The universe is the totality of all space, time, matter, and energy. Astronomy is the study of the universe. A widely used unit of distance in astronomy is the light year, the distance traveled by a beam of light in one year. Early observers grouped the thousands of stars visible to the naked eye into patterns called constellations. These patterns have no physical significance, although they are a very useful means of labeling regions of the sky. The nightly motion of the stars across the sky is the result of Earth's rotation on its axis. Early astronomers, however, imagined that the stars were attached to a vast celestial sphere centered on Earth and that the motions of the heavens were caused by the rotation of the celestial sphere about a fixed Earth. The points where Earth's rotation axis intersects the celestial sphere are called the north and south celestial poles. The line where Earth's equatorial plane cuts the celestial sphere is the celestial equator.
The time from one sunrise to the next is called a solar day. The time between successive risings of any given star is one sidereal day. Because of Earth's revolution around the Sun, the solar day is a few minutes longer than the sidereal day.
The Sun's yearly path around the celestial sphere, or, equivalently, the plane of Earth's orbit around the Sun, is called the ecliptic. Because Earth's axis is inclined to the ecliptic plane, we experienceseasons, depending on which hemisphere (Northern or Southern) happens to be "tipped" toward the Sun. At the summer solstice, the Sun is highest in the sky, and the length of the day is greatest. At the winter solstice, the Sun is lowest, and the day is shortest. At the vernal and autumnal equinoxes, Earth's rotation axis is perpendicular to the line joining Earth to the Sun, and so day and night are of equal length. The interval of time from one vernal equinox to the next is one tropical year.