Field notes

Earth spins, or rotates, on an imaginary line called its axis. One rotation takes about 24 hours. The side facing the Sun has daytime; the side turned away has nighttime. Earth also travels around the Sun. One trip takes about 365¼ days. Earth's axis is tilted about 23.5° and points in nearly the same direction throughout its orbit. As Earth travels around the Sun, each hemisphere takes turns leaning toward the Sun. A hemisphere tilted toward the Sun gets more direct sunlight and longer days, bringing summer; when tilted away, it gets less direct sunlight and shorter days, bringing winter. Seasons are caused by Earth's tilt and orbit—not by Earth being much closer to the Sun. At the two equinoxes each year, daylight and darkness are close to equal. Around the June and December solstices, one hemisphere has its longest or shortest day of the year. The Moon's gravity pulls on Earth's oceans, creating tidal bulges. As Earth rotates through these bulges, many coastal places experience two high tides and two low tides in about a day. The Sun's gravity also affects tides. When the Sun, Earth, and Moon line up at new moon or full moon, their pulls combine to make a larger tidal range: spring tides. At first- and third-quarter moon, the pulls partly work against each other, making a smaller tidal range: neap tides. “Spring tide” describes a larger tidal range; it does not mean the season of spring.

Investigation 1: Earth's movements

Investigation 2: Ocean tides