Table of Contents
- How Is the Moon Dependent on the Sun?
- What Causes the Moon to Shine?
- Moon Phases with the Earth and Sun
- What Happens When the Moon Is in Front of the Sun?
- Lesson Summary
The two brightest objects shining in the sky are the sun and the moon. However, they shine for entirely different reasons. The sun, a star at the center of the solar system, shines because of nuclear fusion within its massive core. Vast amounts of energy are released in this process which the earth experiences as light. The moon, on the other hand, is dependent on the sun. It shines only because its surface reflects sunlight. Without the sun, the moon and the earth would be very cold, dark rocks floating in space.
Just as the earth revolves around the sun, the moon revolves around the earth. Any object that revolves around another is called a satellite. The moon is therefore considered a natural satellite of the earth. The path an object takes as it revolves is called its orbit. All orbiting or moving objects in space tend to remain in motion; this is called inertia. When the inertia of an object is in balance with the gravitational force of another object, it will orbit it. The changing location of the moon as it orbits causes tides, moon phases, and eclipses.
Is the moon a reflection of the sun? The moon shines because the side facing the sun reflects the sunlight light into space. One side of the moon is always facing the sun and is lit up. However, the entire lit-up face of the moon is not always visible. Depending on where the moon is in its orbit, there are times when the moon's lit side faces away from earth. The visibility amount of the moon constantly changes as the moon makes its monthly orbit around the earth.
The visibility amount of the lit surface of the moon changes each month predictably. However, humans are always seeing the same side of the moon. The back or far side of the moon has never been visible; this is because the moon is in synchronous rotation meaning it takes the moon the same amount of time to rotate once on its axis as it does to make one revolution around the earth. The result is the same side of the moon facing the earth. The side of the moon that is never visible is called the 'dark side' of the moon.
Every so often, the earth or moon blocks sunlight from reaching one or the other. When this happens, it is called an eclipse. Eclipses occur in either the new moon or full moon phases and are described more thoroughly later. In new and full moon phases, the moon, earth, and sun line up in a straight line called syzygy.
The changing appearance of the moon as it orbits the earth is called phases. The photograph shows a sequence of moon phases. The moon phase shown in number 1 is the new moon phase. The moon is not visible from earth in a new moon phase because its lit side faces away from earth. Moon number 5 on the photo shows the moon with the entire lit side facing the earth; this is the full moon phase.
The moon's location on its path around the earth is what determines the phases of the moon. Scientists have given names to the changing appearance or phases of the moon. A waxing moon occurs before a full moon when the lit surface of the moon is increasing in size. A waning moon occurs after a full moon when the lit surface of the moon is decreasing in size. More precise names are labeled and described on both the table and the diagram.
The table lists the moon phases in order starting with a new moon. Note that numbers 1-5 on the table also correspond to numbers 1-5 on the moon phase photograph.
|Moon Phase||Amount of Lit Surface Visible|
|1. new moon||none|
|2. waxing crescent||1/4|
|3. first quarter||1/2|
|4. waxing gibbous||3/4|
|5. full moon||all|
|6. waning gibbous||3/4|
|7. third quarter||1/2|
|8. waning crescent||1/4|
|1. new moon||none|
The same phases listed on the table can also be seen on the diagram. The diagram shows the actual position of the moon relative to the earth and the sun. To analyze the diagram, here are important points of reference to keep in mind:
Moon phases with sun depend on the amount of reflected light off the moon reaches earth.
The earth and moon are in constant, predictable motion relative to each other and the sun. A few times a year, sunlight is blocked from reaching the earth or the moon. These occurrences are called eclipses. An eclipse happens when the earth, moon, and sun are in a straight line. Many people wonder why there is not an eclipse once a month. It is because the orbital plane of the moon and earth is tilted about 5 degrees off the orbital plane of the earth and sun.
A lunar eclipse, where the earth blocks sunlight from reaching the moon, can occur 0-3 times per year. It occurs when the moon passes behind the earth and partially or totally blocks sunlight from reaching it. The diagram shows the earth casting a shadow off into space. The moon passing through the shadow will show either a partial or total eclipse.
A total lunar eclipse can only happen in syzygy during the full moon phase. The earth blocks all light from reaching the moon. Interestingly, in a total lunar eclipse, the moon, instead of disappearing due to lack of light, actually turns red. It does this because instead of reflecting sunlight, the moon reflects the longer frequency red-colored light waves refracted from the earth's atmosphere.
Solar eclipses are different. They occur only in the new moon phase when the moon blocks sunlight from reaching the earth. Solar eclipses like lunar eclipses happen just a few times per year. Sun phases do not exist like moon phases, where its appearance regularly changes. However, it has higher and lower energy output phases and solar storms, which are mostly unpredictable.
The moon is a natural satellite making one orbit around the earth each month. Its inertia keeps it moving, and gravitational force keeps it in orbit. The moon is dependent on the sun to reflect sunlight as it does not have its own light source. The amount of it that is visible changes as the moon orbits the earth in what is called phases. The new moon phase is when the moon is not visible because its lit surface is facing away from earth. The lit surface appears to grow in a waxing moon as it orbits the earth. When it reaches the full moon phase, the entire lit surface of the moon is facing towards the earth. The lit surface then grows smaller in a waning moon until it reaches the new moon phase when the cycle begins again. The moon is also in synchronous rotation meaning it rotates at the same speed it revolves, so the same side of the moon always faces the earth.
Twice a month, the moon, earth, and sun are in a straight line called syzygy. A few times per year, this can result in an eclipse. When the moon blocks sunlight from reaching the earth, it is a solar eclipse, and if the earth blocks sunlight from reaching the moon, it is a lunar eclipse. Eclipses occur only a few times per year and not every month because the moon's orbital plane is slightly tilted compared to the earth's orbital plane.
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The moon can shine during the day also, but it is not as obvious. The moon reflects light from the sun, and sometimes it is visible even during the day.
The earth and sun have roles in moon phases. The sun is constantly lighting up half of the moon. However, the amount of visible moonlight depends on the moon's location in its orbit around earth.
When the moon is located between the earth and the sun, it is called a new moon phase. During this phase, the moon is not visible because the lit-up side faces away from the earth.
The sun does not go through phases as the moon does. However, it has times and periods where it puts out more energy or has more storms on its surface.
When the sun, moon, and earth are in a straight line, it is called syzygy. Syzygy is the arrangement that will occur in both full and new moons.
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