Unit 2The Sky and Celestial Motions • The sky and the constellations • Daily motion of the Earth • Annual motion of the Earth • Motion of the Moon • Eclipses • Motions of the planets • Precession
Cosmology • Cosmology is the study of the structure and evolution of the Universe. • Ancient world models represent the earliest cosmologies. • Ancient Egyptians and Babylonians both thought that Earth is flat and the sky is a dome arching above it. • See also the Figures on p. 62 of Bless.
The Horizon System • The horizon • The cardinal directions, N, S, E, W • The point directly above is the zenith • The point directly below is the nadir • Imagine a sphere all around the sky on which are “pinned” the sun, moon, planets and the stars -- this is the celestial sphere • Coordinates altitude and azimuth
The Daily Motion of the Earth • Earth spins on its axis. • This causes the change of day and night, and the rising and setting of the Sun, stars, Moon and planets. • The extensions of the Earth’s spin axis onto the celestial sphere mark the north celestial pole and the south celestial pole.
The Equatorial System • The zenith and the nadir • The (Earth) celestial poles • The (Earth) celestial equator • Latitudes north and south of equator (on Earth) - Declination on the celestial sphere • Longitudes east and west of the Greenwich meridian (on Earth) or the spring equinox (on the celestial sphere - Right Ascension)
The Annual Motion of the Earth • Earth revolves around the Sun. • The Sun appears to move constantly eastward among the stars. • The Sun blocks out different constellations throughout the year. • We see different constellations in the night sky during different seasons. • The apparent path of the Sun through the sky is called the ecliptic. The constellations located along the ecliptic are the constellations of the zodiac.
Equinoxes and Solstices • The apparent path of the Sun through the constellations, the ecliptic, crosses the celestial equator in two points. These are the equinoxes. • On the day of the spring/fall equinox, the sun rises in the east and sets in the west. Day and night have equal length. • The Sun is furthest away from the celestial equator on the days of the solstices.
During the day of the summer solstice, the Sun rises due N of E and sets due N of W. It reaches the highest point in the sky. This is the longest day of the year. • During the day of the winter solstice, the Sun rises due S of E and sets due S of W. It makes its shortest and lowest arc through the sky. This is the shortest day and the longest night of the year. • The time from one vernal equinox to the next is called the tropical year.
The Seasons • The reason for the seasons is the tilt of Earth’s spin axis by 23.5o with respect to its orbital plane, the ecliptic. • During summer on the northern hemisphere, the northern half of Earth is tilted toward the Sun. The northern hemisphere has longer days. • Sunlight strikes the ground more from overhead, heating it up.
The Motion of the Moon • The Moon spins on its axis with the same rate as it orbits around the Earth. • Therefore, the Moon always shows us the same face. • The length of the month derives itself from the lunar phase cycle.