As Earth's only natural satellite, the Moon has long been an object of fascination and confusion. Over the course of a day cycle, the Moon shows us many different "faces". These different "faces" are called phases and they are the result of the way the Sun lights the Moon's surface as the Moon orbits Earth. The Moon can only be seen as a result of the Sun's light reflecting off it.
It does not produce any light of its own. This demonstration will illustrate why the Moon has so many different looks within that day period known as the lunar cycle. Prior to or after the demonstration, allow the students to go on-line and read additional background information related to the Moon Solar System section and man's exploration of this satellite Space Stuff section. Put the lamp in the middle of the room. After the lamp has been turned on, darken all other lights.
The lamp represents the Sun while the orange represents the Moon and the volunteer represents the Earth. Only the volunteer can see the intended results of this demonstration, so each student will have to take their turn in order to understand the phases of the Moon. Earth will face the Sun holding the Moon in the left hand. The Moon should be held in front at arm's length and slightly elevated overhead. Make sure the students understand that it is because of the Moon's slightly inclined orbit around Earth that we usually see a full Moon when the Earth is between the Sun and Moon.
Notice that the lamp has lit up the side of the Moon away from Earth. No one on Earth can see the lit side at this point. Objects that create light tend to also reflect ambient light, so that they tend to be the brightest objects around. Examples include campfires, light bulbs, candle flames, and computer screens. In terms of astronomical bodies, stars are the main objects that create significant amounts of visible light, and therefore are some of the brightest objects in the universe.
If a planet somehow became large enough to initiate nuclear fusion and begin glowing, it would no longer be a planet. It would be a star. Since planets and moons do not emit light, the only reason we can see them is because they reflect light from some other source.
The strongest source of light in our solar system is the sun, so usually we see planets and moons because they are reflecting sunlight. The amount of sunlight incident on a moon or planet that gets reflected depends on the materials in its surface and atmosphere as well as its surface roughness. Snow, rough ice, and clouds are highly reflective. Most types of rock are not. Therefore, a planet that is covered with clouds, such as Earth or Venus, is generally brighter than a rocky moon or planet that has no atmosphere.
There are two main types of reflectivity: specular reflectivity and diffuse reflectivity. Specular reflectivity measures how much of the incoming light gets reflected by the object in the direction given by the mirror angle. In contrast, diffuse reflectivity measures how much light gets reflected in all directions. A mirror has high specular reflectivity and low diffuse reflectivity. In contrast, sand has low specular reflectivity and high diffuse reflectivity.
In everyday life, we experience specular reflectivity as the perception of mirror images and glare spots on the surface of objects. We experience diffuse reflectivity as a somewhat uniform brightness and color that exists on the surface of the object and is roughly the same no matter what our viewing angle is.
Many objects display significant amounts of both specular reflectivity and diffuse reflectivity. If the moon's surface were like a perfectly smooth billiard ball, its surface brightness would be the same all over. In such a case, it would indeed appear half as bright. But the moon has a very rough topography. Also, the moon's face is splotched with dark regions. The end result is that at first quarter, the moon appears only one eleventh as bright as when it's full.
The moon is actually a little brighter at first quarter than at last quarter, since at that phase some parts of the moon reflect sunlight better than others. Believe it or not, the moon is half as bright as a full moon about 2. Even though about 95 percent of the moon is illuminated at this time, and to most casual observers it might still look like a "full" moon, its brightness is roughly 0. However, they are opposite to the lunar phases that we see from the Earth. It's a full Earth when it's new moon for us; last-quarter Earth when we're seeing a first-quarter moon; a crescent Earth when we're seeing a gibbous moon, and when the Earth is at new phase we're seeing a full moon.
From any spot on the moon except on the far side, where you cannot see the Earth , the Earth would always be in the same place in the sky. From the moon, our Earth appears nearly four times larger than a full moon appears to us, and — depending on the state of our atmosphere — shines anywhere from 45 to times brighter than a full moon.
So when a full or nearly full Earth appears in the lunar sky , it illuminates the surrounding lunar landscape with a bluish-gray glow. From here on the Earth, we can see that glow when the moon appears to us as a crescent; sunlight illuminates but a sliver of the moon, while the rest of its outline is dimly visible by virtue of earthlight. Leonardo da Vinci was the first to figure out what that eerie glow appearing on the moon really was.
Phases aren't the only things that are seen in reverse from the moon. An eclipse of the moon for us is an eclipse of the sun from the moon.
In this case, the disk of the Earth appears to block out the sun. If it completely blocks the sun, a narrow ring of light surrounds the dark disk of the Earth; our atmosphere backlighted by the sun. The ring appears to have a ruddy hue, since it's the combined light of all the sunrises and sunsets occurring at that particular moment.
That's why during a total lunar eclipse, the moon takes on a ruddy or coppery glow. When a total eclipse of the sun is taking place here on Earth, an observer on the moon can watch over the course of two or three hours as a small, distinct patch of darkness works its way slowly across the surface of the Earth.
It's the moon's dark shadow, called the umbra, that falls on the Earth, but unlike in a lunar eclipse, where the moon can be completely engulfed by the Earth's shadow, the moon's shadow is less than a couple of hundred miles wide when it touches the Earth, appearing only as a dark blotch.
The lunar craters were formed by asteroids and comets that collided with the moon. Roughly , craters wider than 1 km 0. These are named for scholars, scientists, artists and explorers. For example, Copernicus Crater is named for Nicolaus Copernicus, a Polish astronomer who realized in the s that the planets move about the sun.
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