Why Do We Have Seasons? It Is Not About Distance

Many people assume summer happens when Earth is closest to the Sun and winter when it is farthest. In fact, the reverse is true for the Northern Hemisphere, and distance is not the main reason for seasons at all. The real cause is the tilt of Earth's axis.

Key takeaways

• Earth's rotation axis is tilted about 23.5 degrees relative to its orbit around the Sun.
• Tilt changes how directly sunlight strikes each hemisphere across the year.
• Longer days and more direct sunlight bring summer, while shorter days and slanting sunlight bring winter.
• Earth is slightly closest to the Sun in early January, during Northern Hemisphere winter.
• The hemispheres have opposite seasons.

The tilt of the axis

Earth spins on an axis that points in nearly the same direction in space as it travels around the Sun. That axis is tilted by roughly 23.5 degrees from perpendicular to the plane of its orbit. As a result, for part of the year the Northern Hemisphere leans toward the Sun, and for another part it leans away. Six months later, the situation reverses.

Why tilt changes the weather

Two effects work together. First, when a hemisphere leans toward the Sun, the Sun rises higher in the sky at midday, so sunlight strikes the ground more directly and spreads its energy over a smaller area, heating the surface more. Second, days are longer, giving more hours of warming. When the hemisphere leans away, the Sun is lower, sunlight is more spread out and days are shorter, so there is less heating.

Solstices and equinoxes

The solstices mark the extremes. In June, the Northern Hemisphere is tilted most toward the Sun, giving the longest day of the year, while the Southern Hemisphere has its shortest day. In December, this is reversed. At the equinoxes, in March and September, neither hemisphere is tilted toward the Sun, and day and night are close to equal in length almost everywhere.

The distance myth

Earth's orbit is slightly elliptical, but only a little. Earth is closest to the Sun, at perihelion, in early January, and farthest, at aphelion, in early July. The difference in distance is only a few percent, which has a much smaller effect than the tilt. If distance were the cause, both hemispheres would have summer at the same time, which does not happen.

Seasons near the equator and poles

Near the equator, the Sun is high all year, so temperature changes little and the main seasonal change may be wet and dry periods. Near the poles, the effect is extreme: in summer the Sun may not set for weeks or months, and in winter it may not rise at all.

Why other planets differ

Planets with different tilts have different seasons. Uranus is tilted nearly on its side, so its poles can face the Sun for decades. Mars has a tilt similar to Earth's, so it also has seasons, though they last about twice as long because its year is longer.

Daylight hours around the world

The tilt of Earth's axis changes the length of the day more at higher latitudes. At the equator, daylight stays close to twelve hours all year. At around 40 degrees north, which includes cities such as Madrid and New York, the longest day is roughly fifteen hours and the shortest roughly nine. Closer to the poles, the difference grows until the Sun stays above the horizon for weeks in summer and below it for weeks in winter. Inside the Arctic and Antarctic circles, there is at least one day a year with no sunset and one with no sunrise.

The seasonal lag

The hottest and coldest times of year do not coincide with the solstices. In the Northern Hemisphere, the longest day is in June, but July and August are usually hotter. This seasonal lag happens because land, and especially the oceans, take time to warm up. Even after the days start shortening, the Earth is still receiving more energy than it loses, so temperatures keep rising for a few weeks. The same effect works in reverse in winter, when the coldest weather often comes in January or February, after the shortest day.

Different ways to define seasons

There is more than one way to divide the year. Astronomical seasons are marked by the solstices and equinoxes, so spring begins at the March equinox in the Northern Hemisphere. Meteorologists often use calendar months instead, treating March to May as spring, because it makes climate records easier to compare. Many tropical regions have wet and dry seasons rather than the familiar four. Some cultures describe more than four seasons. In parts of India, a traditional calendar has six seasons, and several Aboriginal Australian groups describe seasonal cycles based on local plants, animals and weather.

Seasons and living things

Many organisms respond to changes in day length, called photoperiod. Trees drop their leaves, birds migrate, some mammals hibernate and many plants flower at particular times of year. Gardeners and farmers use these cues to plan sowing and harvests. Day length is a more reliable signal than temperature, because temperature can swing from week to week, while day length follows a fixed pattern.

Seasons in a changing climate

Records from many regions show that spring events, such as leaf-out and flowering, have been arriving earlier in recent decades, and that growing seasons are lengthening in some places. These changes can disrupt relationships between species, for example if insects emerge before the birds that feed on them arrive. Scientists track these shifts, called phenology, using long records kept by observers and satellites.

Frequently asked questions

Why is it summer in Australia when it is winter in Europe?

The hemispheres tilt toward the Sun at opposite times of year.

Does the Moon cause seasons?

No. The Moon affects tides but does not change the seasonal pattern.

Is the tilt constant?

It changes slightly over tens of thousands of years, which is one factor in long-term climate cycles.

Why are day and night not exactly equal on the equinox?

Day and night are very close to equal, but not exact. The atmosphere bends sunlight so that the Sun appears slightly above the horizon before it has truly risen, and sunrise and sunset are measured at the Sun's upper edge. Both effects add a few minutes of daylight.

Is the summer solstice the hottest day of the year?

No. It is the day with the most daylight, but temperatures usually peak a few weeks later because land and oceans keep absorbing more heat than they release.

Do all places have four seasons?

No. Many tropical places have wet and dry seasons, polar regions have long dark and light periods, and some regions are mild all year. The four-season pattern is typical of temperate latitudes.

Sources and further reading

• Season (Wikipedia)
• NASA Science: Earth's seasons