The Sky, Earth's Motion, Moon Phases & Eclipses

This lecture sets up the language astronomers use to describe positions and motions in the sky. The sky is divided into 88 constellations — fixed sectors named for star patterns. Within each, the brightest stars are labeled by Greek letter (Bayer system): α for the brightest, β for the second, and so on. So Betelgeuse is α Orionis, Rigel β Orionis.

Because Earth rotates, the sky appears to spin around the celestial poles once every sidereal day (23 h 56 min). The North Star, Polaris, sits almost on the celestial north pole, so it appears stationary while other stars trace concentric arcs. Locations on the celestial sphere are given by right ascension (the sky's longitude, measured in hours east of the vernal equinox) and declination (the sky's latitude, in degrees north/south of the celestial equator).

The solar day (24 h) is slightly longer than the sidereal day because Earth also moves ~1° along its orbit each day and must rotate a bit further to bring the Sun back to the meridian. Earth's 23.5° axial tilt — not its distance from the Sun — causes the seasons. Over ~26,000 years, the tilted axis precesses, slowly shifting which star sits at the pole.

The Moon's full phase cycle takes ~29.5 days: new → waxing crescent → first quarter → waxing gibbous → full → waning gibbous → third quarter → waning crescent. When Sun–Earth–Moon align, eclipses occur: lunar (Earth's shadow on the full Moon) or solar (the new Moon's shadow on Earth).

Key distance units introduced: AU (~1.5 × 10⁸ km), light-year (~10¹³ km), and c = 3 × 10⁵ km/s.