Gravity is the underlying reason for the spin of the earth, the motions of stars within galaxies, and the evolution of stars and the universe. The apparent motions of stars in the equatorial coordinate system arise from precession and nutation of the coordinate system, from parallax and stellar aberration due to the orbital motion of the earth about the sun, and from proper motion, the projection onto the celestial sphere of the peculiar motion of a star relative to the local standard of rest. Precession and nutation of the earth arise from gravitational torques on its equatorial bulge applied by the sun, moon and planets.

The calendar is tied to the seasons such that the first day of spring occurs when the sun moving north crosses the (precessing) vernal equinox. The non-integral number of days in the tropical year (equinox to equinox) was accommodated with the addition of a leap day every 4 yr in Caesar's Julian calendar (46 BCE). The Gregorian calendar (1542) of Pope Gregory XIII removes some of these leap years to obtain a more precise agreement.

Eclipses of the sun and moon are a consequence of the motions of the earth and moon in their respective orbits about the sun and earth. The 18-yr saros cycles of lunar eclipses allowed the ancients and early astronomers to predict when they would occur. Total solar eclipses are wonderful to behold. […]