A to E
Aphelion
The point in an orbit around the Sun where a planet or comet is farthest from the Sun. Earth reaches aphelion in early July.
Apogee
The point in the Moon’s orbit, or a satellite’s orbit, where it is farthest from Earth. See how far is the Moon.
Asteroid
A small rocky or metallic body orbiting the Sun, mostly found between Mars and Jupiter. Unlike comets, asteroids usually show no tail.
Celestial equator
The projection of Earth’s equator onto the sky. Objects on it rise due east and set due west for most observers.
Comet
A small icy body that releases gas and dust when it nears the Sun, forming a coma and often tails. Halley’s Comet is the best-known example.
Conjunction
When two celestial objects appear close together in the sky, or when a planet lines up with the Sun as seen from Earth.
Eclipse
When one body passes into the shadow of another, or blocks it from view. A solar eclipse happens when the Moon blocks the Sun; a lunar eclipse when the Moon passes through Earth’s shadow. See the eclipses page.
Ecliptic
The apparent yearly path of the Sun across the sky, which is the plane of Earth’s orbit. The planets and the Moon stay close to it.
Equinox
One of two moments each year when the Sun crosses the celestial equator, around March 20 and September 22–23. Day and night are then roughly equal in length.
F to O
Fireball
A meteor brighter than the planet Venus.
Gibbous
A Moon phase in which more than half, but not all, of the visible disk is lit.
Magnitude
A measure of brightness. Lower numbers mean brighter objects, and very bright objects have negative magnitudes. A difference of 5 magnitudes equals a factor of 100 in brightness.
Meteor
The streak of light made when a small particle from space burns up in the atmosphere. See what is a meteor.
Meteor shower
Many meteors that appear to come from the same point in the sky over a few days or weeks, as Earth passes through a stream of debris. See the meteor shower calendar.
Meteorite
A piece of a meteoroid that survives its fall and lands on the ground.
Occultation
When one object passes in front of another and hides it, such as the Moon covering a star or planet.
Opposition
When a planet is on the opposite side of Earth from the Sun. It rises at sunset and is usually at its brightest.
P to Z
Penumbra
The outer, partial part of a shadow, where the light source is only partly blocked.
Perigee
The point in the Moon’s orbit, or a satellite’s orbit, where it is closest to Earth.
Perihelion
The point in an orbit where a planet or comet is closest to the Sun. Earth reaches perihelion in early January.
Radiant
The point in the sky from which the meteors of a shower appear to come. Showers are named after the constellation that contains it.
Solstice
One of two moments each year when the Sun reaches its farthest point north or south of the celestial equator, around June 21 and December 21–22. They mark the longest and shortest days.
Sporadic meteor
A meteor that does not belong to any known shower.
Synodic month
The time from one new Moon to the next, about 29.5 days. It is the cycle of the moon phases.
Terminator
The line between the lit and dark parts of the Moon or a planet.
Transit
When a smaller body passes across the face of a larger one, such as Mercury or Venus crossing the Sun’s disk.
Umbra
The dark central part of a shadow, where the light source is completely blocked. A total eclipse is seen from inside the umbra.
Waxing and waning
Waxing means the lit part of the Moon is growing, from new Moon to full Moon. Waning means it is shrinking, from full Moon back to new Moon.
Zenith
The point in the sky directly above the observer.
Zenithal hourly rate (ZHR)
The number of meteors a single observer would see in an hour under a perfectly dark sky with the shower’s radiant at the zenith. Real rates are almost always lower.