There’s something in regards to the geometry of space that’s never made sense to me. I know that the axial tilt of the Earth is the rationale we've got seasons, but when a slight angle away from the sun could make me see my breath in winter, why does being 5 million kilometers (km) or 3 million miles nearer not make me melt in a pool of my own sweat? Is it really not any hotter when we are at our shortest distance from our hot star? And if it is, then why does the 23.5° tilt of our axis matter at all?
I've to research these questions yearly at the perihelion, the point at which we’re the closest to the sun (the opposite is called aphelion, which is when the distance of the earth is farthest from the sun). Regardless of 21st December being the shortest day of the year in the northern hemisphere, it’s not till January 4th that we attain perihelion, It is the time when Earth gets essentially the most intense dose of photovoltaic rays it'll receive all year. Averaged across the entire globe, we’re getting sunlight 7% stronger in January than what we get in July.
However it turns out that our distance to the sun really has very little to do with the temperatures we experience. It has way more to do with the angle at which the sun rays hit us.
The sun's ray coming in at 90° hits as directly as possible. At the peak of winter in the northern hemisphere, the sun’s rays are pointed right on the Tropic of Capricorn, which lies 23.5° beneath the equator. That band is getting the most direct light essentially. In all other places, it is getting hit at an angle, and that implies that the same amount of energy in each metaphorical ray is spread over a larger area, weakening the warming effects at any given point.
Let’s think about a one-km-wide beam of sunshine (and overlook that there’s a 3rd dimension for a second, only for simplicity’s sake). At 90°, that incoming beam is putting all its power into warming up a one-km stretch. However, at a 30° angle, that very same light shall be spread across two km, thus halving the intensity at each point. You may visualize this even better with a small torch or flashlight. Point the beam at a vertical piece of paper, and you'll witness a direct circle of light. Turn the paper at an angle, and you'll get a more diffuse ellipse. That is almost just what’s taking place on Earth, except that our planet is spherical and made mainly of water and rock.
Our proximity to the sun would matter more if it heated us through convection. Like a sizzling oven, convection depends on a medium like water or air to carry heat to the target. However space is a vacuum, and with no liquid or gas or liquid to hold convection heat, the sun has to make use of radiant heat. Electromagnetic waves carry energy that heats molecules of air, water, and earth on their arrival, rather than being hot when they arrive and transferring that heat. It is the same sort of heating that makes a bonfire so scorching on your face however not on your back—the fireplace isn’t warming the air, it’s sending out energy waves that cause your skin to warm up. The sun is over 147.5 million km away at any given time, so adding or subtracting 5 million km does not make a significant difference in the radiant warmth that we can notice.
The more intense the sun's rays, the more energy it is carrying and the more radiant heat it contains, so it is actually the intensity of the beams that matter, not how close we're to the supply. Less intense daylight in the northern hemisphere during January, along with fewer hours of heating every day, means a colder winter the farther north you go.
Is The Summer In The Southern Hemisphere Hotter?
However all this brings us to a different question: if the southern hemisphere gets extra intense light throughout its summer, does that imply January is hotter down there than July is up here?
No! Surprisingly — after telling you categorically that light intensity is what matters — it’s hotter during northern hemisphere summers. Despite the fact that we get much less heating potential from the sun, there is extra land mass up here. The south has much more water in its oceans, and since water can soak up plenty of heat without increasing in temperature a lot (that is known as high specific heat capacity), a place with plenty of water will likely be more chill. Land heats up pretty rapidly, so regardless of a lower light intensity, northern summers end up hotter.
There you have it. We’re now getting farther and farther away from the sun until we reach our most distant point—in the dead heat of July. That’s space geometry for you.