The Moon is Earth’s natural satellite and the closest object to our planet in space. Earth’s gravitational pull on the Moon causes it to move in an orbit around Earth. The changing positions of the Moon, Earth, and the Sun lead to several observable events, including moon phases, eclipses, and tides.

The different forms of the Moon that we see from Earth are called phases. The sequence of these phases is known as the lunar cycle, which lasts about 29.5 days. The phase of the Moon we observe depends on how much of the Moon’s sunlit side is facing Earth at a given time. The cycle begins with the new moon, when the side of the Moon facing Earth is not illuminated. As the Moon moves in its orbit, a small portion of the lit side becomes visible, forming a waxing crescent. The Moon then reaches the first quarter, when half of the Moon’s visible surface is lit. As more of the lit portion becomes visible, the Moon enters the waxing gibbous phase. When the entire face of the Moon is illuminated, it is called a full moon.  After the full moon, the illuminated portion begins to decrease. This stage is called waning. The Moon moves through the waning gibbous phase and then reaches the last quarter, when half of the Moon’s visible surface is again lit. Finally, the Moon becomes a waning crescent before returning to the new moon, and the cycle begins again.

Sometimes the Moon, Earth, and Sun line up in a way that causes an eclipse. An eclipse occurs when either Earth or the Moon casts a shadow on the other. From Earth, we observe two types of eclipses. A solar eclipse occurs when the Moon moves between Earth and the Sun, causing the Moon’s shadow to fall on Earth’s surface. A lunar eclipse occurs when the Moon moves into Earth’s shadow.

The Moon also has several distinctive surface features. These include craters, maria, and highlands. Craters are large, round pits formed when meteoroids—chunks of rock from space—strike the Moon’s surface. The dark, flat regions called maria formed billions of years ago when lava spread across the surface and hardened. The highlands are older, lighter-colored regions that contain many craters and are the most heavily cratered areas on the Moon.

Review:

1. Explain an eclipse, a solar eclipse, and a lunar eclipse.
2. How are craters formed?
3. Draw a picture of the different phases of the moon.