Glossary
Last updated: March 29, 2026
AI assistant
Stellara's built-in astronomy helper. It answers questions using your telescope, experience level, location, and current sky context.
Altitude
How high something is above the horizon. Zero degrees means it is on the horizon. Ninety degrees means it is at the zenith.
APOD
Astronomy Picture of the Day, NASA's daily astronomy image or video with a short explanation.
Astronomical twilight
The stage of twilight when the Sun is between 12 and 18 degrees below the horizon. Deep-sky observing usually becomes practical after astronomical twilight ends.
Azimuth
The compass direction of an object in the sky, measured around the horizon. North is one reference point, then east, south, and west around the sky.
Bortle scale
A simple scale for light pollution. Lower numbers mean darker skies. Higher numbers mean brighter urban skies.
Civil twilight
The stage of twilight when the Sun is up to 6 degrees below the horizon. The sky is still bright enough for many outdoor activities without full darkness.
Conjunction
When two objects appear close together in the sky from Earth's point of view.
Declination
The sky version of latitude. It tells you how far north or south an object is on the celestial sphere.
ISS
The International Space Station, a spacecraft orbiting Earth that can be visible from the ground during some passes.
ISS pass
A period when the International Space Station moves above your horizon and can be visible from your location.
Light pollution zone
A brightness zone taken from a light-pollution atlas. Stellara uses it to estimate how bright your sky is at zenith.
mag/arcsec²
Magnitudes per square arcsecond, a standard way to describe sky brightness. Higher values usually mean darker skies.
Moon Phase
The current illuminated shape of the Moon as seen from Earth.
Nautical twilight
The stage of twilight when the Sun is between 6 and 12 degrees below the horizon. The horizon is still partly visible, but the sky is getting much darker.
Observing index
Stellara's summary score for how usable the sky looks for observing. It combines factors like clouds, transparency, seeing, darkness, and moonlight.
Opposition
When an outer planet is opposite the Sun in the sky. This is usually when that planet is highest, brightest, and easiest to observe.
Right ascension
The sky version of longitude. It helps describe an object's position on the celestial sphere.
TLE
Two-Line Element data used to calculate a satellite orbit, including ISS tracking.
Seeing
How steady the atmosphere is. Better seeing means less shimmer and sharper detail on planets and stars.
Transparency
How clear the sky is. Haze, thin cloud, smoke, and humidity can lower transparency and make faint objects harder to see.
Zenith
The point directly overhead in the sky.