Planets orbit the Sun {orbit, planet}| {planetary orbit}.
Planets and asteroid belt lie at 4, 7, 10, 16, 28, 52, 100, 196, 388, and 772 units from Sun {Bode's law} {Bode law}.
Planets can be in line with Sun and Earth {conjunction, planet}|, on far side of Sun {inferior conjunction} or on same side as Earth {superior conjunction}.
planet-orbit plane {ecliptic}|.
Seen from Earth, Venus or Mercury has distance {elongation} from Sun.
Bodies can pass in front of other bodies {occultation}|.
Seen from Earth, planets and their moons periodically appear to move backward {retrograde motion}.
Mercury and Venus can pass in front of Sun {transit}|.
In ancient theories of Earth-centered universe, orbits were circles and circles of circles {epicycle} {epicycle construction} {eccentric construction}.
Perhaps, Earth is near solar-system center {geocentric system}.
Sun is near solar-system center {heliocentric system}.
Earth axis points slightly more counterclockwise, looking north, each year {precession of the equinoxes}| {equinox precession} and completes circle in 22,000 to 26,000 years. Moon nutation orbital-plane changes cause precession rate to vary over an 18.6-year cycle. Orbit variations also have 41,000-year and 100,000-year cycles.
Moon orbital-plane changes cause precession rate to vary over an 18.6-year cycle {nutation, Earth orbit}|.
Orbit around Earth has point {apogee} farthest from Earth.
Orbit around Earth has point {perigee} nearest Earth.
Orbit around Sun has point {aphelion} farthest from Sun.
Orbit around Sun has point {perihelion} nearest Sun.
At vernal equinox, Sun is at a celestial-sphere point {first point of Aries}.
For two objects in space, five points {Lagrangian point} {libration point} have equal competing gravities. Three are unstable, but two are stable.
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Date Modified: 2022.0225