By Scott Hamilton
Great news! The winter solstice occurred just before Christmas, which means our days have begun to get longer. This, of course, leads to a question much like the ones I discussed about the Moon over the last few weeks. What exactly is the winter solstice and why do we have longer days in the summer and shorter days in the winter?
It may sound a bit odd, but it has to do with the tilt of the Earth’s axis. You see, Earth does not spin on an axis that is perpendicular to its orbit. In fact, there is only one planet in our solar system that does. If we lived on Mercury, we would not have a solstice, or seasons for that matter, as there is no tilt in Mercury’s axis.
We have one planet that is at the other extreme. Uranus has a tilt of 98 degrees, which makes for really extreme seasons. The winter temperature on Uranus can reach as low as -370 degrees and summer temperatures can reach 116 degrees. However, because it takes the planet 84 years to orbit the Sun, each season lasts 21 years, so it might not be too bad in the spring and fall.
The earth lands somewhere in the middle of the axis tilt spectrum with a tilt of 23.5 degrees. The temperature swing between winter and summer is heavily dependent on how close to the equator you are, as it depends on the amount of solar radiation reaching the surface of the Earth. In the summer for our hemisphere, the North Pole is closer to the Sun, and we have more hours of daylight and higher concentrations of radiation, leading to higher temperatures. If we live further south, we are closer to the equator and have less of a change in the hours of daylight, resulting in less temperature changes.
Let’s take a couple of examples from the extremes. San Antonio, Texas, will have 10 hours and 15 minutes of daylight on the winter solstice and 14 hours, 1 minute of daylight on the summer solstice. If we look at Utqiagvik, Alaska, the northernmost city in the U.S., the Sun never rises during the winter solstice, nor for the entire month of December. Likewise, for the summer solstice the Sun never sets during the entire month of June. As you can imagine there is a larger temperature difference between summer and winter in Utqigvik than there is in San Antonio.
While we have covered some interesting facts about the hours of daylight and the temperature differences between the summer and winter, we have not really explained why this happens, other than to claim it is due to the tilt of the Earth’s axis. So how exactly does this tilt impact the hours of daylight and the temperature of the region? To answer this question, we have to think very hard about not only how the Earth spins, but also the position of the Earth’s orbit around the Sun. To make things easier for you to observe, we will just talk about how the orbit changes the position of the Sun in the sky.
Because of the 23.5 degree tilt in the axis, the Sun’s position in the sky changes with the seasons, or at least it appears to change due to our view point. In reality the Earth is moving in regards to its position with the Sun. If you looked at the Sun at the same time every day it would be in a different position in the sky every day. The change is so subtle from one day to the next that we never really notice it, but if we look at it over time, it won’t take long to notice a big difference. Viral photographer Ciro Russo, did exactly that and his photos are not too hard to find. (See https://www.pitribe.com/posts/sun-tilt-south-north-year-monthly-photos-103431659916259.) You can see in his collage of 12 photos that the position of the sun is significantly different each month. His photos were taken facing southeast shortly after sunrise and the Sun moves from the left edge of his panorama to just past the center of the image and back again. If you were to take measurements on his images and calculate the angular change of the Sun’s position, you would find it to be exactly 47 degrees.
This apparent wobble in the position of the Sun happens because as Earth orbits the Sun, the angle of the Sun in the sky shifts as the position of the axis changes with regards to the Sun. When our tip of the axis is closest to the Sun, our days last longer and the Sun shifts to the north by exactly 23.5 degrees at the summer solstice (the longest day of the year). As we orbit the Sun, our tip of the axis moves slowly away from the Sun until we reach the autumn equinox, where the Sun is directly centered over the equator (no shift based on the Earth’s tilt). We then continue around, moving further away from the Sun until the winter solstice (the shortest day of the year) and the Sun shifts to the south by exactly 23.5 degrees. This cycle completes when we reach the spring equinox and the Sun is exactly over the equator again before completing our orbit back at the summer solstice.
Until next week, stay safe and learn something new.
Scott Hamilton is an Expert in Emerging Technologies at ATOS and can be reached with questions and comments via email to firstname.lastname@example.org or through his website at https://www.techshepherd.org.