“Perihelion” occurs during the first week of January and it’s when the Earth is as close to the Sun as it gets in its orbit around it. “Aphelion” occurs during the first week of July and it’s when the Earth is as far away from the Sun as it gets in its orbit around it.
This may lead those in the Southern hemisphere to think that’s the reason for the heat of summer and chill of winter. However, if you live in the Northern hemisphere you’re likely thinking that things are completely opposite of what they should be. After all, how is it winter when we are closest to the Sun?! Well, as we will explore, perihelion & aphelion are in fact not the reason for the seasonal differences but instead it comes down to what side of the Sun the Earth is on.
Seasons change because of what’s called axial tilt, which for Earth is 23.45 degrees. This tilt combined with Earth’s orbit is what actually creates the seasons. In the winter, the Northern hemisphere is tilted away from the Sun which means the Southern hemisphere is tilted toward the Sun, creating summer in the Southern hemisphere and winter in the Northern hemisphere. Six months later the roles are reversed and the Northern hemisphere’s tilted toward the Sun and the Southern hemisphere, away from it, creating Northern hemisphere summer and Southern hemisphere winter.
It’s also important to realize that the Earth doesn’t actually tilt back and forth every season, it stays relatively the same. What happens is, as the Earth orbits the Sun throughout the year; that 23.45 degree tilt places the Earth in a position to where different hemispheres get different amounts of light.
Just to illustrate; even though the earth has moved 186 million miles to the opposite side of the Sun in 6 months-time, Polaris is still the North Star so you know the tilt hasn’t changed, just the Earth has. Try this the next time you have a school room globe and a light handy to simulate the Sun. Start by placing the Globe with the North Pole facing away from the Sun to simulate Northern hemisphere winter. Now, while keeping the North Pole pointed in the same direction place the Earth on the opposite side of the Sun. Now you can see that the Northern hemisphere gets the majority of the Sun’s light to simulate summer and the poles are still pointing in the same general directions. This illustrates that it’s really not the Earth that tilts, but in fact the Earth IS TILTED; thus, the change from one side of the Sun to the other places the different hemispheres in a position to receive more radiation from the Sun. Keep in mind that if you do this experiment indoors and you take the time to simulate a North Star it will move. The reason that this doesn’t happen with the real Earth and Polaris is because your simulated north star is but a few feet away and the real Polaris is about 435 light years away. The 180 million mile shift that the Earth partakes in every six months isn’t enough to create a change.
If you’re really observant you may say to yourself, “In the Northern hemisphere the winter solstice in on December 21st but the coldest months of the year are in January and February. And summer solstice falls on June 21st while the hottest months of the year fall on July and August…..what gives?!!” First of all, great observation, and there’s an answer for that as well. It’s called the “Lag of the Seasons” and it’s basically because the earth is a gigantic heat sink. Think of it as your diet, if you change your diet, your body doesn’t change immediately but instead it changes slowly over weeks and months. Another great analogy is an oil tanker; to change direction, speed up or slow down you have to make your corrections and input before you actually need the result because there’s a large lag time between the manual action and that desired result for such a large object.
The same applies to the Earth; the weeks of long days during late May and June finally heat the Earth’s oceans and land, which in turn heats the atmosphere. The result is felt with hot July and August months. In the winter the darkest nights of the year in late November and December finally chill the oceans and land, which in turn chills the atmosphere enough create the bitterly cold months in January and February. Try to pay attention to it this year, the coldest months lag behind the darkest days and the hottest months lag behind the longest days as the Earth takes time to react to the forces that the Sun is placing upon it.
Let’s break this down one step further to help you understand temperatures relative to Sun/Earth interaction. We’re going to take a quick look at individual days. The same actions that create the “Lag of the Seasons” also creates the “Lag of the Days”. You may have noticed this before but if you haven’t try to pay attention to it.
In the summer the Sun’s highest moments of radiation delivery are around noon time and surrounding hours because it’s directly overhead. Thus, it’s as close to you as it will be all day and its energy has the least resistance in the form of atmosphere that it has to contend with all day. So why aren’t the hottest times of day around noon? It’s because it takes time for the Sun’s radiation to heat the land. That land heating then heats the atmosphere and that is why the hottest times of the day are usually between 1400 and 1600, not around noon.
The only thing that I would add to this is that we’re talking about heat only. Remember that I said the highest levels of radiation from the Sun are when it’s as high in the sky as it can be? So for sunburn and sunscreen usage, the noontime and surrounding hours are the most dangerous.