The Moon
The Moon's Origin
Before the Apollo space missions in the 1960s and 1970s, there were three leading theories about the Moon's origin. According to one theroy, the Moon was captured by Earth's gravity. Another held that material surrounding Earth condensed to produce the Moon. An alternative theory proposed that Earth ejected molten material that became the Moon.
The Impact Theory
The data gathered by the Apollo missions have led many scientists tosupport a new theory, known as the impact theory. I states that the Moon formed billions of years ago from condensing gas and debris thrown off when Earth collided with a Mars-sized object.
Motions of the Moon
Doesn't it seem as if the Moon's shape changes night after night? Sometimes, just after sunset, you can see a full, round Moon low in the sky. Other times, only half of the Moon is visible, and it's high in the sky at sunset. At times, the Moon is even visible during the day. What causes the Moon to change in appearance and position in the sky?
Rotation and Revolution
Just as Earth rotates on its axis and revolves around the Sun, the Moon rotates on its axis and revolves around Earth. The Moon's revolution around Earth is responsible for the changes in its appearance. If the Moon rotates on its axis, why can't you see it spin around in space? The reason is that the Moon's rotation takes 27.3 days, the same amount of time it takes to revolve around Earth. Because these two motions take the same amount of time, the same side of the Moon always faces Earth.
Reflection of the Sun
The Moon seems to shine because its surface reflects sunlight. Just as half of Earth experiences day as the other half experiences night, half of the Moon is lighted while the other half is dark. As the Moon revolves around Earth, you see different portions of its lighted side, causing the Moon's appearance to change.
Rotation and Revolution
Just as Earth rotates on its axis and revolves around the Sun, the Moon rotates on its axis and revolves around Earth. The Moon's revolution around Earth is responsible for the changes in its appearance. If the Moon rotates on its axis, why can't you see it spin around in space? The reason is that the Moon's rotation takes 27.3 days, the same amount of time it takes to revolve around Earth. Because these two motions take the same amount of time, the same side of the Moon always faces Earth.
Reflection of the Sun
The Moon seems to shine because its surface reflects sunlight. Just as half of Earth experiences day as the other half experiences night, half of the Moon is lighted while the other half is dark. As the Moon revolves around Earth, you see different portions of its lighted side, causing the Moon's appearance to change.
Phases of the Moon
Moon phases are the different forms that the Moon takes in its appearance from Earth. The phase depends on the relative positions of the Moon, Earth, and the Sun. A new moon occurs when the Moon is between Earth and the Sun. During a new moon, the lighted half of the Moon is facing the Sun and the dark side faces Earth. The Moon is in the sky, but it cannot be seen. The new moon rises and sets with the Sun.
Waxing Phases
After a new moon, the phases begin waxing. Waxing means that more of the illuminated half of the Moon can be seen each night. About 24 hours after a new moon, you can see a thin slice of the Moon. This phase is called the waxing crescent. About a week after a new moon, you can see half of the lighted side of the Moon, or one quarter of the Moon's surface. This is the first quarter phase.
The phases continue to wax. When more than one quarter is visible, it is called waxing gibbous after the Latin word for "humphacked." A full moon occurs when all of the Moon's surface facing Earth reflects light.
Waning Phases
After a full moon, the phases are said to be waning. When the Moon's phases are waning, you see less of its illuminated half each night. Waning gibbous begins just after a full moon. When you can see only half of the lighted side, it is the third-quarter phase. The Moon continues to appear to shrink. Waning crescent occurs just befor another new moon. Once again, you can see only a small slice of the Moon
It takes about 29.5 days for the Moon to complete its cycle of phases. Recall that it takes about 27.3 days for the Moon to revolve around Earth. The discrepancy between these two numbers is due to Earth's revolution. The roughly two extra days are what it takes for the Moon to keep up constantly with Earth as it orbits the Sun.
Waxing Phases
After a new moon, the phases begin waxing. Waxing means that more of the illuminated half of the Moon can be seen each night. About 24 hours after a new moon, you can see a thin slice of the Moon. This phase is called the waxing crescent. About a week after a new moon, you can see half of the lighted side of the Moon, or one quarter of the Moon's surface. This is the first quarter phase.
The phases continue to wax. When more than one quarter is visible, it is called waxing gibbous after the Latin word for "humphacked." A full moon occurs when all of the Moon's surface facing Earth reflects light.
Waning Phases
After a full moon, the phases are said to be waning. When the Moon's phases are waning, you see less of its illuminated half each night. Waning gibbous begins just after a full moon. When you can see only half of the lighted side, it is the third-quarter phase. The Moon continues to appear to shrink. Waning crescent occurs just befor another new moon. Once again, you can see only a small slice of the Moon
It takes about 29.5 days for the Moon to complete its cycle of phases. Recall that it takes about 27.3 days for the Moon to revolve around Earth. The discrepancy between these two numbers is due to Earth's revolution. The roughly two extra days are what it takes for the Moon to keep up constantly with Earth as it orbits the Sun.
Eclipses
Imagine living 10,000 years ago. You are foraging for nuts and fruit when unexpectedly the Sun disappears from the sky. The dardness lasts only a short time, and the Sun soon returns to full brightness. You know something strange has happened, but you don't know why. It will be almost 8,000 years before anyone can explain what you just experienced.
The event just described was a total solar eclipse (ih KLIPS). Today, most people know what causes such eclipses, but without this knowledge, they would have been terrifying events. During a solar eclipse, many animals act as if it is nightime. Cows return to their barns and chickens go to sleep. What causes the day to become night and thhen change back into day?
What causes an eclipse?
The revolution of the Moon causes eclipses. Eclipses occur when Earth or the Moon temporarliy blocks the sunlight from reaching the other. Sometimes, during a new moon, the Moon's shadow falls on Earth and causes a solar eclipse. During a full moon, Earth's shadow can be cast on the Moon, resulting in a lunar eclipse.
An eclipse an occur only when the Sun, the Moon, and Earth are lined up perfectly. Because the Moon's orbit is not in the same plane as Earth's orbit around the Sun, eclipses occur only a few times each year.
Eclipses of the Sun
A solar eclipse occurs when the Moon moves directly between the Sun and Earth and casts its shadow over part of Earth. Depending on where you are on Earth, you may experience a total eclipse or a partial eclipse. The darkest portion of the Moon's shadow is called the umbra (UM bruh). A person standing within the umbra experiences a total solar eclipse. During a total eclipse, the only visible portion of the Sun is a pearly white glow around the edge of the eclipsing Moon.
Surrounding the umbra is a lighter shadow on Earth's surface called th penumbra (puh NUM bruh). Persons standing in the penumbra experience a partial solar eclipse. WARNING: Regardless of where you stand, never look directly at the Sun during an eclipse. The light can permanently damage your eyes.
Eclipses of the Moon
When Earth's shadow falls on the Moon, a lunar eclipse occurs. A lunar eclipse begins when the Moon moves into Earth's penumbra. As the Moon continues to move, it enters Earth's umbra and you see a curved shadow on the Moon's surface. Upon moving completely into Earth's umbra, the Moon goes dark, signaling that a total lunar eclipse has occurred. Sometimes sunlight bent through Earth's atmosphere causes the eclipsed Moon to appear red.
A partial lunar eclipse occurs when only a portion of the Moon moves into Earth's umbra. the remainder of the Moon is in Earth's penumbra and, therefore, receives some direct sumlight. A penumbral lunar eclipse occurs when the Moon is totally within Earth's penumbra. However, it is difficult to tell when a penumbral lunar eclipse occurs because some direct sunlight is falling on the side facing Earth.
A total lunar eclipse can be seen be anyone on the nighttime side of Earth where the Moon is not hidden by clouds. In contrast, only a lucky few people get to witness a total solar eclipse. Only those people in the small region where the Moon's umbra strikes Earth can witness one.
The event just described was a total solar eclipse (ih KLIPS). Today, most people know what causes such eclipses, but without this knowledge, they would have been terrifying events. During a solar eclipse, many animals act as if it is nightime. Cows return to their barns and chickens go to sleep. What causes the day to become night and thhen change back into day?
What causes an eclipse?
The revolution of the Moon causes eclipses. Eclipses occur when Earth or the Moon temporarliy blocks the sunlight from reaching the other. Sometimes, during a new moon, the Moon's shadow falls on Earth and causes a solar eclipse. During a full moon, Earth's shadow can be cast on the Moon, resulting in a lunar eclipse.
An eclipse an occur only when the Sun, the Moon, and Earth are lined up perfectly. Because the Moon's orbit is not in the same plane as Earth's orbit around the Sun, eclipses occur only a few times each year.
Eclipses of the Sun
A solar eclipse occurs when the Moon moves directly between the Sun and Earth and casts its shadow over part of Earth. Depending on where you are on Earth, you may experience a total eclipse or a partial eclipse. The darkest portion of the Moon's shadow is called the umbra (UM bruh). A person standing within the umbra experiences a total solar eclipse. During a total eclipse, the only visible portion of the Sun is a pearly white glow around the edge of the eclipsing Moon.
Surrounding the umbra is a lighter shadow on Earth's surface called th penumbra (puh NUM bruh). Persons standing in the penumbra experience a partial solar eclipse. WARNING: Regardless of where you stand, never look directly at the Sun during an eclipse. The light can permanently damage your eyes.
Eclipses of the Moon
When Earth's shadow falls on the Moon, a lunar eclipse occurs. A lunar eclipse begins when the Moon moves into Earth's penumbra. As the Moon continues to move, it enters Earth's umbra and you see a curved shadow on the Moon's surface. Upon moving completely into Earth's umbra, the Moon goes dark, signaling that a total lunar eclipse has occurred. Sometimes sunlight bent through Earth's atmosphere causes the eclipsed Moon to appear red.
A partial lunar eclipse occurs when only a portion of the Moon moves into Earth's umbra. the remainder of the Moon is in Earth's penumbra and, therefore, receives some direct sumlight. A penumbral lunar eclipse occurs when the Moon is totally within Earth's penumbra. However, it is difficult to tell when a penumbral lunar eclipse occurs because some direct sunlight is falling on the side facing Earth.
A total lunar eclipse can be seen be anyone on the nighttime side of Earth where the Moon is not hidden by clouds. In contrast, only a lucky few people get to witness a total solar eclipse. Only those people in the small region where the Moon's umbra strikes Earth can witness one.