Have you ever wondered why the Earth’ seasons change throughout the year? Or why when it is summer in the US it is winter in Australia? Today we will explore why the Earth has seasons! It is all about the tilt of the Earth’s axis.
What is an Axis?
Before we dive too deep into things, let’s cover some basics. First things first, the Earth is shaped like a sphere. Now, imagine you put a pole straight through one side of the sphere to the other, like putting a round bead on a straight wire. Now imagine flicking that bead so it spins on the wire. This is an axis of rotation. The Earth has an axis of rotation from the North pole to the South pole. It is about this axis that the Earth spins, giving us night and day as one side faces the Sun and the other does not.
Image credit: Slyavula Education
It’s not necessarily the axis itself that gives us seasons, but its tilt. The Earth’s axis is tilted at a 23.4-degree angle from being perfectly straight up and down.
Image Credit: timeanddate.com
This tilt in relation to the Sun is what gives us seasons!
How Does a Tilt Result in Seasons?
Because the Earth is a sphere, the Sun’s rays do not hit the surface with the same intensity at all points on the Earth’s surface. The angle at which light hits a surface determines its intensity at the point. Light will have the highest intensity if it hits a point perpendicularly, or 90o. Because the Earth is round, it experiences the highest intensity sunlight at the equator, and less intense light as you move closer to the poles. The more intense light a surface receives, the higher temperatures it experiences. This is why the weather is much hotter at the equator than at the poles.
Image Credit: National Geographic, Mary Crooks
But how does this give us seasons? This is where the tilt comes in. The top of Earth’s axis is not always pointed at the Sun as it travels around it. At some points in the year, it is pointed away from the Sun. This changes which part of the Earth is getting more intense light throughout the year. This is our seasons!
Image Credit: timeanddate.com
During June through August, the North Pole is pointed towards the Sun, making it summer in the Northern Hemisphere, and winter in the Southern Hemisphere. During December through February, the North Pole is pointed away from the Sun, making it winter in the Norther Hemisphere and summer in the Southern Hemisphere. It is called Summer Solstice when the North Pole is pointed towards the Sun, and Winter Solstice when the South Pole is pointed towards the Sun.
Does Earth’s Elliptical Path Affect Our Seasons?
Earth’s orbit around the Sun is not perfectly circular, it is elliptical, meaning that at some points in the year the Earth is further away from the Sun. It is reasonable to think that the difference in distance may make an impact on our seasons, making the Earth hotter when it is closer to the Sun and colder when it is further. However, this is not the case. The change in distance from the Sun does not impact our seasons, because while the Earth’s orbit is not perfectly circular, it is not too far off. Eccentricity is the measure of how elliptical something is. An eccentricity measure of 0 is a perfect circle and 1 is a parabola. Ellipses are anything in between 0 and 1. The Earth’s eccentricity is 0.02. Making the difference between the furthest distance and closest distance to the Sun very small.
Image Credit: Swinburne University
When the Earth is closest to the Sun it is about 91.4 million miles (147.1 million km) from the Sun. At its furthest, Earth is 94.5 million miles (152.1 million km). While a 3.1-million-mile difference may seem large to us, it is very small on the scale of the solar system.
In fact, it is winter in the Norther Hemisphere when the Earth is closest to the Sun. The Earth is furthest from the Sun when it is summer in the Northern Hemisphere. Does this mean that the summers in the Northern Hemisphere are not as hot as the summers in the Southern Hemisphere? Nope! While the Earth does receive more energy from the Sun when it is closest during the Southern Hemisphere’s summer, this does not make Southern Hemisphere summers hotter. This is because there are fewer landmasses in the Southern Hemisphere than the Norther Hemisphere. It takes oceans longer to heat up than land mass, so with less land mass to heat up, the Southern Hemisphere does not get hotter just because it is closer to the Sun during its summer. Overall, this makes the summers in the Northern and Southern Hemispheres relatively the same temperature.
Why is the Earth’s Axis Tilted?
Astronomers’ best hypothesis as to why the Earth’s axis is tilted, is that long ago, when the Earth was young, something large hit Earth and knocked it off-kilter. This is why it leans at 23.4o instead of rotating straight up and down. Also, most scientists think that when the giant object hit Earth, chunks of Earth broke off, one of which became our Moon. And they call the giant object that hit Earth, Theia.
Image Credit: NASA
Do Other Planets Have Seasons?
Yes! All planets in our solar system have seasons. Most planets have the four that Earth experiences. Some planets have such a small change between the seasons it is hardly noticeable when a new one begins, this is what happens on Venus. Other planets, like Uranus, have very extremely seasons. While others like Mercury have seasons that are really hard to define. When astronomers refer to a planet’s season, they are referring to what season its Norther Hemisphere is experiencing.
If we remember that the Earth’s tilt is 23.4o and we have very distinguishable seasons it will be easy to compare how distinguishable seasons are on other planets. Jupiter and Venus have axis tilts of only about 3o, making their seasons very similar to one another.
While the Earth’s changes in distance from the Sun throughout the year do not affect our seasons because our eccentricity is not large, Mars has a very eccentric orbit of 0.09, compared to Earth. This means that both the tilt of Mar’s axis, 24o, and its changing distance form the Sun affects its seasons rather drastically.
Image Credit: NASA/JPL-Caltech/Richard Barkus
Even more extreme are the seasons on Uranus. Uranus’s orbit is nearly circular like Earth’s, meaning it keeps relatively the same distance from the Sun throughout its year. However, Uranus’s axis is tilted 82o! It is basically laying on its side. Just like what happened to Earth, it is hypothesized that a very large body collided with Uranus and knocked it to its side. For a shift this dramatic the colliding body is expected to be about the size of Earth! This extreme tilt gives Uranus extreme 20-year-long seasons and crazy weather. For about a quarter of Uranus’s year (84 Earth years) the Sun is directly hitting over each pole, leaving the rest of the planet in complete darkness and a very cold winter.
As we get closer and closer to winter let’s all be thankful it won’t last from more than a few months!