All-time heat records have been set around the world this week, in North American cities including Denver, Colorado; Burlington, Vermont; and Montreal, Quebec; as well as in cities in Europe and Russia. But here’s something that might confuse you: Today is also aphelion day — the day in Earth’s orbit that where it’s farthest from the Sun. What gives?
I know that you, a Gizmodo reader, are well aware of how the seasons work. But maybe someone you know thinks that the hottest part of the year is when the Earth is closest to the Sun. So let’s break it down.
The Earth travels in an ellipse around the Sun. At its farthest point, which occurred today, July 7 at 2:46AM AEDT, the Earth was 152,095,566km away from the Sun. On average, it’s around 150 million km from the Sun, and on 4 January 2019, it will be 147,099,761km from the Sun.
That isn't a huge difference, if you think about the vastness of space. Instead, the seasons are determined by the directness of the Sun’s rays, which is determined by Earth’s tilt.
The Earth is tilted at a 23.5 degree angle in its orbit around the Sun. That means during June and the surrounding months, Earth’s northern half points at the Sun, and during January and its surrounding months, Earth’s southern half points at the Sun. During March and September, the sunlight hits both hemispheres equally.
The more direct the Sun’s rays, the more light reaches Earth per unit area. Think about holding a bunch of straws, where each straw is a ray of sunlight, and trying to fit them into the glass, the Earth’s surface. If your glass’ rim lines up with the straws, more of them will fit than if you hold the glass at an angle to the straws. More straws, more rays; more rays, more energy; more energy, more heat.
As to why the northern hemisphere is so particularly hot right now, well, it’s complicated. It is the summertime there, so you should expect it to be hot, and you should also expect there to be random fluctuations such that sometimes it’s hotter than normal — and no, you can’t blame a single statistical fluke in the Earth’s heat on climate change. Greg Porter at The Washington Post explains that there’s a heat dome over much of the Earth’s north half — a region of high pressure a few kilometres up in the atmosphere is compressing the air, adding to the summer heat.
But you can’t leave climate change out of it. As things trend warmer, we can expect to see oppressive heat waves such as these more often. And we do — 2015 was the second-warmest year on record, 2016 was the warmest year on record, and 2017 was the third-warmest year on record, according to NOAA.
Someone on Twitter pointed me to another interesting fact — the entire Earth is hotter during aphelion. It’s actually true that the sunlight is a little bit less intense when the Earth is farther from the Sun, perhaps seven per cent less, according to an old NASA document. But there’s a lot more land in the Northern hemisphere, and land generally heats up more easily than water (it has a lower heat capacity). Even though the Earth is closer to the Sun in January, that heat is spread over the harder-to-heat ocean.
Also, as astronomer Kelly Lepo from McGill pointed out to me, the days are longer when the axis is tilted to the Sun, more time for the Earth to receive the energy from the rays and get hotter.
So, I know you’re probably thinking, “uh, I knew this already.” But I promise you that you have coworkers, friends and relatives who think that it’s hottest when the Earth is closest to the Sun. Sharing this knowledge will make you very popular at parties.