Interactions in the Sun-Earth-Moon System - Discovery Education

Interactions in the Sun-Earth-Moon SystemJoey and his sister are on vacation in Turkey. Theyare very excited because on this sunny day theywill get to see a total eclipse of the sun. Theirfriends back home will not be able to see any of it.Joey has made special glasses for everyone to useto safely observe the eclipse.He explains to his sister, “In just about an hour, the moon will pass in front of the sunand completely block it out! It will get very dark, like at sunset, and then we’ll seesomething really special!”His sister looks up into the sky and is very puzzled. She can see the sun, but the moonis nowhere in sight.How can an eclipse happen if the moon doesn’t seem to be there?Why will they have to wear these special glasses if it will be dark during the eclipse?What is the “something special” that they will see during the eclipse?How can it be that they will be able to see the eclipse, but none of their friends backhome in the United States can see it?Background InformationThe sun is the center of the solar system. All of the planets and planetary objects orbit aroundit. It takes the Earth 365 days or one year to orbit around the sun. While the Earth is orbitingaround the sun it is spinning on its tilted axis. It takes 24 hours or one day for the Earth tocomplete one rotation on its axis.The moon is a satellite of the Earth and takes 28 days to orbit the Earth. It is much smaller thanthe Earth and is the closest object in space to the Earth. The moon does not produce its ownlight. It reflects light from the sun. The phases of the moon are dependent on where the moonis in its orbit around the Earth. When the moon appears to become fuller as the days go by thenthe moon is a waxing moon. As days go by and the moon appears to become a crescent shapeit is called a waning moon. The Lunar cycle ranges from the new moon where a moon is notvisible in the sky to a full moon where the moon seems fully illuminated. The same side of themoon always faces the Earth and people are not able to see the other side of the moon fromEarth.Discovery Education 2006. All Rights Reserved. page 1

A solar eclipse occurs when the moon passes between the Earth and the sun. This is alwaysduring a new moon. The moon can’t be seen because its lighted side is facing away from theEarth, toward the sun it is about to pass in front of. Due to the distance and shape of the moonthere are times when it completely covers the sun during an eclipse. Because of the timing ofthe Earth’s rotation and the lunar revolution, eclipses are seen at different points on the Earth’ssurface. The size of the moon’s umbral shadow (where the sun appears to be completelyblocked) is relatively small about 100 miles wide. In addition, the moon’s orbit is not exactly inline with the Earth’s. This means that in many months no eclipse is visible anywhere on Earth.All this means that viewing a solar eclipse is a rare thing. During a total solar eclipse, the moonblocks out all parts of the sun except its glowing atmosphere. This is known as the corona.When this happens, people describe it as a ring of fire around the moon with “diamond sparkles”where the sun’s light peeks through the mountain gaps on the edge of the moon.When observing a solar eclipse it is important to protect the eyes. On a normal day it is toodifficult to look directly at the sun because it hurts and damages the retinas of the eyes. Duringthe total eclipse, people are tempted to look at the sun because the moon is blocking most ofthe light. The problem is in knowing when the moon’s edge will slip past the sun and theharmful light will return. Doctors recommend using special smoked lens glasses – sunglassesdo not provide nearly enough protection. Another safe way to view the eclipse is with a pin-holecard. With the sun at your back, hold the card out and focus the light of the sun through the pinholeonto another surface. The entire eclipse can be viewed this way and it’s easier to takepictures of the image.Of course, the Earth can come between the sun and the moon as well. This is called a lunareclipse. These always occur during a full moon, when the moon is on the opposite side of Earthfrom the sun and all of the visible lunar surface is reflecting light back to the Earth. Earth’sshadow passes completely across the whole moon (total lunar eclipse) or, more often, passesacross part of the moon (partial lunar eclipse). Again, the difference in the angle of the orbits ofthe moon and the Earth determine when a lunar eclipse will be full or partial. In a total lunareclipse, the moon turns a dull orange-redThe moon’s and sun’s gravity also has an influence on the Earth’s tides. The moon has astronger tidal force and is responsible for low and high tides in the ocean depending on wherethe moon is in the sky. The moon, because it is closer, has a greater effect on the tides, eventhough the sun is much larger. Gravitational force decreases exponentially with distance. Still,when both the sun and the moon are on the same side of Earth (during a new moon phase) thegravitational force is strongest and the tide changes are much greater.Finally, the tilt of the Earth’s axis is responsible for seasons. This is a very difficult concept forstudents to grasp and requires a lot of explanation and exploration. Simply put, the Earth’s axispoints in the same direction, toward the North Star, as it revolves around the sun. This meansDiscovery Education 2006. All Rights Reserved. page 2

that during one part of it’s orbit, the northern hemisphere is more exposed to the sun (beingcloser does not have a noticeable effect). This hemisphere receives more direct sunlight, andwarms up accordingly. At the same time, the southern hemisphere gets less sunlight (it’s polegets none for a while). So the southern hemisphere remains cooler. The Earth progresses inits orbit to the opposite side of the sun, with Earths’ axis still pointing in the same direction. Nowthe southern hemisphere receives more direct sunlight and warms up while the northernhemisphere is getting less sunlight and cools down.Inquiry Problem SolutionWhile Joey’s sister cannot see the moon, it is there near the sun in the sky. The moon does notreflect light to the Earth during a solar eclipse because the sun is further away, on the other sideof the moon. Only the moon’s dark side is facing the Earth just before it crosses the sun.When observing a solar eclipse it is important to protect your eyes. On a normal day it is toodifficult to look directly at the sun as it hurts and damages the eyes. It seems much easier tolook at the sun during a solar eclipse because the moon is blocking most of the light. Butpeople should always where protective eye ware when looking at eclipses. Another way tosafely look at an eclipse is with a pin-hole camera. The image of the sun is projected ontoanother surface and is not as bright, but just as clear.There is also “something special” that they will see. During a total solar eclipse, the moonblocks out all parts of the sun except its glowing atmosphere. This is known as the sun’scorona. When this happens, people describe it as a ring of fire around the moon with “diamondsparkles” where the corona’s light peeks between the mountains on the edge of the moon.Solar eclipses are very rare. It is not unusual to wait for twenty or thirty years to see one. Eventhough they occur at least once a year, they can only be seen from a small portion of the Earth’ssurface, usually a path about 100 miles wide and a thousand miles long, about the area of a thinstrip of paper placed on a globe. In addition, Turkey is on the other side of the planet from us,so people in the United States would not likely be seeing the sun at that time – it would be night.Discovery Education 2006. All Rights Reserved. page 3