Earth, Moon, And Sun
Earth moves in 2 major ways: rotation and revolution. The imaginary line that passes through Earth's center and the North and South poles is Earth's axis. The spinning of Earth on its axis is called rotation. Earth's rotation causes day and night. As Earth rotates eastward, the sun appears to move west across the sky. As Earth continues to turn east the sun appears to set in the west. Sunlight can't reach the side of Earth facing away from the sun, so it is night there. It takes about 24 hours to rotate once and 24 hours is one whole day. Recall that Earth revolves around the sun because the sun's gravity pulls on it while Earth's inertia keeps moving ahead. Revolution is the movement of one object around another. One revolution of Earth around the sun is called a year. In summer the north end of Earth's axis is tilted toward the sun. In the Northern Hemisphere the noon sun is high in the sky and there are more hours of daylight than darkness. The sun's rays are concentrated. During the winter people in the Southern Hemisphere receive the most direct sunlight so it is summer for them. At the same time, the sun's rays in the Northern Hemisphere are more slanted and there are fewer hours of daylight. The different shapes of the moon you see from Earth are called phases. Phases are caused by the motions of the moon around Earth. As the moon orbits Earth, the relative positions of the moon, Earth, and sun change. The lunar phases occur at different position compared to Earth and the Sun. For example if the part of the moon facing Earth is not lit the moon is a full moon. The whole cycle starts with a new moon, waxing crescent, First Quarter, Waxing gibbous, Full Moon, Waning gibbous, Third quarter, and Waning crescent. All of the lunar phases depend on where the moon is. The moon's orbit around Earth is slightly tilted with respect to Earth's orbit around the sun. As a result, the moon travels above and below Earth's orbit. But on rare occasions, Earth, the moon, and the sun line up. When an object in space comes between the sun and a third object, it casts a shadow on that object, causing an eclipse. There are two types of eclipses: solar and lunar eclipses. A solar eclipse occurs when the moon passes directly between Earth and the sun, blocking sunlight from Earth. The moon's shadow then hits Earth. A lunar eclipse occurs at a full moon when Earth is directly between the moon and the sun. During a lunar eclipse, Earth blocks sunlight from reaching the moon. Lunar eclipses occur only when there is a full moon because the moon is closest to Earth's shadow at that time.