Skip Navigation
×
This site requires JavaScript for most of its functionality. See
these instructions
for enabling JavaScript in your web browser.
Home
Menu
About
Center for
NEO Studies
NEO Basics
Search Program
Orbits
Introduction
NEO Lookup
NEO DB Query
Horizons
Orbit Elements
Accessible NEAs
Close
Approaches
Introduction
NEOs
Fireballs
Pre-Impact Detections
Impact
Risk
Introduction
Sentry
Scout
Planetary
Defense
Introduction
NEO Deflection App
Impact Scenarios
NASA Planetary Defense
Coordination Office
Discovery
Statistics
Introduction
Cumulative Totals
by Size
by Survey (all)
by Survey (140m)
by Survey (km)
WISE/NEOWISE
Tools
Asteroid Size Estimator
Sky Search
What's Observable
Horizons
NEO Lookup
NEO DB Query
Extras
News Archive
NEO Missions
Documents
FAQ
Glossary
Related Links
RSS Feeds
Site Map
Home
Extras
Glossary
semi-major axis
Glossary
Terms
albedo
aphelion
argument of perihelion
ascending node
au (Astronomical Unit)
B-plane
descending node
eccentricity
ecliptic plane
ephemeris
H (absolute magnitude)
inclination
keyhole
LD (Lunar Distance)
line of nodes
longitude of the ascending node
NEO (Near-Earth Object)
nodal points
perigee
perihelion
period
PHA (Potentially Hazardous Asteroid)
semi-major axis
true anomaly
semi-major axis
Definition
One half of the major axis of the elliptical orbit; also the mean distance from the Sun.
