Night-sky glossary
Short, honest definitions. Where a word has a lesson behind it, follow the link.
- Altitude and azimuth
- Altitude is the height above the horizon in degrees (0–90°); azimuth is the compass direction (0° north, 90° east). A fist at arm’s length is about 10°.
- Asterism
- A familiar star pattern that is not an official constellation, such as the Big Dipper or the Summer Triangle. →
- Auroral oval
- The ring around each magnetic pole where the aurora usually glows. It widens towards the equator during storms. →
- Bz
- The north–south direction of the magnetic field carried by the solar wind. A strongly southward (negative) Bz lets energy into Earth’s magnetosphere and fuels the aurora.
- Circumpolar
- A star or constellation that never sets from your latitude because it circles close enough to the celestial pole. →
- CME
- Coronal mass ejection: a billion-tonne cloud of solar plasma. When one hits Earth 1–4 days later, it can trigger a strong geomagnetic storm.
- Ecliptic
- The Sun’s yearly path across the sky. The Moon and planets always stay close to it, which is why they pass through the zodiac constellations.
- Elongation
- The angle between a planet and the Sun as seen from Earth. Mercury and Venus are easiest to see at greatest elongation. →
- G-scale
- NOAA’s storm scale: G1 (minor, Kp 5) to G5 (extreme, Kp 9). →
- Kp index
- A 0–9 scale of global geomagnetic disturbance, measured every three hours. Higher Kp pushes the aurora further from the poles. →
- Light year
- The distance light travels in a year: about 9.46 trillion km. It measures distance, not time.
- Magnitude
- Brightness on a reverse logarithmic scale: smaller is brighter, and 5 magnitudes is a factor of 100. Sirius is −1.5, the faintest naked-eye stars about 6. →
- Opposition
- When an outer planet is opposite the Sun in our sky: it rises at sunset, is up all night and is at its brightest. →
- Precession
- The slow wobble of Earth’s axis over about 26,000 years. It changes which star is the pole star and shifts the zodiac. →
- Radiant
- The point in the sky that a shower’s meteors seem to fly out of. It names the shower: the Perseids come from Perseus. →
- Supermoon
- A full Moon near perigee, the closest point of its orbit. It looks up to about 14% larger and 30% brighter than a full Moon at its farthest. →
- Terminator
- The line between day and night on the Moon or a planet. Craters along it cast long shadows and look sharpest in binoculars. →
- Twilight
- The time when the Sun is below the horizon but still lights the sky: civil (to −6°), nautical (to −12°) and astronomical (to −18°). True darkness starts below −18°.
- Zenith
- The point straight above your head (altitude 90°). The opposite point, under your feet, is the nadir.
- ZHR
- Zenithal hourly rate: meteors per hour one observer would see under a perfectly dark sky with the radiant overhead. Real counts are lower. →