# 6th Grade Astronomy Earth in Space.

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6th Grade Astronomy Earth in Space

Earth in Space How does Earth move in space?
Key concepts Key Terms How does Earth move in space? What causes the cycle of seasons on Earth? Astronomy Revolution Axis Rotation Calendar Solstice Equinox Orbit

How Earth Moves Earth moves through space in two major ways: rotation and revolution. The imaginary line that passes through Earth’s center and the North and South pole is Earth’s axis. The spinning of Earth on its axis is called rotation. Earth’s rotation causes day and night. As the Earth rotates eastward, the sun appears to move westward. It is day on the side of the Earth facing the sun. As Earth continues to the east, the sun appears to set in the west. Sunlight cannot reach the side facing away from the sun, so it is night. It takes Earth 24 hours to rotate once. We call that cycle a day.

How Earth Moves cont… In addition to rotating on its axis, Earth travels around the sun. Revolution is the movement of one object around another. One complete revolution of Earth around the sun is called a year. Earth follows a path , or orbit, as it revolves around the sun. Earth's orbit is a slightly elongated circle, or ellipse.

Speed of rotation You might be surprised to know that a spot on the surface of the Earth is moving at 1675 km/h or 465 meters/second. That’s 1,040 miles/hour. Just think, for every second, you’re moving almost half a kilometer through space, and you don’t even feel it.

Figure it out! Want to do the calculation for yourself?
The Earth’s circumference at the equator is 40,075 km. And the length of time the Earth takes to complete one full turn on its axis is hours. So 40,075/23.93 = 1,675 km.

Does it slow down? The speed of the Earth’s rotation changes as you go North or South away from the equator. Finally, when you reach one of the Earth’s poles, you’re taking a whole day to just turn once in place – that’s not very fast.

Don’t fall off! Because you’re spinning around and around on the Earth, there’s a force that wants to spin you off into space; like when you spin a weight on a string. The force is inertia. But don’t worry, that force isn’t very strong, and it’s totally overwhelmed by the force of gravity holding you down. The force that wants to throw you into space is only 0.3% the force of gravity. In other words, if the Earth wasn’t spinning, you would weigh 0.3% more than you do right now.

Brain Check What is rotation? Which direction does the Earth rotate?
What causes day and night? What force wants to fling you off into space? Is the Earth spinning faster at the equator, or at the poles? How long does it take the Earth to revolve around the sun?

Calendars A calendar is a system of organizing time that defines the beginning, length, and divisions of a year. The ancient Egyptians created one of the first calendars. Egyptian astronomers counted the number of days between each first appearance of the star Sirius in the morning. They found out that there are about 365 days in a year. Dividing the year into smaller parts was also difficult. Early people used the moon cycles to divide the year. The time from a full moon to the next is 29 ½ days. A year of 12 of these adds up to 354 days. The ancient Egyptian calendar had 12 months of 30 days each with an extra 5 days at the end.

Calendars cont… The Romans borrowed the Egyptian calendar.
The Earth orbits the sun in about 365 ¼ days. The Romans adjusted the Egyptian calendar by adding one day every four years. We know this fourth year as “leap year.” During a leap year February is given 29 days instead of 28. Using this system makes sure that events such as summer, occurs on the same date each year. The Roman calendar was off by a little more than 11 minutes a year.

Brain Check Who invented the first known calendar?
What star did they observe? What was the problem with this calendar? How did the Romans adjust the Egyptian calendar?

Seasons on Earth The sun hits the Earth from various angles. The sunlight hits the Earth most directly near the equator. At the poles, sunlight arrives at a steep angle. That is why it is warmer near the equator. If the Earth’s axis were straight up and down relative to its orbit, temperatures would remain constant year-round. There would be no seasons. Earth has seasons because its axis is tilted as it revolves around the sun. It is tilted as a 23.5 degree angle. As it revolves around the sun, the north end is titled toward the sun for part of the year and away from the sun the other part of the year.

Seasons of the Earth cont…
Summer and winter are caused by the Earth’s tilt as it revolves around the sun. The change in season is not caused by changes in the Earth’s distance from the sun. In fact, Earth is farthest away from the sun when it is summer in the Northern Hemisphere.

Earth in June In June, the north end of Earth’s axis is tilted toward the sun. In the Northern Hemisphere the noon sun is high in the sky and there are more hours of daylight than dark. The combination of direct rays and more hours of sunlight heats the surface more in June than any other time of the year. It is summer in the Northern Hemisphere.

Earth in June cont.. At the same time south of the equator, the sun’s energy is spread over a larger area. The sun is low in the sky and the days are shorter than the nights. The combination of less direct rays and fewer hours of sunlight, heats the Earth’s surface less than any other time of the year. It is winter in the Southern Hemisphere.

Earth in December In December, people in the Southern Hemisphere receive the most direct sunlight, so it is summer there. At the same time, the sun’s rays in the Northern Hemisphere are more slanted and there are fewer hours of daylight. So it is winter in the Northern Hemisphere.

Solstices The sun reaches its greatest distance north or south of the equator twice each year. These days are known as a solstice. The day when the sun is farthest north of the equator is the summer solstice in the Northern Hemisphere. It is also the winter solstice in the Southern Hemisphere. This solstice occurs June 21 each year.

Solstices cont… It is the longest day of the year in the Northern Hemisphere and the shortest day in the Southern Hemisphere. On December 21, the sun is farthest south of the equator. This is the winter solstice in the Northern Hemisphere and the summer solstice in the Southern Hemisphere.

Equinoxes Halfway between the solstices, neither hemisphere is tilted toward or away from the sun. This occurs twice a year, when the moon is directly overhead at the equator. Each of these days is known as an equinox, which means “equal night.” During and equinox, day and night are about 12 hours long everywhere on Earth. The vernal (spring) equinox occurs around March 21 and marks the beginning of spring in the Northern Hemisphere. The autumnal equinox occurs around September 22. It marks the beginning of fall.

Check your knowledge! What are the two major motions of Earth as it travels through space? Which motion causes day and night? What causes the seasons? What are solstices? What are equinoxes? How are they related to seasons? How would the seasons be different if the Earth was not tilted on its axis?

Motions of the Moon The changing relative positions of the moon, Earth, and sun causes the phases of the moon, eclipses, and tides. The moon rotates once on its axis in the same amount of time as it revolves around Earth. A day and a year on the moon are the same length. For this reason, the same side of the moon always faces Earth.

Phases of the Moon The different shapes of the moon you see from Earth are called phases. The moon goes through a different set of phases each time it makes a complete revolution around Earth. Half of the moon is always in sunlight. The half of the moon that faces Earth is not always the half that is sunlit. The phase of the moon you see depends on how much of the sunlit side of the moon faces Earth.

Eclipses When the moon’s shadow hits the Earth or Earth’s shadow hits the moon, an eclipse occurs. There are two types of eclipses: solar and lunar. (solar and lunar come from the Latin words for “sun” and “moon.”) A solar eclipse occurs when the moon passes directly between Earth and the sun, blocking sunlight from Earth. This happens during the new moon phase. A solar eclipse occurs when the new moon blocks our view of the sun.

Eclipses The very darkest of the moon’s shadow, the umbra is cone-shaped. From any point in the umbra, light from the sun is completely blocked. Only the people in the umbra experience a total solar eclipse. The larger part of the shadow is the penumbra. In the penumbra, part of the sun is visible from Earth. During a solar eclipse, people in the penumbra see only a partial eclipse.

Lunar Eclipse A lunar eclipse occurs at a full moon when the Earth is directly between the moon and the sun. During a lunar eclipse the Earth blocks the sunlight from reaching the moon. Only during a full moon, because the moon is closest to Earth’s shadow at that time. Like the moon’s shadow during a solar eclipse, Earth’s shadow has an umbra and penumbra. Unlike a total solar eclipse, a total lunar eclipse can be seen anywhere on Earth. A partial lunar eclipse occurs when the moon passes partly into the umbra of Earth’s shadow.

Past Eclipses During the 20th century 375 eclipses have taken or will take place: 228 solar and 147 lunar. The last total eclipse of the sun visible in the U.S. in this century occurred over the state of Hawaii on July 11, The prior such eclipse occurred over the state of Washington on Feb. 26, The next total eclipse will be visible from the U.S. in 2017.

Tides Tides- the rise and fall of ocean water that occurs every 12.5 hours. The water rises for about 6 hours, then falls for about 6 hours. Tides are caused mainly by differences in how much the moon’s gravity pulls on different parts of the Earth. At any one time there are two high tides and two places with low tides on Earth. As Earth rotates, one high tide stays on the side of Earth facing the moon. The second high tide is on the opposites side of Earth. Each location on Earth sweeps through those tides every 25 hours.

Spring Tides The sun’s gravity also pulls on Earth’s waters.
The sun, moon, and Earth are nearly in a line during a new moon. The gravity of the sun and moon pull in the same direction. Their combined forces produce a spring tide. This tide has the greatest difference between high and low. At a full moon the sun and moon are on opposite sides of the Earth. This also causes a spring tide. It doesn’t matter the order in which they line up. Spring tides occur twice a month, at new moon and at full moon.

Neap Tides During the moon’s first quarter and third-quarter phases, the line between Earth and the sun are at right angles to the line between Earth and the moon. The sun’s pull is at right angles to the moons pull. They are pulling in different directions, producing the least difference between low and high tides. Neap tides occur twice a month.

Brain Check Think to yourself, what is the difference between spring and neap tides? On your white board, draw a Venn diagram comparing and contrasting spring and neap tides. Using your white board as a guide, discuss with your neighbor the similarities and differences of spring and neap tides.

Earth’s Moon The Greeks thought the moon was perfectly smooth.
1609- Galileo Galilei made his own telescope by putting two lenses in a wooden tube. He was able to see details of the moon, that had never been seen before. Features of the moon include- maria, craters, and highlands.

The moon’s surface The moon’s surface has dark, flat areas, which GaIileo called maria, the Latin word for “seas.” Galileo thought the maria were oceans. He was wrong. The maria are hardened rock formed from lava flows that occurred between 3-4 billion years ago. The moon has many craters covering the surface. Some craters are hundreds of kilometers across. A long time ago scientist thought volcanoes caused these craters. Today they know that meteoroids hitting the surface caused these holes. Meteoroids are chunks of rock or dusk from space. On Earth our craters have been worn away over time by water, wind and other forces. Since the moon has no liquid water or atmosphere, its surface has changed very little over billions of years.

Moon’s surface cont… Galileo correctly guessed that some of the light colored features of the moon were highlands, or mountains. The peaks of the highlands and the rims of the craters cast dark shadows. The rugged lunar highlands cover much of the moon’s surface.