Lecture on Precession

Definition and Basics

Precession: Movement of the rotational axis of an object.
- Commonly observed in toy tops.
- All spinning objects can demonstrate precession.

Earth's Precession: Most significant for astronomy.
Orbital Tilt (Obliquity): Earth has an obliquity of 23.5 degrees.
- The rotational axis traces a cone with a half-angle of 23.5 degrees over ~26,000 years.
- Known as "precession of the equinoxes."

Earth's Shape: Not spherically symmetric, but oblate (wider at the equator).
Gravitational Influence: Sun and Moon's gravitational pull on Earth's equatorial bulge causes precession.

Device: Air gyroscope by Ealing Corporation (1956).
Setup:
- 4-inch steel ball used to observe precession.
- Air pump reduces friction, allowing the ball to spin freely.
- A rod is attached for visibility of the spin.
- Weight adjusted to balance the center of mass over the ball's center.
- Ball spins stably when balanced.
Observation:
- When weight is shifted, the ball mimics Earth's oblate shape.
- Precession is observed as a slow change in rotational axis.

Earth's axis is the basis for the celestial equatorial coordinate system.
Precession Effects:
- Changes in the vernal equinox and celestial poles affect star positions in right ascension and declination.
- Polaris, near the north celestial pole now, will not be in about a thousand years.
Astronomical Catalogs:
- Updated every 50-100 years to current celestial equatorial system definitions.
- Essential for accurate telescope pointing systems.

Understanding precession is crucial for accurate astronomical observations and updating positional catalogs.