the.com/nutation
the earth doesn't just spin and tilt, it also nods, like it's agreeing with the moon.
means a small periodic wobble in the tilt of a rotating body's axis, layered on top of its slower long-term wobble.
from from latin nutare, to nod — 18th-century astronomer james bradley caught earth's axis nodding as he tried to measure starlight and stumbled onto the wobble instead.
discovery accidentbradley found it hunting for stellar parallax
causemoon's orbital tilt tugging unevenly on earth's bulge
periodmain cycle repeats roughly every 18.6 years
plants toobotanists call growing-tip circling nutation as well