We all know the four seasons. Traditionally, spring, summer, autumn, and winter. Of course here in Texas your family, like mine, may have a running joke that would add a slight variation or two to that list. The seasons are historically associated with cycles of life: birth, growth, harvest, and death. On the scientific side, they are the result of the earth’s Axial Tilt and Elliptical Orbit. Often we think the seasons are because of the physical distance of the earth from the sun, but our distance from the sun is actually not the reason for the seasons.
Axial Tilt means that the earth’s North Pole and South Pole do not point straight “up” or “down” as the earth rotates. Our earth is in fact tilted about 21-24 degrees to the side. As the earth travels around the sun, that tilt combined with our atmosphere and Texas’ location in the Northern Hemisphere are the cause of our seasons. During the summer, because we face the sun more directly, the Northern Hemisphere gets more direct sunlight, while the Southern Hemisphere gets indirect sunlight. As we continue our orbit, during our winter time, we are getting that indirect sunlight, while the Southern Hemisphere gets the direct sunlight. Yes, this means in Australia, New Years is in the middle of summer.
Not only is our axis not completely straight, but our orbit around the sun isn’t a perfect circle. We have an Elliptical Orbit, which means our annual orbit around the sun is more of an oval or elliptical shape. This oval is what directly affects the Apsis, or one of two points of extreme distance of the earth to the sun.
Surprisingly in the summer time, the earth is physically farther away from the sun than at any other point, and in winter, we are physically closer to the sun than at any other point. Those two extreme points of distance, pluralized as apsides (app-sid-deez), are the Aphelion and Perihelion respectively.
Aphelion is what we call the farthest point that earth will be from the sun. Next year in 2020, we will reach aphelion on July 4th and at this point, we will be about 94,507,635 miles from the sun. Perihelion is the point at which we are closest to the sun. The next perihelion will occur quite soon, on January 5, 2020, and we will be about 91,398,199 miles from the sun.
If you need a reason to extend your celebrations in the coming years, don’t worry, the apsides are not the same every year. Thanks to the gravitational pull of the sun, the earth’s orbit is not exactly the same path from year to year. Because of those slight variations in our orbit, the apsides shift around from year to year. For example, in 2021, the perihelion will fall on January 2, and the aphelion on July 5. Hundreds of years ago in the 1200s, perihelion fell in December, and scientists estimate that in 4000 years from now, it will fall in March.
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So if you, like me, want one more reason to celebrate and eat good food, then come January 5th, I wish you a very Happy Perihelion. If you would like to celebrate the season with my team at the Perot Museum, we have very special holiday event taking place December 21 – January 5. Festively called “Tech the Halls,” we will be celebrating the winter season with engineering-themed programming like augmented reality robot hockey, simple machines, coat design, snowflake races, and special Arctic paleo chats.
For more information on this special program, more info can be found at perotmuseum.org/TechtheHalls.