# Kālakriyāpāda III.17–21 — the eccentric–epicycle model as the geo/helio duality
# Āryabhaṭa's śīghra correction is, geometrically, the change of centre from Earth to Sun.
model.geocentric
reset
show.bodies
show.bodyLabels
show.connectors
show.orbitRing
show.eclipticBelt
show.constellations
now
speed.set 20
play
overlay.show Geocentric — the Sun and its orbit wheel about the Earth
wait 6
overlay.show This is the sky as it is actually seen: everything referred to the observer's own place
wait.next Now recentre it on the Sun

model.heliocentric
overlay.clear
overlay.show Heliocentric — the same motion, recentred on the Sun
wait 6
overlay.show Nothing about the motion changed. Only the point everything is measured from did
wait.next Switch back and tell me why it matters

model.geocentric
overlay.clear
overlay.show That switch, done as a correction rather than a change of diagram, is the śīghra
overlay.show An epicycle carrying a planet about a point that itself moves is a change of centre in disguise
overlay.show Which is why the correction works without ever committing to which body is at rest
wait.next Done

pause
overlay.clear