The Sun and the Earth-Moon System
Lesson Objectives
Describe how Earth’s movements affect seasons and cause day and night.
Explain solar and lunar eclipses.
Describe the phases of the Moon and explain why they occur.
Explain how movements of the Earth and Moon affect Earth’s tides.
Vocabulary
crescent
gibbous
lunar eclipse
penumbra
shadow
solar eclipse
umbra
Introduction
The motions of bodies in the solar system are, for the most part, regular and understandable. From Earth, the Sun rises in the eastern sky in the morning and sets in the western sky in the evening. If the Moon is full on Day 1, it will be full again on Day 28, and new on Day 14. The motions of Earth relative to the Sun, and the motions of the Moon and Sun relative to Earth affect different phenomena on Earth, including day and night, the seasons, tides, and phases of the Moon.
Day-Night Cycle
Earth rotates once on its axis about every 24 hours. To an observer cooling down on the North Pole, the rotation appears counterclockwise. From nearly all points on Earth, the Sun appears to move across the sky from east to west each day. Of course, the Sun is not moving from east to west at all; Earth is rotating. The Moon and stars also seem to rise in the east and set in the west.
Earth’s rotation means that there is a cycle of daylight and darkness approximately every 24 hours, the length of a day. Different places experience sunset and sunrise at different times and the amount of time a location is in daylight and darkness also differs by location.
Shadows are areas where an object obstructs a light source so that darkness takes on the form of the object. On Earth, a shadow can be cast by the Sun, Moon or, rarely, Mercury or Venus.
Earth’s Seasons
A common misconception is that the Sun is closer to Earth in the summer and farther away from it during the winter. Instead, the seasons are caused by the 23.5° tilt of Earth’s axis of rotation relative to its plane of orbit around the Sun (Figure below). At summer solstice, June 21 or 22, Earth’s axis points toward the Sun and so the Sun is directly overhead at its furthest north point of the year, the Tropic of Cancer (23.5° N).
The Earth’s tilt on its axis leads to one hemisphere facing the Sun more than the other hemisphere and gives rise to seasons.
During the summer, areas north of the equator experience longer days and shorter nights. In the Southern Hemisphere, the Sun is as far away as it will be and so it is their winter. Locations will have longer nights and shorter days. The opposite occurs on winter solstice, which begins on December 21. More about seasons can be found in the Earth’s Atmosphere chapter.
