Glossary
Plain-language definitions of the terms used across Look Up Nights.
- Activity period
- The span of dates when a meteor shower produces meteors, as listed by the International Meteor Organization.
- Annular eclipse
- A solar eclipse when the Moon looks slightly smaller than the Sun, leaving a bright ring. Eye protection is needed throughout.
- Astronomical twilight
- The period when the Sun's center is 12° to 18° below the horizon. After it ends, scattered sunlight is fainter than starlight. Read more
- Civil twilight
- The period when the Sun's center is between sunset and 6° below the horizon. After it ends, artificial light is normally needed outdoors (USNO). Read more
- Corona
- The Sun's faint outer atmosphere, visible to the eye only during the total phase of a total solar eclipse.
- Dark adaptation
- Your eyes' adjustment to darkness. The American Meteor Society suggests at least 20–30 minutes before meteor watching.
- Fireball
- An exceptionally bright meteor. The Geminids are known for producing them.
- First quarter
- The phase when half of the Moon's disk is lit, about a quarter of the way through the cycle. It sets around the middle of the night.
- Full Moon
- The phase when the Moon is opposite the Sun and its whole face is lit. It rises around sunset and sets around sunrise.
- Gibbous
- A phase in which more than half of the Moon's disk is lit but it isn't full.
- ISO 12312-2
- The international standard NASA says safe eclipse glasses and handheld solar viewers ought to comply with. Read more
- Last quarter
- Also called third quarter. Half the disk is lit; the Moon rises around the middle of the night and sets around midday.
- Meteoroid, meteor, meteorite
- A meteoroid is a small object in space; a meteor is the streak of light when it enters the atmosphere; a meteorite is any piece that reaches the ground.
- Nautical twilight
- The period when the Sun's center is 6° to 12° below the horizon. The horizon is still visible for star sights at sea.
- New Moon
- The phase when the Moon's lit side faces away from Earth and the Moon is near the Sun in the sky. Evening skies are darkest around this time. Read more
- Path of totality
- The narrow track on Earth's surface where a solar eclipse is total. Outside it, people see a partial eclipse. Read more
- Peak (maximum)
- The time of a shower's greatest expected activity. It's a forecast, and nearby nights are often good too.
- Radiant
- The point in the sky from which a shower's meteors appear to come. Showers are named for the constellation containing it. Read more
- Sporadic meteor
- A random meteor not associated with any recognized shower. They can appear on any night.
- Terminator
- The line between the lit and dark parts of the Moon, where long shadows make craters stand out.
- Totality
- The phase of a total solar eclipse when the Moon completely covers the Sun's bright face. Only then is it safe to look without eye protection. Read more
- Umbra and penumbra
- The dark inner shadow and the fainter outer shadow. A penumbral lunar eclipse involves only the outer shadow, so the dimming is subtle. Read more
- Universal Time (UT)
- The time standard used for astronomical events, colloquially Greenwich Mean Time. Add your UTC offset to get local time.
- Waxing and waning
- Waxing means the lit part is growing (New Moon to Full Moon); waning means it is shrinking (Full Moon to New Moon).
- ZHR (zenithal hourly rate)
- The number of meteors one observer would see in an hour under a very dark, clear sky with the radiant directly overhead. Real counts are usually lower. Read more