Earth’s Aphelion and Perihelion
The orbit of the planets around their corresponding sun is elliptical in shape. The other interesting thing about this arrangement is that the sun will be at one of the ellipse’s foci, meaning, that at one point in the orbit, the planet will be further away from the sun than at other times of its year. Our Earth follows these same rules with our star, the Sun. The vocabulary here is helpful…. Aphelion is when the planet is furthest from the Sun, while Perihelion is when the planet is at its closest approach to the Sun.
The Earth’s Aphelion takes place in the early portion of January, while perihelion takes place in early July. The dates change a little back and forth with things like leap years and such. Yep – the calendar is a quirky thing. At the 2019 aphelion the Earth was 94,513,221 miles from the Sun (July 4th 2019). At the 2019 perihelion on January 3rd, the Earth was 91,403,554 miles from the Sun… those changes are very small, so small in fact that the two foci of the Earth’s elliptical orbit are within the diameter of the Sun!
With the distance changing between the Earth and the Sun, will the Sun appear to change in size over the course of the year? The answer is a clear “YES”! In fact, anyone with modest camera equipment and a safe solar filter can prove this out easily enough. How?
- Get a camera you can use with the same lens throughout the year.
- The lens should have a reasonable, long focal length… 200-500mm. The longer the focal length is, the more easily the Sun’s apparent diameter change will be to see.
- Cover the lens with a safe, approved solar filter. I recommend Baader Solar Film. WARNING!! Do NOT mess around with this, as a failure to use proper filtration will damage your camera and your eyes permanently!
- Mount the camera on a tripod for stability.
- Take photos of the Sun on clear days throughout the year.
- You can then overlay the images using editing applications to see and measure these changes.
I took these images below using a Nikon D-810a, a 300mm telephoto lens and a Baader Solar Film filter. The image on the left was on aphelion July 7, 2018. The image on the right was taken during perihelion on January 3, 2019. I had to run around a little to find the correct lens but managed to get it right! Take notes. That helps! The apparent diameters of the Sun are obviously different in the two images… and, if you do the math, the change is just a little bit more than 3%.