Earth’s Rotation • How can we prove that the earth rotates? • We can say that it must rotate since the stars seem to move in a circle over 24 hours and rise and set • However, ancient people thought this showed that the “heavens” were moving around the Earth. So what else can we use to prove earth’s rotation? • A pendulum at the North Pole will appear to trace a circle over 24 hours • Called a Foucault pendulum after the scientist who developed this concept • It is not actually the pendulum moving • The pendulum goes straight back and forth, due to inertia, while the earth rotates beneath it. Link
Precession • As the earth rotates it wobbles, like a top • The North pole makes a circle over 23,000 years, so that it points in different directions over time • Today it points at Polaris, but in 11,000 years Vega will be out North Star • This precession of the earth’s axis is due to the moon’s pull on the earth
Earth’s Revolution • The earth also revolves around the sun • How do know this? • For one thing we can see changing stars over the year. Link • The combined earth rotation and revolution around the sun cause changes nightly and yearly in the position of constellations Classzone hyperlink • Of course, the stars could be going around the earth instead – this was the belief of earlier people • This has been disproved over time. One piece of evidence is stellar parallax.
Stellar Parallax • Briefly, stellar parallax occurs as the earth goes around the sun and we see stars at different positions against the farther stars. • This was impossible for earlier astronomers to see due to the great distances to the stars. We will discuss this more in another chapter. • New technology makes it possible to measure parallax today.
Earth’s orbit • The earth’s orbit around the sun is an ellipse. • At the point when the earth is closest to the sun, it is at perihelion. • Aphelion, on the other hand, is the point in a planet’s orbit when it is farthest from the sun. The earth’s orbit is not this elliptical. It is much closer to a circle than this sketch.
NOT THE REASON FOR SEASONS • Many people mistakenly think that we have summer because that is when we are closest to the sun. • This is not true! How is this possible if Australia has summer when we have winter? • Besides, the earth is actually at perihelion in January. • So, what does cause the seasons?
Earth’s Ecliptic and Tilt • Since the planets condensed from the solar nebula, which flattened as it spun, they mostly lie in a flat plane • This plane of the planets is called the ecliptic • The earth orbits the sun in the plane of the ecliptic • But the earth is tilted from the ecliptic by 23.5 degrees – probably due to the moon-forming collision.
Earth’s Tilt and Seasons • The earth’s axis is tilted from the ecliptic by 23.5 degrees, at the present time. • This tilt varies slightly over time. • The axis tilt is important because it determines our change in seasons. • If we didn’t have a tilted axis, every day would be the same – there would be no seasons. Demonstration. • We also have varied hours of sunlight in each season Link • Classzone animation
Solstices and Equinoxes • There are four times in earth’s orbit which have special meaning for us. • These are the winter and summer solstices and the spring and fall equinoxes • The earth tilts most directly towards the sun on the summer solstice in June, giving us in the Northern Hemisphere most sunlight • And tilts away from the sun on the winter solstice in December • Hyperlink to classzone.com • (Short class demo with flashlights changing angle and intensity of light)
The Skydome • The sun’s path across the sky varies during the year at different latitudes, due to the tilt of the earth and the earth’s path around the sun • This is a sketch of a skydome, which shows the sun’s path • On which day of the year would this be the apparent path of the sun for an observer near our location? (Let us try the solar motion modeler to figure these out. Then we will do a worksheet.)
Lunar Phases – let’s review them. Video Link
Phases again lunar phases video.classzone Can you identify each of these moon phases?
Perigee and Apogee • An object orbits Earth in an elliptical path, so it gets closer and farther from the earth. The point at which it is closest to the earth is called perigee. The farthest point in its orbit is called apogee. The moon, as a natural satellite follows the same orbit
Eclipses • The moon has no light of its own, so the light you see is a reflection of sunlight. • The moon, earth and sun are not exactly in the same plane. The moon is sometimes above the plane or below it. Link • But, when the moon, earth and sun are in the same plane, we get eclipses.
Solar eclipses • Solar eclipses occur when the moon blocks light from the sun causing a shadow on the earth. • During total eclipses the whole sun is blocked and we can see the sun’s corona. Link • Partial eclipses show part of the sun. Link • Annular eclipses show a “ring” of light around the moon’s shadow. This hybrid picture combines total and annular eclipse pictures. Which is which? • Classzone hyperlink
Lunar eclipses • Lunar eclipses occur when the earth blocks light from getting to the moon and the moon is in the earth’s shadow. • Total lunar eclipses are more common than total solar eclipses because the earth’s shadow is much larger than the moon’s shadow. • Partial eclipses also occur.
Lunar eclipse • The moon does not get totally black during a total lunar eclipse however, because light from the sun is refracted through the earth’s atmosphere, and shines on the moon, giving it a red color. The color varies for each eclipse, due to pollution, etc. • Eclipse video • Classzone eclipse
Tides • Tides are caused by the gravitational pull of the moon and sun. • The moon is closer, and effects tides more. • Notice that there are high tides on the side nearest the moon (direct) and another on the opposite (opposite) side. Why is there an opposite tide? Your hypothesis?
Opposite tide, etc. • The opposite tide is caused partly by the moon pulling the whole earth away from the water on the far side of the earth - like when you pull on something and the loose part behind it is left behind. (Picture pulling someone towards you who is wearing a cape. What does the cape do?) • Also, the part of the earth furthest from the moon is less attracted by lunar gravity (inverse square law) • The opposite tide is not quite as high as the direct high tide. • Locations between high tides have low tides.
Spring and Neap tides • Since the sun also pulls on us, high and low tides vary in their height, depending on the relative positions of the earth, moon and sun. • When sun and moon pull the same direction, we have “spring” tides • When sun and moon pull in opposite directions (at 90 degrees) we have “neap” tides. (Pictures follow.)
Spring and Neap tides 2 Notice that spring tides have higher high tides and lower low tides. Tides are more extreme. Neap tides, however, are less extreme. Which lunar phase causes which tides? Do you think that new and full phase tides will have the same height?
Monthly change in tides, spring to neap. On the graph here, which days show spring and which show neap tides? How do you know which is which? When is the full moon? New moon? Quarter moons?
Tides and location • Tides also change from place to place, due to differences in coastlines. • At the end of the presentation we may do an activity based on this difference.
Extreme Tides • A couple of years ago I was up in Nova Scotia, location of some of the highest tides in the world. Here are some pictures and 2 videos of a place called “Reversing Falls” in New Brunswick, where the tide coming in actually causes the river to flow up stream for a while during the day.
Pictures Low tide Low Tide High tide St John’s Reversing Falls link
Extreme Tides 2 • The tides are so high in Nova Scotia that they use them to generate electric power at the Tidal Generating Plant shown here
Credits • Tarbuck and Lutgens Earth Science pictures • Spaulding and Namowitz Earth Science pictures and animations • NOAA – tidal graphs • Starkins - pictures • Movies: • Origins • McMillan Chaisson Astronomy