This lesson will require one 30-minute period.
301-20: observe and explain how the relative positions of Earth, the Moon, and the Sun are responsible for the moon phases, eclipses, and tides.
Students in this lesson will be introduced to the concept of tides and tidal forces, and will be asked to discuss the benefits of tides to life on Earth.
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Open class by asking students to hypothesize as to the cause of the tides; have them discuss how they believe the Moon to be the cause, and ask if the Sun is involved in any way.
Introduce the Tides applet. Explain the forces at work:
For the millions of people in the world living along the ocean, the daily fluctuations of the water level are an important fact of life. Tides occur because of the gravitational attraction between the water in the oceans and the Sun and Moon. The Sun and Moon actually pull at the oceans and cause a tidal bulge (the tidal influence of the Moon is about twice that of the Sun). There are approximately two high tides and two low tides each day, and when it is high tide at one coastal location, it is low tide along a different coast a quarter of the way around the Earth. Because tides occur due to both the Sun and Moon, there are two kinds of tides which depend on the orientation of the Sun and Moon. A spring tide occurs near full or new moon, and causes the greatest tidal differences because the Sun and Moon act together to create one large tidal bulge. A neap tide occurs near a quarter moon when the Sun and Moon are at right angles from each other, causing two smaller tidal bulges. In addition to the effect of the orientation of the Sun and Moon, the distance to the Moon will also affect the tide levels. During perigee, the gravitational pull of the Moon is about 40% greater than if it were at apogee. The worlds greatest tides occur in Canada, in the Bay of Fundy in Nova Scotia. If the Moon is near perigee during a spring tide, the water level at high tide can be as much as 16 metres higher than at low tide at a place called Minas Basin. The location and shape of the shoreline combined with the depth of the water are the key reasons that the Bay experiences such dramatic tidal variations.
Solicit student questions to be sure this concept is understood.
Conduct a discussion regarding the effects of Earth's tides. What would our Earth be like without them? What are their benefits, and how would life on Earth be hampered without them?
No evaluation of this lesson is necessary.