Overview
This lecture explains the astronomical concepts of precession and parallax, focusing on how the Earth's axial wobble affects the night sky and celestial coordinate systems.
Precession of the Earth's Axis
- The Earth spins on its axis once every 24 hours and orbits the Sun once a year.
- Precession is the slow movement of the Earth's rotational axis, tracing a circle over ~26,000 years.
- This motion is similar to the wobble seen in spinning tops or coins as they slow down.
- Nutation is a smaller periodic wobble superimposed on precession; Earth's axial tilt varies between 22.5° and 24.5°.
- Changes in Earth's tilt due to precession and nutation can contribute to long-term climate changes like ice ages.
Effects of Precession
- The celestial coordinate system (North/South Celestial Poles and Celestial Equator) shifts slowly over time.
- The current Pole Star is Polaris, but 5,000 years ago it was Thuban; in 10,000 years it will be Vega.
- The "first point of Aries" (vernal equinox) moves through the zodiac, spending about 2,100 years in each sign.
- Star maps and celestial coordinates have a "sell-by date" since star positions change over time.
- Professional astronomers update star coordinates yearly to account for precession.
- Alignments of ancient structures (such as the pyramids of Giza) to stars become inaccurate due to precession.
- Alignments to solar events (like Stonehenge's heel stone and the solstice sun) are unaffected by precession.
Celestial Coordinate System and the Ecliptic
- The celestial equator is Earth's equator projected into space; the ecliptic is the Sun's apparent annual path.
- The two cross at the vernal (first point of Aries) and autumnal equinoxes, which shift due to precession.
- The first point of Aries is currently in Pisces and will enter Aquarius in about 600 years.
- This shift defines astrological "ages," but has no astronomical significance.
Key Terms & Definitions
- Precession — Slow, conical motion of Earth's axis, completing a cycle every ~26,000 years.
- Nutation — Small, periodic oscillation (wobble) of Earth's axis superimposed on precession.
- Celestial Equator — The Earth's equator projected outward onto the celestial sphere.
- Ecliptic — The apparent annual path of the Sun against the background stars.
- First Point of Aries — The position in the sky where the Sun crosses the celestial equator at the vernal equinox.
- Vernal Equinox — The moment when the Sun crosses the celestial equator moving northward.
- Pole Star — The star closest to the direction of Earth's rotational axis (currently Polaris).
Action Items / Next Steps
- Review updated star maps regularly, noting their date of validity.
- Learn to identify the celestial equator, ecliptic, and first point of Aries in star charts.