# Golapāda IV.36 — precession of the equinoxes (ayana), swept across millennia
model.geocentric
pause
reset
show.bodies
show.earthAxis
show.nep
show.constellations
show.eclipticBelt
show.celestialEquator
show.cardinals
year.set -6000
overlay.show The Earth's axis is tilted, and it does not hold still — it wheels slowly around the ecliptic pole
overlay.show One turn takes about 25,800 years. Starting at 6000 BCE, stepping 40 years at a time
wait.next Start the sweep

repeat 400
  year.relative 40
  overlay.clear
  overlay.show {date.year} — the equinox drifts along the ecliptic
  wait 0.04
end
overlay.clear
overlay.show The tilt never changed. What moved was where the equator cuts the ecliptic
wait.next What does that cost the calendar?

overlay.clear
overlay.show That crossing is the vernal point, and every tropical longitude is measured from it
overlay.show It slips back about 1° in 72 years, so a fixed-star longitude and a tropical one drift apart
overlay.show The gap between them is the ayanāṃśa — the whole reason sāyana and nirayaṇa reckonings differ
wait.next Done

overlay.clear