By Tony Rice, NASA Ambassador
With temperatures rising into the 90s, you might assume that the Earth is closer to the Sun now than it is during the winter months, but the opposite is true.
Earth is at aphelion, the furthest point from the Sun in its not-quite-circular orbit, on July 4, 2022. On January 4, we’ll be 3.1 million miles closer at perihelion.
Aphelion has an effect on our weather, but not what you might think.
The amount of energy the Earth receives from the Sun varies by only about 3.5% during 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,
It’s easier to see when you look at it from the Sun’s perspective in these images from the GOES-16 weather satellite, which the WRAL Severe Weather team uses every day to compile forecasts.
The increased distance slows Earth’s travel around the Sun by about 1,000 m/s (2,237 mph) at this time of year compared to perihelion in January. This affects the length of the seasons. Spring and summer last about 93 days, autumn and winter last about 89 days.
Aphelion of other planets
Astronomers describe the shape of an orbit as eccentricity, a number between zero and one. Zero is a perfect circle, and until that number reaches 1 (a parabola), the planet, moon, or whatever remains in orbit.
Earth has an eccentricity of 0.017, Venus is the roundest at 0.007. Mars has an eccentricity of 0.094, which greatly affects its seasons. Martian summer 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?
One would think that even the small increase in solar energy the Earth receives around perihelion would lead to slightly warmer summers 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 compared to here in the Northern Hemisphere). This is easier to see when you look at Earth from the poles:
Water requires significantly more energy to raise its temperature than land. Think about that the next time you step out of the cool surf at the beach onto the hot sand.
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