CHAPTER 1.3 – PHASES, ECLIPSES, AND TIDES

FACTS ABOUT THE MOON When you see the moon from Earth, you are seeing a pattern of light-colored and dark-colored areas of the moon’s surface. The near side of the moon always faces the Earth and the far side of the moon always faces away from Earth.

I. MOTIONS OF THE MOON The moon moves through space in two ways: 1. It revolves around the earth and 2. It rotates on its axis. As the moon revolves around the Earth, the positions of the Earth, moon, and sun change. The changes are called phases of the moon, eclipses, and tides.

The moon rotates once on its axis in the same amount of time it takes to revolve around the Earth. It is 27.3 days. In other words, its day and its year are equal. This also causes us to see the same side of the moon each time.

II. PHASES OF THE MOON The moon does not produce its own light. It just reflects the light from the sun. When you look at the moon during different nights, it shows different shapes in the sky. The shapes are round or some sort of crescent. This is called moon phases and there are 8 phases. The phases differ depending upon when the moon is in its revolution around Earth. The phase of the moon you see depends on how much of the sunlit side of the moon faces Earth.

The 8 Moon Phases

The first phase of the moon cycle is New Moon The first phase of the moon cycle is New Moon. We see nothing in the sky because the lighted side of the moon is facing back to the sun. As the moon revolves around the Earth, more and more of the lighted moon is seen until it reached a full moon. After it reaches a full moon, more and more of the lighted moon is less seen until it is a New Moon again. Each of the 8 phases takes one week to complete.

III. ECLIPSES The moon’s orbit around Earth is slightly tilted. When the moon’s shadow hits Earth or Earth’s shadow hits the moon, an eclipse occurs. When it involves the moon, it is called a lunar eclipse. Also, when a third object comes between the sun and the Earth to cast a shadow, an eclipse occurs. This is called a solar eclipse.

A. SUN (Solar Eclipse) 1. WHEN DO SOLAR ECLIPSES OCCUR? During a new moon, the moon lies between the Earth and the sun. At any other time, the moon is a little above or below the sun due to its titled axis. When the moon passes directly between the sun and the Earth, it blocks the sun and casts a shadow on the Earth. This is called a solar eclipse.

2. TOTAL SOLAR ECLIPSES During a total solar eclipse, the darkest part of the moon’s shadow is blocking the sun’s light. This is called the umbra. During a total solar eclipse, day turns into night and the air cools. You can even see stars and the solar corona (the faint outer atmosphere of the sun).

Total Solar Eclipse by the Moon’s Shadow

3. PARTIAL SOLAR ECLIPSES During a partial solar eclipse, a less dark part of the moon’s shadow covers the sun. This is called the penumbra. In a partial solar eclipse, you will still be able to see part of the sun.

B. MOON (LUNAR) 1. WHEN DO LUNAR ECLIPSES OCCUR? A lunar eclipse only occurs during a full moon. At that time, the earth is directly in between the moon and the sun. As a result, the Earth blocks sunlight from reaching the moon.

2. TOTAL LUNAR ECLIPSES A total lunar eclipse occurs when the Earth’s umbra (darkest part of the shadow) block the sun’s light from reaching the moon.

3. PARTIAL LUNAR ECLIPSES During a partial lunar eclipse, the Earth, moon, and sun are not quite in line. As a result, the Earth is not completely blocking some of the sun’s light to reach the moon and reflect. (The moon goes into partially into the Earth’s umbra.)

Partial Lunar Eclipse

IV. TIDES Def. of tides: The rise and fall of ocean water that occurs every 12.5 hours. The water rises for 6 hours and falls for 6 hours. Tides are caused mainly by differences in how much the moon’s gravity pulls on different parts of the Earth.

A. THE TIDE CYCLE Look at the diagram on p. 26. There are always two spots on Earth that have high tides and two spots on Earth that have low tides. (They are opposite from each other.) Point A experience high tide since it is closest to the moon and the moon has the greatest pull on Point A. Its opposite point is C where high tide occurs as well because the moon has the weakest pull at that spot so the water ‘ pools’ there. Points B & D are in between the points with the strongest pull toward the moon and weakest pull toward the moon. As a result, there is low tide here because water is either being pulled to point A or C.

B. SPRING TIDES Besides the moon having gravitational pull on the Earth’s waters, so does the sun. When the sun’s gravity and the moon’s gravity pull together, it produces a spring tide. Spring tides happen twice a month on the new moon phase and on the full moon phase.

C. NEAP TIDES During the 1st and 3rd quarters of the moon cycle, the Earth, moon, and sun are at right angles to each other. In other words, the sun’s gravitational pull on the water is at a right angle to the moon’s gravitational pull on the water. It is called a neap tide and it occurs twice a month.