Planet Cycles
An interactive 3D planetarium: drag to rotate, scroll or pinch to zoom, tap a planet and move through time.
Planetary orbits
Planets move in ellipses around the Sun, each with its own small tilt (inclination) relative to the ecliptic. Positions in this model are calculated for every moment with the astronomy-engine library (based on VSOP87), the same source as our lunar calculations.
Orbital periods
Mercury orbits the Sun in 88 days, Earth in one year, Jupiter in almost 12 years and Neptune in almost 165 years. Two planets line up again after their synodic period: Jupiter and Saturn roughly every 19.9 years.
Heliocentric versus geocentric
Heliocentric puts the Sun at the centre and shows the real orbits. Geocentric puts Earth at the centre and shows where the planets appear in our sky. They are two reference frames of the same reality.
Conjunctions
A conjunction is the moment two planets share the same ecliptic longitude. Seen from Earth they then appear close together, like Jupiter and Saturn on 21 December 2020. The model finds conjunctions from the calculated positions.
Retrograde motion
When Earth overtakes an outer planet, that planet seems to drift backwards against the stars for a while. It is a perspective effect, not a real reversal of the orbit. Switch on Geocentric and one-year trails to see the loops.
About the astrology layer
The optional astrology layer shows signs and aspects as interpretive terminology based on the same calculated longitudes. There is no scientific evidence that planetary positions affect people or events.