It's aphelion day, the Sun is at its farthest

While it may seem odd that the Sun is at its farthest during the summer, that distance has relatively little impact on temperatures.

Posted 6:00 p.m. Jul 5, 2021 — Updated 1:59 p.m. Jul 6, 2021

Tony Rice

, NASA Ambassador

Each year, about two weeks after the Summer Solstice, the Sun is at its farthest from Earth. That point, also known as aphelion, occurs at 6:27 pm EDT tonight when the Earth is over 94.5 million miles (15,2098,463 km or 94,510,888 miles to be exact) from the Sun.

The opposite happens on January 2 when Earth reaches perihelion and the Sun will be 3.1 million miles closer.

This happens because the Earth's orbit is stretched into a slightly elongated ellipse.

This image from NASA schematically illustrates the elliptical nature of our orbit about the sun (and the moon's orbit around the Earth), showing the points of aphelion and perihelion. Note that the eccentricity of the orbit is exaggerated somewhat in order to highlight the concept.

Earth-Sun distance impact on weather

It seems backwards that Earth is at its closest in the winter and furthest during the summer (our neighbors in the southern hemisphere might disagree). But the Sun-Earth distance doesn't have much impact on how much of the Sun's energy reaches us, about 1.361 kilowatts per square meter (kW/m²).

This number varies so little (about 3.5% over the year) that scientist call this number the solar constant.

Sun's angle is the reason for the seasons

Summers are warmer than winters because of the Earth's tilt, about 23.44°. This brings the Sun higher in the sky during the summer, concentrating more of the sun's energy into a given area, producing more heating. The lower Sun angle during the winter months spreads that energy over a larger area, reducing the energy any given area receives.

You can demonstrate this with a flashlight held directly overhead and then at an increasingly sharp angle. The brightness of the flashlight hasn't changed, but as you change the angle you shine the light, you change how the concentrated the energy is.

The Sun also spends more time above the horizon during summer months, so we receive those more direct rays for longer, building up more heat. Conversely, less heat is radiated from the surface over those shorter nights keeping summer months warmer.

Longer summers, shorter winters

The way aphelion does affect our weather is on the length of seasons.

Because Earth is farther away from the Sun during summer months it moves a little slower around the Sun, more than 2200 miles per hour slower. It takes more time to travel from the June solstice to the September equinox than from the December solstice to the March equinox. That makes summer nearly 5 days longer than winter.

But what about down under?

All this talk of seasons is relative to hemisphere of course. While last month's solstice marked the beginning of astronomical summer here, it brought the the beginning of winter south of the equator. With the Sun a little closer during southern hemisphere summers, are they a little warmer?

more than 2/3rds of Earth's land is in the northern hemisphere

Much more of the southern hemisphere is covered by water. Water has a much greater heat capacity, the amount of energy required to raise its temperature, than land. The meager increase in solar energy around perihelion is more than offset.

Heat energy stored in the oceans is the subject of study by many of NASA's Earth observing satellites including Aqua and Suomi NPP

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