(This blog post is just a holiday interest story. It does not concern anything legal.)
On January 5, 2020 at 2:48 am Eastern time, the Earth and Sun will be closer to each other than at any other time of year. We call it Perihelion Day. Because the Sun is so close to the Earth on this day, you might think it should coincide with summer. But when it comes to the amount of heat generated from Sunlight, it turns out that distance doesn’t have nearly the same effect as angle. In winter, the northern hemisphere of the Earth is tilted about 23 degrees away from the Sun. At this angle, the light from the Sun is much less intense, which is the main reason we have winter at this time of year.
Perihelion Day happens because the Earth’s orbit around the Sun is not quite circular. Instead, it is elliptical, and the position of the Sun is not exactly in the center of the ellipse. The not-quite-roundness of the Earth’s orbit, combined with the off-center position of the Sun, creates an orbit where the Earth kind of wobbles through its orbit. In the winter, we have Perihelion Day, and in the summer, we have Aphelion Day, where the Earth and Sun are farthest apart. The Greek prefix “peri” means near; the prefix “apo” means far; and the root “helios” means Sun.
The Perihelion distance changes every year, mostly because of gravitational interference from the Moon and, to a lesser extent, the other planets. This means the exact shape of Earth’s orbit is slightly different every year. Sometimes the orbit is nearly circular. Sometimes it is more elliptical. The Sun itself constantly wobbles as well, due in large part to the planets’ different gravitational pulls, especially Jupiter.
Happy Perihelion Day!