Precession of the Equinoxes: A slow, 26,000-year cycle where Earth's rotational axis traces a cone caused by gravitational forces from the Sun and Moon.
Earth's Tilt: The axial tilt is 23.5 degrees causing season changes.
Rotational Axis vs. Orbital Axis: Axis of rotation is tilted relative to Earth's orbital plane around the Sun.
Effects of Precession
Pole Star Movement: The North Celestial Pole (aligned with Polaris now) shifts over time, changing the position of the pole star.
Current Pole Star: Polaris, approx. 41 arcminutes from North Celestial Pole.
Other Historical North Stars: Thuban (3000 BC), Vega (13,000 years ago).
Cycle Completion: Every 26,000 years, returning around Polaris.
Equinoxes Shift: Positions on the celestial sphere where equinoxes occur move over time.
Marked changes in equinox point relative to stars used for astronomical measurements.
Coordinates for celestial objects adjusted periodically (every 50 years in catalogs).
Practical Observations
Using Stellarium: Demonstrated effects in New Haven, February 2016.
Finding Polaris: Big Dipper's pointer stars lead to Polaris.
Seasonal Sky Movement: Sky rotates around the North Celestial Pole.
Historical Observations: Movement shown back to the time of Shakespeare and Julius Caesar.
Example: Spring equinox was in Aries 3000 years ago, now between Pisces/Aquarius.
Solstice Alignments: Checked using Stellarium for historical changes in solar alignments at solstices.
Stonehenge Alignment: Solstice alignments robust to precession.
Architectural Alignments: Need to consider precession for historically dated buildings aligned with stars.
Observations and Theory
Gyroscopic Effect: Earth acts like a gyroscope with torque causing precession.
Gravitational Forces: Equatorial bulge caused by rotation and torques from the Moon and Sun drive precession.
Long-Term Cycles: Nutation (100,000-year cycle) affects axial tilt over longer periods, not significant for short-term precession.
Conclusion
Comprehensive Understanding: Precession and its effects are critical for astronomy, affecting everything from astrological signs to the alignment of ancient monuments.