Aphelion

Definition

Aphelion (from the Greek apo, away from, and helios, the Sun) is the point of an orbit around the Sun where the body is farthest from it. It lies at the opposite end of the major axis from perihelion. Together the two points are called the apsides, and the line joining them is the line of apsides.

The same idea applies to any orbit, with a different suffix for each central body: apogee for the Earth, apolune or aposelene for the Moon, apojove for Jupiter and apastron for a double star. The generic term is apoapsis.

Q = a × (1 + e)v_perihelion / v_aphelion = (1 + e) / (1 − e) = Q / q

Here a is the semi-major axis, e the eccentricity and q the perihelion distance. The second relation comes from Kepler's second law: the body moves slowest at aphelion, and the ratio of the extreme speeds equals the ratio of the extreme distances.

The Earth at aphelion

The Earth reaches aphelion every year between 3 and 6 July, at about 152.1 million km from the Sun (1.0167 AU). The exact date moves from year to year, mainly because the Moon pulls the Earth slightly around the Earth–Moon barycentre.

The difference with perihelion is small but measurable. The Sun's apparent diameter shrinks by about 3%, and the Earth receives about 7% less sunlight in early July than in early January. The Earth also moves 3.4% more slowly, which is why the northern spring and summer together last about a week longer than autumn and winter.

Aphelion is not the reason for summer: the seasons come from the 23.4° tilt of the Earth's axis. The northern hemisphere has its summer while the Earth is farthest from the Sun.

Aphelion drifts through the calendar

The line of apsides is not fixed. Planetary perturbations turn it slowly, and the equinoxes themselves drift because of precession. Combined, the two effects move the date of aphelion by roughly one day every 58 years. In about 10,000 years aphelion will fall in January, during the northern winter.

These slow changes in the timing and length of the seasons, together with variations in eccentricity and axial tilt, are the Milankovitch cycles, which set the pace of the ice ages over the last few million years.

Aphelion elsewhere in the solar system

• Mars has an eccentric orbit (e = 0.093) and its distance from the Sun varies between 1.38 and 1.67 AU. When an opposition happens near Mars' aphelion, the planet stays around 100 million km from Earth and looks small in a telescope. At the best perihelic oppositions, such as in 2003, the distance drops below 60 million km.
• Mars also reaches aphelion during its southern winter, which makes that winter long and cold. Enough carbon dioxide freezes onto the southern polar cap to lower the planet's atmospheric pressure by about a quarter.
• Halley's comet reached aphelion in December 2023, about 35 AU from the Sun and beyond the orbit of Neptune. It is now falling back towards the inner solar system and will return to perihelion in 2061.
• Long-period comets have aphelia of thousands or tens of thousands of AU, in the Oort cloud, and they spend almost their entire orbit there, moving at only a few hundred metres per second.

On the 3D view

The interactive illustration above marks the aphelion of each planetary orbit. Compare its distance from the Sun with the perihelion on the opposite side: the difference is obvious for Mercury and Mars and almost invisible for Venus and the Earth.

Explore it on AstroAlgos

• Perihelion — the closest point to the Sun
• Orbit — Kepler's laws and orbital elements
• Semidiameter — the apparent size of the Sun and planets
• Heliocentric view — all planetary orbits in 3D