During the lesson, monitor students’ understanding of the following points, and adjust as necessary, to help ensure students master targeted learning goals within the time frame:

• Assess students’ understanding of Earth’s movements (rotation and revolution) through class discussion and student demonstrations.
• The cycle involving the Earth, moon, and the sun will be assessed through group discussions and a review of the Earth-Moon Worksheet
• The relationship between orbits and seasons will be assessed through a review of the Earth-Sun Worksheet.
• Assess student understanding of the history of the theories that have changed over time for the Earth and moon through group discussion.
• Collect the Earth-Moon Worksheet and the Earth-Sun Worksheet for individual assessment.

Ask the class for a show of hands: “Where do you think the sun appears to rise in the morning? Do you think the location the sun rises differs depending on your location on Earth?”

Point that those who agree have their directions correct, but that the sun does not actually come up or go down. Rather, the Earth rotates so that the sun is in view, or out of view. The common-sense perception that the sun is moving is false, but it is built into our language. Anyway, it would be too clumsy to say, “The Earth has rotated so as to bring the sun into our line of sight in the east.”

Hand out copies of the Astronomer PowerPoint to students (S-8-1-2_Astronomer Powerpoint.pdf).

However, the assumption that the Earth did not move, and that the sun, planets, and stars revolved around it, was the foundation of astronomy from prehistoric times until the late Renaissance. Since we don’t feel any movement, to claim that the Earth did move required considerable proof. Certain Greek and Arab astronomers had suggested the possibility, but it was Copernicus who presented the theory as a complete system in 1543. The invention of the astronomical telescope in 1610 by Galileo showed moons orbiting Jupiter, which could be used as a model of the solar system. Kepler’s accurate mathematical analysis of planetary orbits was based on the assumption that Copernicus was correct. Kepler’s analysis was supported by Sir Isaac Newton’s theories of gravity, and Newton’s equations were used to correctly predict the return of Halley’s Comet and other phenomena. Refer to the link below for an in depth timeline covering the contributions made towards the heliocentric theory.

http://www.astronomyfactbook.com/timelines/heliocentrism.htm

But so what? The system that was current before Copernicus also correctly predicted most celestial movements, with precision that was considered adequate at the time. Called the Ptolemaic (pronounced tall-a-May-ic) system, it assumed that the moon, sun, and planets were embedded in huge invisible spheres that were nested around the stationary Earth. The movements of the spheres were controlled by complex gear arrangements and it was up to astronomers to figure out the gearing in order to predict astronomical events.

A simple demonstration that we are deceived by our senses, and that the Earth does move, did not come until 1851. That year French scientist Leon Foucault (foo-Coe) set up a pendulum with a heavy weight that swung freely from a wire and could continue swinging, slowly, for hours. The results were astonishing but entirely predictable if Copernicus was correct: the route of the pendulum’s swing slowly shifted in a circle throughout the day.

That is because the pendulum was trying to go in a straight line, but the Earth was rotating under it.

To illustrate this point, show a video of a Foucault Pendulum, such as the ones listed below:

• https://www.youtube.com/watch?v=aMxLVDuf4VY (Foucalt’s Pendulum 3:21 sec)
• https://www.youtube.com/watch?v=aGMPvJKC1_U&feature=related (1:21 sec)

Point out that, after centuries of work and analysis, the Copernican system is now universally accepted. Assumptions derived from it have been used to successfully send probes to distant planets. The Ptolemaic system is now considered an historical curiosity—yet there was a time when no one saw any reason to question it.

Meanwhile, while Newton’s laws concerning gravitation are now universally accepted, for centuries they proved inadequate for predicting the position of Mercury. Tiny but consistent discrepancies kept creeping in. Only after astronomers added the affects of Einstein’s theories of relativity—especially the part about mass increasing with speed—did the discrepancies go away.

Review the two motions that the lesson will cover, rotation and revolution, and distinguish the two.

• Rotation: the spinning of a body around its centerline, or axis. As noted, the rotation of the Earth around its axis (the line running from the North Pole to the South Pole) is what causes the sun and other celestial bodies to appear to move in the sky.
• Revolution: the path that a body takes while completing a circular orbit around another body. In nearly all cases the path amounts to an elongated circle called an ellipse. The degree that the elliptical path of an orbit varies from a true circle is called its eccentricity. The eccentricity of the orbits of most planets is actually quite small.

Rotation and revolution are independent of each other. Taken together, however, they set up important daily, monthly, and yearly cycles. The lesson will look into rotation and revolution in greater detail, and then consider those cycles.

To open the discussion on rotation, display a world globe, turning it counter-clockwise as seen from the north, and make the following points:

• The Earth spins to the east. That is why the sun appears first (“rises”) in the east and is last seen (“sets”) in the west. Seen from above the Northern Hemisphere, the rotation is counter-clockwise.
• The Earth spins once with respect to the sun every 24 hours, defining “day.”
• The Earth’s rotational speed at the equator is about a thousand miles per hour (or a little more than 1,670 kilometers per hour.)
• With respect to the sun, the Earth is tilted 23.5 degrees. (The world globe may have this tilt built in.) This causes the North Pole to point toward a spot in the sky that happens to be very close to Polaris, the Pole Star, making Polaris useful for navigation. As will be explained later, this tilt also causes the seasons to change.
• All celestial bodies studied so far show some kind of rotation, presumably because of momentum left over from when they were formed. However, some have a rotation period that is synchronous with their orbital period. The best example is the moon, which always shows the same side to the Earth. Seen from the Earth, it does not appear to be rotating. Seen from far out in space, it clearly is.

Discuss revolution by drawing a circle on the interactive whiteboard representing the Earth’s orbit, drawing it in a counter-clockwise direction. Make the following points about revolution:

• The Earth revolves around the sun in one year, and this revolution defines the year.
• The Earth’s speed around the sun is about 108,000 kilometers per hour, or about 67,000 miles per hour.
• The Earth’s orbital path is technically an ellipse, but is close to a perfect circle. The average distance from the sun is about 150 million kilometers (93 million miles), with a variation of no more than 3 million kilometers (2 million miles) either way. This small variation is NOT what causes the seasons.
• Seen from Polaris, the Earth, all the other planets, and most of their moons orbit in a counter-clockwise direction. The sun also rotates in the same direction. Therefore, the sun and planets are assumed to have formed together from a spinning disc of material.
• Any object that orbits in a clockwise direction is said to be in a retrograde orbit. The main example is Triton, the largest moon of Neptune, which is assumed to have been captured into Neptune’s orbit after the formation of Neptune and its other moons. Some smaller, outer moons of the outer planets also have retrograde orbits and are assumed to be captured asteroids. Halley’s Comet is also retrograde, as are some other comets and a few asteroids.
• There is no “standing still” in space. Once a small body comes under the gravitational influences of a larger body, it moves toward the body and either hits it, sling-shots around it and goes back into space, or goes into orbit around it. So objects typically end up orbiting something. The moon orbits the Earth, the Earth orbits the sun, and the sun orbits the center of the Milky Way galaxy, which is about 26,000 light years away. The sun orbits the galaxy with a period of about 225 million years.

The combination of the Earth (and moon’s) rotation and revolution create daily, monthly, seasonal, and yearly cycles. Next, we’ll look at the earth–moon system and the daily and monthly cycles it generates.

Distribute copies of the Earth–Moon Worksheet (S-8-1-2_Earth-Moon Worksheet.doc) to each student, and note that the combination of the Earth’s rotation and the revolution of the moon around Earth generates the tides and the lunar phases.

Make the following points about tides:

• The rotation of the Earth with respect to the moon and sun creates the ocean tides, as the water rises on the part of the Earth facing the moon. On the side of the Earth opposite the moon, the water also rises somewhat, as the Earth itself is pulled toward the moon leaving the surface water “behind”. Both cases are called high tide.

• See a simulation of tides at http://sunshine.chpc.utah.edu/labs/tides/menu_tide.swf

• At points 90 degrees from the moon, water is being pulled away toward the places with high tide, producing low tide.
• Most (but not all) coasts experience two high tides and two low tides daily, with the two high (or low) tides typically being about 12 hours 25 minutes apart. They would be 12 hours apart except that the moon orbits the Earth in the same direction that the Earth rotates.
• The movement of tidal water produces complex patterns. Therefore, the tidal cycle is not always regular.
• Tidal forces are at their highest during the course of a month when the moon and sun are in line, on the same side or on opposite sides of the Earth. The result is called a spring tide, and the name has nothing to do with the season.
• When the moon and sun are at 90 degrees, tidal forces are at their lowest, and the result is called a neap tide.

Instruct students to do the following with their worksheets:

• Assume that the moon is in the New position. Draw bulges on the Earth showing where the high tides should be.
• Add labels for spring tide and neap tide by the lunar phases where they would occur (by New and Full Moon for spring tide, First Quarter and Third Quarter for neap tide.)

Invite students to recount memories of any recent trips to the beach and what tidal movement they saw.

Continue the discussion and point out that the monthly revolution of the moon around the Earth also produces the lunar phases. Still using the Earth–Moon worksheet, discuss what phase the moon is on that day, and mark its position on the worksheet.

Continue the lesson by making the following points:

• The moon takes about 29.5 days to move through a complete cycle of phases. This cycle is the basis of the calendar month, but calendar months vary in length from 28 to 31 days for reasons that have nothing to do with astronomy. (The reasons include political decisions by Roman emperors.)
• As shown on the worksheet, the moon is always half lit by the sun. Our view of the lit area, however, is constantly shifting as the moon revolves around the sun, producing the phases.
• The same side of the moon always faces the Earth. However, this has nothing to do with lunar phases, and we would see the same phases if the moon rotated with respect to the Earth.
• The shadow of the moon falling on the Earth produces solar eclipses. Since the orbit of the moon is tilted with respect to the equator of the Earth, such eclipses occur two to five times per year, somewhere on Earth, rather than once a month. Less than half are total eclipses, where the moon completely covers the sun.
• The moon and the sun happen to be the same apparent size in the sky, as the sun’s diameter is about 400 times larger than the moon but about 400 times farther away. This means that total eclipses can sometimes exactly cover the face of the sun while leaving its atmosphere visible. Looking directly at it can still cause eye damage.
• The shadow of the moon is small by the time it gets to the Earth. A total eclipse can only be seen from a band on the ground about 230 kilometers (143 miles) wide and is over in seven minutes.
• The shadow of the Earth falling across the moon produces lunar eclipses. There are at least two per year. As with solar eclipses, most are partial rather than total. Since the shadow of the Earth is much bigger than the shadow of the moon, a lunar eclipse can last several hours and be seen by anyone with a view of the moon during the event. This makes lunar eclipses seem more common than solar eclipses. Shadowed parts of the moon turn a deep, dull red during the eclipse as they are lit by light reflected from the Earth.

Using the same Earth–Moon worksheet, students should do the following:

• Using a pencil, shade in the “View from Earth” circle by each lunar position, showing what the lunar phase looks like when the moon is in that position.
• Sketch in the shadow that causes a solar eclipse, from the proper lunar position to the Earth. Label it.
• Sketch in the shadow that causes a lunar eclipse, from the Earth to the proper lunar position. Label it.

Students should then gather in small groups to correct each other’s work.

After students have checked their work, continue the discussion by explaining that the combination of the rotation of the Earth and its revolution around the sun also produces the seasonal cycles. Distribute copies of the Earth–Sun Worksheet (S-8-1-2_Earth-Sun Worksheet.doc). Make these points:

• Again, the rotation and revolution of the Earth are separate motions.
• The time it takes for the Earth to revolve around the sun is 365 and a quarter days. Since the calendar has to be figured in complete days, we use leap days to synchronize the two motions.
• With the axis of the Earth tilted at 23.5 degrees with respect to the sun, the North Pole points toward the same spot in the sky (near the star Polaris) no matter where the Earth is during its orbit around the sun.
• Therefore, at some point during the year the North Pole is tilted 23.5 degrees toward the sun. Six months later it is tilted 23.5 degrees away from the sun.
• When pointed away from the sun, a hemisphere gets less sunlight and the days are shorter, since the sun is 23.5 degrees lower in the sky. This produces cooler weather, resulting in winter.
• At the same time the opposite hemisphere will be pointed toward the sun and will experience warmer conditions, resulting in summer.
• The moment that the tilt is the maximum toward or away from the sun is called the solstice.
• One solstice marks the start of summer in the Northern Hemisphere, typically on June 20 or 21. The other marks the start of winter, typically on December 21 or 22. These are not the hottest or coldest days of the year, since it takes time for the weather to change.
• At the halfway point between the solstices the Earth’s tilt is sideways with respect to the sun, neither toward nor away from it.
• The halfway point between the solstices is called the equinox, when day and night are of equal length.
• One equinox marks the start of spring, typically on March 20 or 21, and is also called the vernal equinox. The other marks the start of fall, typically on September 22 or 23, and is also called the autumnal equinox.

Using the same Earth–Sun worksheet, students should do these two things:

• Label the part of the orbit with the name of the season that occurs during that time (summer for the lower left, fall for the lower right, winter for the upper right, spring for the upper left.)
• Label a typical date beside the two solstices and equinoxes (June 20 or 21 for the summer solstice, December 21 or 22 for the winter solstice, March 20 or 21 for the spring equinox, and September 22 or 23 for the fall equinox.)

Students should then gather in small groups to correct each other’s work.

They should keep both worksheets for reference.

Extension:

• Students can pick out a favorite place on the coast and, using the Internet, research its tidal situation for that day. They should try to find its expected high and low tides. A likely starting point is http://www.saltwatertides.com.
• Students can gather this data for a full month and decide what days represent a spring tide and a neap tide.