# How do we define the aphelion of Earth?

After seeing this question, I was curious on how we define the aphelion of Earth. On a quick search it said that the aphelion is the point when Earth is the furthest way from the sun. However, all the objects in the solar system effect the shape and position of the sun up to some degree.

If the location of the sun constantly changes, then how do we define the aphelion of Earths orbit?

The aphelion is usually considered to be the point of greatest actual separation, so this would include the (very minor!) gravitational effects of other solar system bodies. It's not the slight movement of the Sun however that is most important, the gravity of our very own Moon causes significantly more wobbliness in the Earth's orbit. But all in all, the list of increasingly small effect grows as you consider more and more complications, and the aphelion is the eventual sum of all of them...

On the other hand, the aphelion is also often considered the furthest point in the mathematically perfect 2D conic section approximation of an orbit, which is usually good enough. This would give you a subtly different value.

Both of these meanings are common, and without further context one shouldn't assume either.

As a sidenote about terminology, the generic term is apoapsis, and the word changes depending of the parent body. So apogee would be the furthest point from Earth, relevant to the orbit of the Moon, ISS, etc. "apogee of Earth" would technically not make sense, hence the use of "aphelion" (furthest point from the Sun) in this answer.

• and of course an apogee might contain a further ambiguity - it might be quoted from the center of the Earth, or from the surface of Earth's reference sphere (~6328.137 km) so a 300 x 500 km orbit for example would be said to have an apogee of 500 km, though just down the hall they might be calling it 6878 km. (see also Help understanding negative apogee coinciding with exact center of Earth (Mars InSight launch))
– uhoh
May 3, 2023 at 23:28
• As well as the direct effects due to the Moon's gravity, the exact time of Earth's perihelion & aphelion is a bit wobbly due to the Earth's orbit around the Earth-Moon barycentre. I have some details (including graphs) here: astronomy.stackexchange.com/a/49605/16685 & astronomy.stackexchange.com/a/49546/16685 May 4, 2023 at 19:10
• "Slight movement of the sun"? The Earth-Moon barycenter is inside the Earth, but the Sun-Jupiter barycenter is actually outside the sun -- a much bigger movement! May 4, 2023 at 21:47
• @Dave Sure, Jupiter perturbs the Sun, Earth & Moon, (and everything else in the solar system), but in terms of Earth's aphelion & perihelion we can treat the Sun as a fixed point. The Sun has a complex spiral trajectory around the SSB (solar system barycentre) but that's not relevant for the Earth's orbit. Over the course of a year, the distance from the centre of the Sun to the Sun-EMB barycentre only varies by ~15 km. May 5, 2023 at 12:33