A Common Misconception
Many misconceptions about outer space exist today. One of the more common misconceptions we come in contact with is about where the seasons on Earth come from. It would be logical to assume that as Earth gets closer to the sun, we would be experiencing summer, and as it gets farther from the sun we would be experiencing winter.
Even though there is a difference in distance between Earth and the Sun, the difference is not enough to impact seasonal changes on Earth. In Boise, Idaho (where I’m currently crafting this blog post), it is 99 degrees Fahrenheit. In Wellington, New Zealand, the high for today is 56 degrees Fahrenheit. We see each location in the Northern and Southern hemisphere respectively, experiencing very different temperatures.
Reason for the Seasons
On the 6th of July, Earth was at what astronomers call Aphelion. This means that Earth was the farthest distance from the Sun that it will be all year long (roughly 94.5 million miles). On July 6th, it was almost 100 degrees Fahrenheit in Boise, Idaho. Since Earth is at it’s farthest distance from the Sun on that day, how is it possible that Boise would be experiencing such high temperatures?
The figure above shows us the distance between the Earth and Sun at Perihelion and Aphelion, but it also shows us how the Earth’s axis is tilted. Earth’s axial tilt is 23.5 degrees from the plane of its orbit around the Sun. Because of this tilt, Earth receives more or less direct sunlight depending on the time of year. During the summer, Boise is receiving more direct sunlight and thus experiences higher temperatures than Wellington, New Zealand is at the same time.
What Caused Earth’s Axial Tilt?
Most scientists point to the Giant-Impact Hypothesis to tell us why Earth’s axis is tilted. It is believed that originally, Earth’s axis was perpendicular to its equator (90 degrees). According to the hypothesis, roughly 4.5 billion years ago a Mars-sized planet called Theia collided with a young Earth that knocked its axis off-kilter.
Changes in Axial Orientation
Over long periods of time, Earth’s axis actually changes position in the sky. This change in position is called precession. Precession is the wobble in Earth’s axis that will change the way the axis is oriented. This wobble is very similar to the wobble that occurs in a
spinning top. The figure at the right depicts how the precession of the top is similar to the precession of Earth. As the top spins quickly, we can see the axis of the top spinning at a slower rate in a circular motion. The same thing occurs with Earth, although at a much more gradual rate. In fact, it takes Earth 26,000 years to make a full precession. Currently, our axis is oriented toward Polaris making it our North Star. However, in 13,000 years our axis will be oriented toward Vega which would make Vega our new North Star.
It would make sense to assume that seasonal variances are due to our distance from the Sun, but sometimes space just isn’t that simple. We have Theia and our axial tilt to thank for the beautiful seasonal changes we experience on Earth. What’s your favorite season?