Universe Cycle - Earth (6)
Pre Lab 

  • Exploring tides.
  • Comparing the motion of the Sun, Earth, and Moon.
  • axis
  • equator
  • tides
  • tilt
  • revolve
  • rotate

Students learn how tides are created and review Earth’s motion.

Low tide


Tides caused by the Moon. Note that the length of the arrows is proportional to the strength of gravitational force.  

One consequence of motions of the Sun, Moon, and Earth is the creation of the tides. A tide is the alternating rise and fall of the ocean’s surface. Humans have noticed this change since early times. The tides were initially explained through myths and tales involving supernatural forces. Later, early scientists like Poseidonius of Syria and Pytheas of Marseilles saw a connection between the Moon and the changing ocean levels. Aristotle, Pliny and Caesar also noticed that the Sun had an affect on the level. However, the reasons for these connections remained a mystery.

The problem was solved by Sir Isaac Newton in the latter part of the 17th century. He proposed that the tides were caused by the gravitational interaction between the Earth, Sun, and Moon. The basic premise of Newton’s explanation can be understood by first considering just the gravitational attraction and motions of the Earth and the Moon. In this system, at each point on the Earth, there are generally two high and two low tides per day. As shown in the picture on the next page, the gravitational attraction of the Moon causes the Earth’s oceans to bulge upward toward the Moon. The bulge occurs because the part of the ocean closest to the Moon is pulled more; remember that the gravitational attraction between two objects is greater if they are closer together. On the opposite side of the Earth, inertia from the Earth’s rotation on its axis raises a second bulge in the ocean, opposite the first one. To accommodate these two rises in ocean level, ocean level drops between them, as shown in the diagram. These two bulges and low points remain essentially stationary. As the Earth rotates, completing one spin in 24 hours, each point on the Earth’s surface passes under both bulges and low points. We experience the resulting rise and fall of ocean level as the tides.

Spring Tide - Maximum high and low tides occur when the Moon and sun line up. Note: the Moon can also be on the other side of the Earth. 

The tides are actually much more complex than this simple explanation. Other factors in the rise and fall of the tides include: 1) the gravitational attraction of the Sun; 2) the changing distances between the Earth, Moon, and Sun; 3) variations in the Earth’s rotation; and 4) the shape of the Earth’s ocean basins and coastlines.

Of these, the most significant is the influence of the Sun. When the Sun and Moon are aligned, their gravitational attraction on the Earth’s ocean work together to produce extremely high and low tides. These are a called spring tides. When the Moon and Sun are at right angles, their gravitational attraction works at right angles, producing tides of minimum height. These are called neap tides.


Use the following demonstrations to review how the Earth moves in space and what happens during its movements. 

  1. Hold the globe upright and review the parts of the Earth. Ask students to show the locations of the North and South Poles, the equator, and the Earth's axis. Have them point out lines of longitude and latitude.
  2. Ask students what word(s) describe the shape of the Earth. Ask them if the Earth is a perfect sphere. (No: as the Earth rotates, spin causes the polar regions to flatten out and equatorial areas to bulge, making the Earth vaguely egg- or pear-shaped.)
  3. Hold the globe with the axis upright. Have one student shine the flashlight on the globe. Ask them what the light represents (the Sun). Ask the students if the Earth is correctly positioned in space. (No, the Earth's axis is tilted 23 1/2 degrees.) Next, tilt the globe so that the North Pole is inclined toward the "Sun".

    Neap tides - Minimum high and low tides when Sun, Earth, and Moon are at right angles. 


  4. Ask the students what difference this makes to temperatures on the Earth’s surface. (The North Pole and Northern Hemisphere receive more direct light, so are warmer, while the South Pole and Southern Hemisphere receive less, so are cooler.) This is the basic cause of the seasons, which will be covered more in Lab.
  5. Ask the following questions about motions in the Earth/Moon/Sun system. Demonstrate the answers. What revolves around the Sun? (the Earth with its Moon.) What revolves around the Earth? (The Moon). Which bodies rotate? (the Earth, Moon, and Sun)
Here are other, optional general questions: 
  1. Is the Sun a planet? (No, a star.)
  2. Is the Moon a planet? (No, a satellite.)
  3. How long does it take for Earth to make one full rotation? (24 hours.)
  4. How long does it take for Earth to revolve one time around the Sun? (365 1/4 days or 1 year.)
  5. How long does it take the Moon to rotate once around the Earth? (almost 28 days.)
  6. How long does it take for the Moon to revolve around the Earth? (almost 28 days.)
  7. Does the Sun revolve around anything? (Yes - the core of the Milky Way Galaxy.)
  1. Explain tides to the students.



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