# When is Earth closest to the Sun?

When is Earth closest to the Sun?

Well, you say, perihelion of course. But perihelion is when the Earth-Moon barycentre is closest to the Sun. Not Earth itself.

The barycentre is on average 4,671 km from the Earth, which should mean that even if the orbit of the Earth-Moon barycentre around the Sun were perfectly circular, the Earth-Moon orbit around the barycentre would cause an oscillation in the Earth-Sun distance by twice that amount on a monthly basis. That should be enough to move Earth's closest approach to the Sun away from perihelion.

Considering this, how many days from perihelion may Earth's closest approach to the Sun be?

Question inspired by: What place on Earth is closest to the Sun?

• I just had a very quick look at the linked question, but noticed something odd in the formula for approximate distance: $\cos(days \frac{365.25}{360})$. That conversion ratio is inverted! It's also inverted on the Cornell page that Camilo took it from. Jan 21, 2019 at 22:09
• @PM2Ring You are right, I'll fix it in my answer and add a comment. Well spotted! Thanks Jan 21, 2019 at 23:02
• Perihelion times are typically already referring to the moment when the Earth's center and the Sun's center are closest, not the barycenter. See this explanation on US Naval Observatory for details.The dates of perihelion can vary by up to about 2 days from the average, mostly because of the Moon. Jan 21, 2019 at 23:48
• Re But perihelion is when the Earth-Moon barycentre is closest to the Sun. Not Earth itself. This question is based on this thesis, which is incorrect. That Earth perihelion and aphelion are the times at which the distance between the center of the Sun to the center of the Earth rather than to the Earth-Moon barycenter reach a local extremum are why perihelion and aphelion bounce around much more so than do the equinoxes and solstices. Jan 22, 2019 at 11:40
• @DavidHammen Aha. I stand corrected. Jan 22, 2019 at 16:09