt is much larger.
Solstices
- June solstice (around June 20 to 21): the Northern Hemisphere tilts most toward the Sun. Longest day in the north, shortest in the south.
- December solstice (around December 21 to 22): the opposite.
Equinoxes
Around March 20 and September 22 or 23, neither hemisphere tilts toward the Sun. Day and night are roughly equal around the world.
Seasonal lag
The warmest days usually come weeks after the June solstice, and the coldest weeks after the December solstice. Oceans and land take time to heat and cool.
Seasons near the equator and poles
Near the equator, the Sun is high all year, and seasons are often defined by rainfall rather than temperature. Near the poles, tilt produces extreme seasons, with months of continuous daylight or darkness.
Other planets
Uranus is tilted about 98 degrees, so it orbits almost on its side, producing extreme seasons. Mars has a tilt similar to Earth’s and has seasons too.
Frequently Asked Questions
Does Earth’s tilt change? It varies slowly between about 22.1 and 24.5 degrees over a cycle of roughly 41,000 years.
Are meteorological and astronomical seasons the same? No. Meteorologists use whole months, starting seasons on March 1, June 1, September 1 and December 1.
Why is the Southern Hemisphere summer not much hotter if Earth is closer? The Southern Hemisphere has more ocean, which moderates temperatures.
The bottom line
Seasons come from Earth’s tilt, which changes the angle of sunlight and the length of the day. Distance from the Sun plays only a minor part.
