The Reasons for the Seasons

earth and its seasons
Earth as seen by orbiting satellites through all four seasons. NASA

The change of seasons is one of those phenomena that people take for granted. They know it happens in most places, but don't always stop to think about why we have seasons. The answer lies in the realm of astronomy and planetary science.

The biggest reason for the seasons is that Earth's axis is tilted relative to its orbital plane. Think of the orbital plane of the solar system as a flat plate. Most of the planets orbit around the Sun on the "surface" of the plate. Rather than having their north and south poles point directly perpendicular to the plate, most planets have their poles at a slant. This is particularly true of Earth, whose poles are tilted 23.5 degrees.

Earth may have a tilt because of a large impact in our planet's history that likely caused the creation of our Moon. During that event, infant Earth was smacked pretty heavily by a Mars-sized impactor. That caused it to tip over on its side for a while until the system settled down. 

One idea of the Moon's formation.
The best theory about the formation of the Moon says that the infant Earth and a Mars-sized body called Theia collided early in the history of the solar system. The remnants were blasted to space and eventually coalesced to form the Moon. NASA/JPL-Caltech 

 

Eventually, the Moon formed and Earth's tilt settled to the 23.5 degrees it is today. It means that during part of the year, half of the planet is tilted away from the Sun, while the other half is tilted toward it. Both hemispheres still get sunlight, but one gets it more directly when it's tilted toward the Sun in summer, while the other gets it less directly during winter (when it is tilted away). 

This diagram shows Earth's axial tilt and how it affects the hemispheres that are tilted toward the Sun through different parts of the year.  NASA/CMGlee

When the northern hemisphere is tilted toward the Sun, people in that part of the world experience summer. At the same time, the southern hemisphere gets less light, so winter occurs there. The solstices and equinoxes are used mostly in calendars to mark the beginning and end of seasons but are not themselves related to the causes of the seasons.

Seasonal Changes

Our year is divided up into four seasons: summer, fall, winter, spring. Unless someone lives at the equator, each season delivers different weather patterns. Generally, it's warmer in spring and summer, and cooler in autumn and winter. Ask most people why it is cold in the winter and warm in the summer and they'll likely say that Earth must be closer to the Sun in the summer and farther away in the winter. This seems to make common sense. After all, as someone gets close to a fire, they feel more heat. So why wouldn't closeness to the Sun cause the warm summer season?

While this is an interesting observation, it actually leads to the wrong conclusion. Here's why: Earth is farthest from the Sun in July each year and closest in December, so the "closeness" reason is wrong. Also, when it is summer in the northern hemisphere, winter is happening in the southern hemisphere, and vice versa. If the reason for the seasons was solely due to our proximity to the Sun, then it should be warm in both the northern and southern hemispheres at the same time of year. That doesn't happen. It's really the tilt that is the main reason we have seasons. But, there is another factor to consider.

the jovian worlds of jupiter, saturn, uranus, and neptune
All planets have an axial tilt, including the gas giants. The Uranus tilt is so severe it "rolls" around the Sun on it side. NASA

It's Hotter at High Noon Too

Earth's tilt also means that the Sun will appear to rise and set in different parts of the sky during different times of the year. In the summertime the Sun peaks almost directly overhead, and generally speaking will be above the horizon (i.e. there will be daylight) during more of the day. This means that the Sun will have more time to heat the surface of the Earth in the summer, making it even warmer. In the winter, there's less time to heat the surface, and things are a bit chillier.

Observers can generally see this change of apparent sky positions for quite easily. Over the course of a year, it's fairly easy to note the position of the Sun in the sky. In the summertime, it will be higher up and rise and set at different positions than it does in the wintertime. It's a great project for anyone to try, and all they need is a rough drawing or picture of the local horizon to the east and west. Observers can glance out at the sunrise or sunset each day, and mark the positions of sunrise and sunset each day to get the full idea.

Back to Proximity

So, does it matter how close Earth is to the Sun? Well, yes, in a sense, it does, just not the way people expect. Earth's orbit around the Sun is only slightly elliptical. The difference between its closest point to the Sun and the most distant is a little more than three percent. That isn't enough to cause huge temperature swings. It translates to a difference of a few degrees Celsius on average. The temperature difference between summer and winter is a lot more than that. So, closeness doesn't make as much of a difference as the amount of sunlight the planet receives. That's why just simply assuming that Earth is closer during one part of the year than another is wrong. The reasons for our seasons are easy to understand with a good mental image of our planet's tilt and its orbit around the Sun.

Key Takeways

  • Earth's axial tilt plays a large role in creating seasons on our planet.
  • The hemisphere (north or south) tilted toward the Sun receives more heat during that time.
  • Closeness to the Sun is NOT a reason for the seasons.

Sources

  • "Earth's Tilt Is the Reason for the Seasons!” Ice-Albedo Feedback: How Melting Ice Causes More Ice to Melt - Windows to the Universe, www.windows2universe.org/earth/climate/cli_seasons.html.
  • Greicius, Tony. “NASA Study Solves Two Mysteries About Wobbling Earth.” NASA, NASA, 8 Apr. 2016, www.nasa.gov/feature/nasa-study-solves-two-mysteries-about-wobbling-earth.
  • “In Depth | Earth – Solar System Exploration: NASA Science.” NASA, NASA, 9 Apr. 2018, solarsystem.nasa.gov/planets/earth/in-depth/.