Aphelion and perihelion

In an elliptical orbit around the Sun, two specific points define the extreme distances: the aphelion is the point farthest from the Sun, and the perihelion is the point closest to it. These terms derive directly from Kepler’s first law, which states that planets trace out ellipses in which the Sun occupies one of the foci— and not a perfect circle centered on the Sun.

Where do these terms come from?

The names are derived from Greek: helios meaning “Sun,” apo meaning “far from,” and peri meaning “near.” For orbits around another body, the suffix changes: around the Earth, we speak of apogee and perigee; around any star, we use apoaster and periaster; around the Moon, apolune and perilune.

Variations in speed

Kepler’s second law states that the line connecting a planet to the Sun sweeps out equal areas in equal times. Direct consequence: a planet moves faster at perihelion than at aphelion. Earth is at perihelion in early January (147 million km from the Sun) and at aphelion in early July (152 million km). The difference of only 3.3% explains why the seasons are caused by the axial tilt, not by this variation in distance.

Orbital eccentricity

The difference between aphelion and perihelion depends on the eccentricity of the orbit, a measure of its flattening. Earth has a low eccentricity (0.017), so its orbital extremes differ only slightly. Comets have highly elliptical orbits: Halley’s Comet ranges from 0.59 AU (between Venus and the Sun) to 35 AU (beyond Neptune), with an eccentricity of 0.967.

Importance in Astronomy

Knowing the precise aphelion and perihelion of an object is essential for celestial mechanics: predicting positions, calculating orbital periods, planning space missions, and assessing the risk of asteroid impacts. Orbital maneuvers by space probes exploit the energy differences between perihelion (maximum velocity) and aphelion (minimum velocity) to optimize transfers.

A concrete example: the Parker probe

The Parker Solar Probe, launched in 2018, reaches perihelion distances of less than 7 million km from the Sun—less than 10 solar radii. At this distance, it travels at approximately 700,000 km/h, making it the fastest object ever built by humankind. Its aphelion, on the other hand, is in the vicinity of Venus, allowing it to use that planet for successive gravity assists.

Did you know?

Mercury’s perihelion exhibits a slight precession— a gradual shift — that could not be explained by Newtonian mechanics. General relativityEinstein’s general theory of relativity solved this puzzle in 1915, which constituted one of the first striking confirmations of his theory.

Frequently Asked Questions

Why is the Earth closer to the Sun in winter?

This is true for the Northern Hemisphere, but it is a coincidence related to the Earth’s current axial tilt. Due to the precession of the Earth’s axis, this alignment gradually changes over tens of thousands of years. The seasons depend primarily on the tilt of the axis, not on the distance from the Sun.

Do all bodies in the solar system have an aphelion?

Yes, any body in a closed orbit around the Sun has an aphelion and a perihelion. Objects in parabolic or hyperbolic orbits (such as certain interstellar comets that pass by only once) do not have a defined aphelion sincethey are not bound to the Sun.

Can we calculate the orbital period from the aphelion and perihelion?

Yes. The sum of the aphelion and perihelion divided by 2 gives the semi-major axis, which determines the orbital period via Kepler’s third law. For the solar system, T² = a³ (where T is in years and a is in AU).