Why Do We Have Seasons?


Many people believe the seasons work in a simple cycle: December and January are cold and snowy, April and May are full of flowers, July and August are hot and sunny, and September and October are full of colorful leaves.

This perception seems magical and like the natural order of the world.

If someone were told that one-third of Earth’s population has never seen snow or that July 4th is not a beach day everywhere, it would sound unbelievable.

The Reality of Seasons

In reality, having four distinct seasons only happens in two regions on Earth, and even there, the seasons are reversed!

But why?

Johannes Kepler and Earth’s Orbit

Most people have heard of Johannes Kepler, an astronomer who discovered that planetary orbits are elliptical and that the sun isn’t at the exact center.

This was a significant discovery several hundred years ago because it solved many mathematical problems astronomers had with measuring planetary orbits.

Earth’s Almost Circular Orbit

Earth’s orbit is not a perfect circle, but it’s very close.

Because it’s technically an ellipse, our distance from the sun changes throughout the year.

It might be assumed that winter happens when Earth is farther from the sun, but actually, Earth is closer to the sun in January than in July by about 5 million kilometers!

In the northern hemisphere, January is in the middle of winter.

Simultaneous Seasons

Another interesting fact is that summer and winter occur simultaneously on different parts of Earth.

For example, when it’s winter in Connecticut, it’s summer in New Zealand.

The Tilt of the Earth

If it’s not the distance from the sun, what causes the seasons?

The Earth doesn’t sit straight up; it tilts.

This tilt, at about 23.5 degrees from vertical, is one of the main reasons for the seasons.

As Earth spins on its tilted axis and revolves around the sun, the tilt causes the number of daylight hours in a region to change throughout the year.

There are more daylight hours in summer and fewer in winter.

Daylight Hours and Seasonal Warming

During the summer, a place like Hartford, Connecticut, which is about 40 degrees north of the equator, gets around 15 hours of daylight and only 9 hours of darkness each day.

This means it warms up for longer than it cools down, leading to an overall warming effect.

In winter, the opposite happens, resulting in a cooling effect.

Variations in Daylight Hours

As one moves further north, the number of daylight hours in summer increases.

For instance, Juneau, Alaska, gets about 19 hours of daylight on a summer day, while Tallahassee, Florida, gets about 14.

At the North Pole, the sun doesn’t set at all in summer.

The Role of Solar Energy

But daylight hours aren’t the only factor.

If they were, the North Pole would be the hottest place on Earth in the northern summer because it gets 24 hours of daylight.

Yet, it remains cold with ice and snow.

Solar Energy Distribution

The Earth is a sphere, so the amount of solar energy an area receives changes based on how high the sun is in the sky.

This height varies during the day and throughout the year, with the greatest height at noon on the summer solstice (June 21 in the northern hemisphere and December 21 in the southern hemisphere).

During summer, the northern hemisphere is tilted towards the sun, making the sun higher in the sky for longer periods.

Combining Factors for Seasonal Change

The increased daylight hours and the higher position of the sun mean more solar energy per square kilometer.

However, at the North Pole, the sunlight is spread out and delivers less energy because the sun is always at an angle, even during summer.

Plus, the North Pole has to make up for six months of cooling down without any sunlight.

Appreciating the Seasons

So, as the seasons change, remember that it’s not just the beauty of each new season to appreciate but also the astronomical complexity that brings them to you.