Chapter 1 Earth, Moon, and Sun Section 3 Phases, Eclipses, and Tides

As the moon revolves around Earth and Earth revolves around the sun, the relative positions of all of them change. These positions cause the phases of the moon, eclipses, and tides.

Motions of the Moon The moon's orbit around Earth is an oval shape. The moon rotates slowly on its own axis once every 27.3 days. Because it revolves around Earth every 27.3 days, a “day” and a “year” on the moon are the same length. The same side of the moon, the “near side,” always faces Earth. The “far side” of the moon always faces away from Earth, so you never see it from Earth.

Phases of the Moon The moon does not produce the light you see. It reflects light from the sun. The different shapes of the moon you see from Earth are called phases. The moon goes through all the phases each time it revolves around Earth…about once a month.

What Causes the Phases? Phases are caused by changes in the relative positions of the moon, Earth, and the sun. The sun lights the moon. The phase of the moon you see depends on how much of the sunlit side of the moon faces Earth.

Cycles of the Moon During the new moon, the side of the moon facing Earth is not lit. As the moon revolves around Earth, you see more and more of the lighted side of the moon every day, until the side of the moon you see is fully lit. About 29.5 days after the last new moon, the cycle is complete, and you see a new moon again.

Eclipses In most months the moon revolves completely around Earth without the moon moving into Earth's shadow or the moon's shadow hitting Earth. When the moon's shadow hits Earth or Earth's shadow hits the moon, an eclipse occurs. An eclipse occurs when an object in space comes between the sun and a third object, and casts a shadow on that object. There are two types of eclipses: solar eclipses and lunar eclipses

Tilt of the Moon The moon's orbit is tilted with respect to Earth's orbit. So the moon rarely goes directly between Earth and the sun.

Solar Eclipse A solar eclipse occurs when the moon passes between Earth and the sun, blocking the sunlight from reaching Earth. The moon's shadow then hits Earth.

Total Solar Eclipse The darkest part of the moon's shadow, the umbra, is cone-shaped. From any point in the umbra, light from the sun is completely blocked by the moon. The moon's umbra happens to be long enough so that the point of the cone can just reach a small part of Earth's surface.

Lunar Eclipse 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. The moon is then in Earth's shadow and looks dark from Earth. Because the moon is closest to Earth's shadow during the full moon, lunar eclipses occur only at full moon

Lunar Eclipse

Total Lunar Eclipse When the moon is in Earth's umbra, you see a total lunar eclipse. You can see Earth's shadow on the moon before and after the total part of a lunar eclipse. Unlike a solar eclipse, a lunar eclipse can be seen anywhere on Earth that the moon is visible.

Tides Tides, the rise and fall of water, every 12.5 hours or so. The force of gravity pulls the moon and Earth (including the water on Earth's surface) toward each other. As Earth rotates, the moon's gravity pulls water toward the point on Earth's surface closest to the moon.

Tides When the gravity of the moon and the sun pull from the same direction (new moon or full moon) it is called a spring tide and cause big differences between high tide and low tide levels. When the sun’s pull and the moon’s pull meet at a right angle (first-quarter moon and third- quarter moon) it is called a neap tide, which have less noticeable high tides and low tides.

Study Guide Section 1 – Axis tilt – Seasons (equinoxes and solstices) – Rotation/revolution – Orbit Section 2 – Gravity Newton’s law of universal gravitation Two factors: mass and distance Weight changes due to gravity changes – Inertia Newton’s first law of motion Earth is in orbit because of two factors: gravity and inertia Section 3 Phases of the moon (8) Near side/ far side of the moon Sunlight reflects off of the moon to make the phases Solar eclipse Lunar eclipse Umbra Tides Spring tide/ Neap tide Moon’s gravity causes the tides