Distant Retrograde Orbit
A distant retrograde orbit is a high-altitude path around a moon that moves opposite the moon's motion around its planet and can be useful for stable cislunar mission design.
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Lyapunov Orbit
A Lyapunov orbit is a planar periodic path near a collinear Lagrange point, used in mission design and as a building block for other Lagrange-region trajectories.
Lissajous Orbit
A Lissajous orbit is a looping spacecraft path near a Lagrange point, often used when a mission needs useful geometry without a fully closed halo orbit.
Halo Orbit
A halo orbit is a three-dimensional spacecraft path around a Lagrange-point region, useful for missions that need steady viewing geometry and manageable station-keeping.
Lagrange Points
Lagrange points are special locations in a two-body system where a small object can keep the same relative position with less station-keeping.
Kepler's Equation
Kepler's equation links time in an elliptical orbit to eccentric anomaly, making it a core tool for finding where an orbiting body is.
Eccentric Anomaly
Eccentric anomaly is a geometric helper angle used to connect orbital time with an object's actual position on an elliptical orbit.
Mean Anomaly
Mean anomaly is a time-based angle that tracks how far an orbiting body has progressed since periapsis in an ideal Keplerian orbit.
True Anomaly
True anomaly is the angle that gives an orbiting object's actual position along an elliptical orbit relative to periapsis.
Argument of Periapsis
The argument of periapsis is the angle that locates an orbit's closest point within its own orbital plane.
Apsidal Precession
Apsidal precession is the slow rotation of an orbit's closest and farthest points, changing where periapsis and apoapsis occur.
Nodal Precession
Nodal precession is the slow rotation of an orbit's line of nodes, changing where a tilted orbit crosses its reference plane.
Ascending Node
The ascending node is the point where an orbiting object crosses a reference plane moving northward, anchoring an orbit's orientation in space.
Orbital Inclination
Orbital inclination measures the tilt of an orbit against a chosen reference plane, helping define where an object travels through space.
Orbital Eccentricity
Orbital eccentricity measures how much an orbit departs from a circle, shaping distances, speeds, and sunlight patterns along a path.
Milankovitch Cycles
Milankovitch cycles are slow changes in Earth's orbit and orientation that alter sunlight patterns and help pace ice-age climate cycles.
Axial Tilt
Axial tilt is the angle between a planet's rotation axis and its orbital plane, and on Earth it drives the pattern of seasons.
Solstice
A solstice is one of the two yearly moments when the Sun reaches its greatest north or south position in the sky.
Equinox
An equinox is one of the two moments each year when the Sun crosses the celestial equator, marking a seasonal turning point.
Ecliptic
The ecliptic is the Sun's apparent yearly path on the celestial sphere and the projection of Earth's orbital plane.
Celestial Sphere
The celestial sphere is an imaginary sky sphere that helps astronomers describe positions and motion in the night sky.
Declination
Declination is the north-south coordinate astronomers use to locate objects above or below the celestial equator.
Hour Angle
Hour angle is an astronomy coordinate that tells how far a celestial object is from crossing an observer's meridian.
Right Ascension
Right ascension is the sky-coordinate measure astronomers use to locate objects eastward around the celestial equator.
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