ADS-B
Automatic Dependent Surveillance–Broadcast: position and flight data transmitted by many aircraft and received by ground or satellite networks.
32 TERMS
Sky evidence glossary
Definitions are written for observers, not specialists. Each term explains how it affects a real sky-sighting investigation.
A
Angular speed
How quickly an object moves across the observer’s field of view, usually more useful visually than its unknown ground speed.
Auroral oval
The broad ring-shaped region around a magnetic pole where auroral activity is most likely.
Azimuth
Horizontal compass angle, usually measured clockwise from north.
B
Bearing
Direction from one location to another; Sky Event Radar uses observer-to-target bearing for nearby aircraft context.
Bolide
A very bright meteor, often used for exceptionally energetic fireballs or events that explode in the atmosphere.
C
CNEOS
NASA/JPL Center for Near Earth Object Studies, which publishes a Fireball Data API containing energetic atmospheric events detected by U.S. government sensors.
Cloud cover
Estimated fraction of the sky covered by cloud. It changes visibility but does not prove a particular line of sight was blocked.
E
Earth shadow
The region behind Earth where direct sunlight is blocked. Satellites can abruptly fade when they enter it.
Elevation
Angle above the horizon: 0° at the horizon and 90° directly overhead.
Ephemeris
Calculated positions of astronomical objects as a function of time.
F
Fireball
An unusually bright meteor. It describes brightness, not a different physical process from an ordinary meteor.
Flare
A temporary increase in brightness, often caused by changing reflection geometry.
G
Geocoding
Converting a place name into coordinates and related location metadata such as timezone.
Geomagnetic Kp index
A planetary measure of geomagnetic disturbance. Useful context for aurora, but not a local visibility forecast.
Great-circle path
The shortest path between two points on a sphere; many satellite tracks look like smooth great-circle-like arcs across the sky.
H
Haversine distance
A formula for approximate distance between two latitude/longitude points on Earth.
I
Impact energy
CNEOS estimate of atmospheric event energy, commonly expressed in kilotons of TNT equivalent.
ISS
International Space Station, a large low-Earth-orbit spacecraft that can become extremely bright during favorable passes.
L
Light pillar
A vertical optical column produced when light reflects from plate-shaped ice crystals.
M
Meteor
Visible atmospheric light produced when a meteoroid enters the atmosphere at high speed.
Meteor shower
A period when Earth encounters a stream of debris, increasing meteors from a particular radiant region.
O
OVATION
NOAA model that estimates current auroral intensity/probability on a geographic grid.
P
Parallax
Apparent positional change caused by a change in observer viewpoint.
Persistent train
A luminous trail that remains after the meteor itself has disappeared and can evolve in upper-atmosphere winds.
R
Reentry
Return of a spacecraft, rocket body or debris into denser atmosphere, sometimes producing a long-lived fragment train.
S
Scintillation
Rapid brightness/color fluctuations caused by atmospheric turbulence, especially noticeable for bright stars near the horizon.
Solar altitude
The Sun’s angle above or below the horizon. Negative values describe twilight or night.
Starlink train
A temporary line-like formation of recently deployed Starlink satellites following similar orbital tracks.
T
TLE / orbital elements
Parameters used to describe an orbit and propagate a satellite’s predicted position.
Twilight
Period when the Sun is below the horizon but still illuminates the atmosphere: civil, nautical and astronomical twilight.
V
Visibility
Meteorological estimate of how far objects can be seen through the lower atmosphere; different from cloud cover.