Earth spins on an axis running between its north and south poles, and it moves in an orbit around the Sun. We will now explore how these motions create the day-night cycle, the yearly cycle, and the seasons.

RotationThe spinning of a body on an axis running through it is called rotation. We do not feel Earth’s rotation because our planet is so physically large compared to us that Earth’s gravitational attraction holds us firmly on its surface. Earth’s rotation causes the stars—as well as the Sun, the Moon, and the planets—to appear to rise along the eastern horizon, move across the sky, and set along the western horizon. Earth’s daily rotation, causing the Sun to rise and set, thereby creates day and night. In general, this rotation creates diurnal motion, or daily motion, which causes all objects to circle around the celestial poles.

Depending on your latitude, some of the stars and constellations never disappear below the horizon. Instead, they trace complete circles in the sky over the course of each night (Figure 1-9). To understand why this happens, imagine that you are standing on Earth’s North Pole at night. Looking straight up, you see Polaris. Because Earth is spinning around its axis directly under your feet, all the stars appear to move from left to right (counterclockwise) in horizontal rings above you. The exception is Polaris, which always remains at the North Pole’s zenith. As seen from the North Pole, no stars rise or set (Figure 1-10). They just seem to revolve around Polaris in horizontal circles. Stars and constellations that never go below the horizon as seen from a given place on Earth are called circumpolar (Figure 1-9). While there is no bright South Pole star equivalent to Polaris, all stars seen from the South Pole are also circumpolar and move from right to left (clockwise).

If you live in the northern hemisphere, Polaris is always located above your northern horizon at an angle equal to your latitude. Only the stars and constellations that pass between Polaris and the land directly below it are circumpolar. As you go farther south in the northern hemisphere, the number of stars and constellations that are circumpolar decreases. Likewise, as you go farther north in the southern hemisphere, the number of stars and constellations that are circumpolar also decreases.

Now visualize yourself at the equator. All the stars appear to rise straight up in the eastern sky and set straight down in the western sky (Figure 1-11). Polaris is barely visible on the northern horizon. While Polaris never sets, all the other stars do, and therefore none of the stars are circumpolar as seen from the equator.

As you can see from these last two mental exercises, the angle at which the stars rise and set depends on your viewing latitude. Figure 1-12 shows stars setting at 35° north latitude. Polaris is fixed at 35° above the horizon to the right of this figure, not at the zenith, as it is at the North Pole, nor on the horizon, as seen from the equator. As another example, except for those stars in the corners, all the stars whose paths are shown in Figure 1-9 are circumpolar because they are visible all night, every night.

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RevolutionWhereas rotation is the spinning of an astronomical object around an axis through it, revolution is the motion of any astronomical object around another astronomical object. Earth takes 1 year, or about 365¼ days, to revolve around the Sun. A year on Earth is measured by the motion of our planet relative to the stars. For example, draw a straight line from the Sun through Earth to some star on the opposite side of Earth from the Sun. As Earth revolves around the Sun, that line inscribes a straight path on the celestial sphere (namely, the ecliptic) and returns to the original star 365¼ days later. The length of any cycle of motion measured with respect to the stars is called a sidereal period. Earth’s annual orbit around the Sun, 1 year, is a sidereal period, as is the period of the diurnal motion of the stars each day in our sky. This latter period is 23 h 56 m 4 s.

Different constellations are visible at night during different times of the year because of Earth’s revolution around the Sun. If Earth were rotating at a fixed place over the Sun, rather than orbiting around it, then every star would always rise and set at the same time every day. Of course, Earth is in motion around the Sun, and as a result the Sun moves around the celestial sphere and the stars rise approximately 4 minutes earlier each day than they did the day (or night) before. This effect accumulates, bringing different constellations up at night throughout the year. Figure 1-13 summarizes this motion. When the Sun is within the boundaries of (colloquially, “in”) Virgo (September 18–November 1), for example, the hemisphere containing the Sun and the constellations around Virgo are in daylight (see Figure 1-13a). When the Sun is up, so are Virgo and the surrounding constellations, so we cannot see them. During that time of year, the constellations on the other side of the celestial sphere, centered on the constellation Pisces, are in darkness. Thus, when the Sun is “in” Virgo, Pisces and the constellations around it are high in our sky at night.

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Six months later, when the Sun is “in” Pisces, that half of the sky is filled with daylight, while Virgo and the constellations around it are high in the night sky (see Figure 1-13b). These arguments apply everywhere on Earth at the same time because the Sun moves along its path very slowly as seen from Earth, taking a year to make one complete circuit around the celestial sphere.