Tidal Cycles: Interactive Model

By Laura Hospitál on Feb 10, 2016

I found that my students were struggling to understand why the tides are later (50 min) each day. As I contemplated how to make this content more clear, this simple activity came to me.  It seemed to help students grasp the effect on tidal cycles of the rotation of the Earth and the Moon.  Students themselves "play" the roles of the rotating Earth and Moon.

I would recommend this activity be completed before reading about the tidal cycles as the content will be more comprehensible after the activity.  It may be repeated after more formal instruction on the content.

Related Vocabulary:

• tidal cycles - The periodic changes in the intensity of tides caused primarily by the varying relations between the Earth, Moon, and Sun.
• tide - the periodic rise and fall of the water level in the oceans and the other large bodies of water
• high tide - water level reaches its highest level
• low tide - water level reaches its lowest level

Preparation:

1. Place a rectangular mat that is tall enough for students to feel tactually on the ground with an object large enough to be felt tactually with the foot on one end. I improvised and used a folded towel and a large magnet. (See picture.)
2. Look up when high tide and low tide occurs on the day on which you are doing this activity at a location near you or familiar to the students for use in the activity.

Materials

• Rectangular mat (kitchen or bathroom) - tall enough for student to feel the edges tactually
• Large rock or other object to indicate the initial position of the location on Earth facing the Moon. (See procedure.)

Procedure

I.  Begin by introducing the activity - Why do the tides occur later each day?  Discuss.

II.  Starting position of the Earth and the Moon

• Tell the students that they will be "playing" the roles of the Moon and the Earth.
• Have the Earth stand on the mat and face the object.
• Choose a location the students know which is close by and which experiences tides.
• Explain that this is the location on Earth  facing the Moon as we begin this activity. For instance, "It is high tide at Galveston Bay,TX at 9:56am this morning."  The mat and the rock will tactually represent this initial position so that when the Earth rotates once and is again facing this direction, a day will have elapsed.
• Have the students locate the  object with their feet and desribe in their own words what this position indicates (to verify that they understand).
• Ask, Why does high tide occur when the Moon and Earth are lined up?  The Moon's gravitational pull - The effect of the gravity of the Moon on the oceans causing high tide.  Say (using a location near you), "So when the Earth and Moon were in this position at 9:56 am, Galveston Bay was experiencing high tide."

III.  Rotation of the Earth and Moon

• Have the Earth begin rotating counterclockwise while the Moon slowly rotates and revolves only a small portion of the orbit around the Earth.  The Moon will also rotate counter clockwise and revolve counterclockwise.  Since it requires 27 days for the Moon to revolve around the Sun, the Moon will only move slightly, one or two steps to the right.  The student should be directed to move enough so the it is clear tactually that he/she is not longer standing right in front of the rock (symbolizing the initial direction that the location on Earth was facing).
• Have both Earth and Moon stop rotating when the Earth reaches the initial position.
• Ask, "How much time has elapsed?" 24 hours
• Ask "How do you know?" The Earth has rotated once on its axis.
• Ask "Is Galveston Bay experiencing high tide again?" No, because the Moon is not lined up with the Earth.  It has moved sligtly farther in its revolution.  Discuss.
• Repeat the rotation of Earth and slow revolution and rotation of the Moon to model another day.  Discuss.
• Ask  "Why will the tides be 50 minutes later each day?"  Because a place on Earth takes 24 hours and 50 min. to rotate to face the Moon again. If necessary, repeat again, taking questions as you proceed.

IV.  Closure:

Students will individually write a description of why the tides are 50 minutes later each day.  This can be used as an assessment measure of their understanding.  For some groups, this could be done as a discussion or students could work in groups of two with one scribing.

Variations

Rather than the teacher looking up the high and low tides, if time allows, students can be asked to do so at the beginning of class.

NGSS Standards:

MIddle School - Space Systems

ESS1.A: The Universe and Its Stars
Patterns of the apparent motion of the sun, the moon, and stars in the sky can be observed, described, predicted, and explained with models. (MS-ESS1-1)