Earth at Aphelion

At 9:32 p.m. MDT on July 4, 2012, Earth will reach the point in its orbit where it is farthest from the Sun. No, that is not an error; Earth will be farthest from the Sun (the technical term is aphelion). That may seem strange given that the average high temperature on that day in Salt Lake City is around 90 °F. These two facts may prompt several questions. How does Earth’s changing distance from the Sun influence the seasons? How much farther from the Sun are we on July 4?

If we look at most diagrams that depict Earth’s orbit, the orbit appears very oval or elliptical in shape. That is because these diagrams usually show the orbit from the side.

Earth’s orbit viewed from the side (Earth and Sun not to scale)

But, that oblique view can leave us with the idea that Earth’s distance from the Sun changes significantly. If we instead view the orbit from directly above, it appears to be a perfect circle.

Earth’s orbit viewed from above (Earth and Sun not to scale)

That is because the orbit is different from a perfect circle by only 1.67%, hardly noticeable. So, on July 4 when Earth is farthest from the Sun, it is only 1.67% farther than average. That small change in distance does not significantly change the amount of energy we receive from the Sun. The effect on the seasons due to Earth’s changing distance is minimal.

The reason the weather is hot in summer has nothing to do with our distance from the Sun. Rather; it is because Earth’s northern hemisphere is leaning toward the Sun due to the tilt of Earth’s axis. This results in more intense sunlight and more hours of daylight for residents of the northern hemisphere. At the same time, the southern hemisphere experiences winter. Tilt trumps distance!

2 thoughts on “Earth at Aphelion”

1. So nice to see your article on the sun.

Thanks for doing a good job with informing students.

Hope you have a great summer, Flo

2. I normally exaserate the elliptical shape for my students, but I like the 1.67%, which is about the width of my chalk line on the black board. Thanks for the % tip, that really helps.