As temperatures soar into the 90s, you might guess that Earth is closer to the Sun now than in the winter months, but the opposite is true. Earth is at aphelion, the farthest point from the Sun in its not-so-circular orbit, on July 4, 2022. On January 4, we’ll be 3.1 million miles closer to the perihelion. Aphelion has an effect on our weather, but it’s not what you might think. The amount of energy the Earth receives from the Sun varies by only about 3.5% over the course of the year. Temperatures are lower in the winter months because the Sun’s rays reach us at a greater angle, spreading that energy over a larger area. When the Sun is more directly overhead, the energy is more concentrated, raising temperatures, This is easier to see when you look at it from the perspective of the Sun in these images from the GOES-16 weather satellite that the WRAL Severe Weather team uses every day to put together forecasts. The increased distance slows Earth’s travel around the Sun by about 1000 m/s (2237 mph) at this time of year compared to perihelion in January. This affects the length of seasons. Spring and summer are about 93 days, autumn and winter are about 89 days.
Aphelion on other planets
Astronomers describe the shape of an orbit as eccentricity, a number between zero and one. Zero is a perfect circle, and as long as that number doesn’t reach 1 (a parabola), the planet, moon, or whatever, stays in orbit. Earth has an eccentricity of 0.017, Venus is the most circular at 0.007. Mars has an eccentricity of 0.094 which has a big impact on its seasons. Summer on Mars is 25 days longer than winter, and spring is 52 days longer than autumn. Pluto is the winner with an eccentricity of 0.244. Its orbit is so elliptical that it was actually closer to the Sun than Neptune between 1979 and 1999.
So summers are hotter in Australia, right?
You would think that even the small increase in the Sun’s energy that the Earth receives around the perihelion would make the summers a little warmer in the southern hemisphere. The opposite is true here as well. The Northern Hemisphere is about 3.5º F warmer during the year because there is much more ocean than land south of the equator (80%/20% water/land vs. here in the Northern Hemisphere). This is easier to see when looking at Earth from the poles: Water requires much more energy to raise its temperature than the earth. Think about that the next time you step out of the cool beach surf onto the hot sand.
