Precession

Definition

Precession is the slow, conical motion of Earth's rotation axis, which causes the celestial poles to trace out circles in the sky over a period of approximately 26,000 years. This phenomenon affects the position of stars and constellations over long time periods.

5 Must Know Facts For Your Next Test

1. The primary cause of precession is gravitational forces exerted by the Sun and Moon on Earth's equatorial bulge.
2. Hipparchus, an ancient Greek astronomer, is credited with discovering precession around 130 BCE.
3. Precession causes the position of the North Star to change over millennia; currently, Polaris serves as the North Star.
4. Due to precession, astrological signs have shifted from their original positions when they were first established thousands of years ago.
5. The precessional cycle affects the timing of equinoxes and solstices, gradually shifting them along the calendar year.

Review Questions

• What are the primary forces responsible for Earth's precession?
• Who discovered precession and when?
• How does precession affect the identification of the North Star?

Related terms

Equinoxes:

The two points in Earth's orbit where day and night are approximately equal in length worldwide.

Celestial Poles:

The imaginary points in space where Earth's rotational axis intersects with its celestial sphere.

Axial Tilt:

The angle between Earth's rotational axis and its orbital plane around the Sun, currently about 23.5 degrees.