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29 September 2026 2518 stories tracked

Glossary · Comets & Asteroids

Perihelion and aphelion

The closest and furthest points of an orbit around the Sun.

Every orbit is an ellipse with the Sun at one focus, so there is a nearest point and a furthest one. For Earth the difference is about 3% and has nothing to do with the seasons, which are caused by axial tilt — Earth is closest to the Sun in early January. For comets the difference is everything: perihelion is when the ices sublimate, the tail grows and the object becomes visible at all, and it is also when it is most likely to break apart.

Related terms

Near-Earth object An asteroid or comet whose orbit brings it inside 1.3 astronomical units of the Sun. Asteroid and comet Rock that stays rock, and ice that grows a tail when it gets warm. Coma and tail A comet's atmosphere, and the two separate tails the Sun pushes off it. Oort cloud A shell of icy bodies at the edge of the Sun's influence, inferred rather than observed.

Questions

What is Perihelion and aphelion?

The closest and furthest points of an orbit around the Sun.

Why does it matter?

For Earth the difference is about 3% and has nothing to do with the seasons, which are caused by axial tilt — Earth is closest to the Sun in early January. For comets the difference is everything: perihelion is when the ices sublimate, the tail grows and the object becomes visible at all, and it is also when it is most likely to break apart.

Written and computed by . Corrections and errors to hello@cantinalabs.dev.