Why Does Earth Have Seasons?

The Short Answer:

Earth has seasons because its axis is tilted. Earth’s axis is always pointed in the same direction, so different parts of Earth get the sun’s direct rays throughout the year. For example, in summer, the sun's rays hit that region more directly than at any other time of the year.

Many people believe that Earth is closest to the sun in summer and farthest in winter...
SAY WHAT?!

Although this idea makes sense, it is incorrect.

It is true that Earth's orbit is not a perfect circle. It is slightly elongated, so that during part of the year, Earth is closer to the sun than at other times. However, in the Northern Hemisphere, we are having winter when Earth is closest to the sun and summer when it is farthest away!

There is a completely different reason for Earth's seasons.

Cartoon of large object hitting Earth, knocking out big chunks of material that become the future Moon, and tilting the Earth's axis.

Earth has seasons because sometime early in its long history, something very big hit the young Earth to knock it off-kilter. So instead of rotating with its axis perpendicular to its orbital plane, it is tilted 23.45 degrees from the perpendicular.

Incidentally, that big something that hit Earth also knocked a chunk of it out that became our Moon. At least that is generally accepted theory.

So, here we are, orbiting the sun, but tilted a bit and always with the axis pointed in the same direction. So different parts of Earth get the sun's direct rays as we travel through the year.

Earth's tilt is the reason for the seasons. View of Earth in relation to the sun during each of the four seasons. The hemisphere receiving the direct rays of the sun has summer while the hemisphere tilted away from the sun, thus getting its rays from more of an angle, has winter.

Thus, sometimes it is the North Pole tilting toward the sun (like in June) and sometimes it is the South Pole tilting toward the sun (like in December). Hence, the seasons. It is summer in June in the Northern Hemisphere because the sun's rays hit that part of Earth more directly than at any other point in Earth's orbit - or, in other words, more directly than at any other time of the year. It is winter in December in the Northern Hemisphere, because that is when it is the South Pole's turn to be tilted toward the sun.

It follows that if you live in North America, during the winter the sun's path across the sky is more toward the south, rising in the southeast and setting in the southwest. During the summer, the sun tracks more directly overhead, rising in the east, overhead at noon, and setting in the west. If you live in the Southern Hemisphere, the sun will be more northerly in the sky during the winter (that is, June, July, and August).

So if you go to South Africa for the winter holidays, bring your swimsuit and leave your skis at home.