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It is not difficult to think of reasons why it is important for students to understand
that a round globe represents our planet. After all, Earth is our home in space.
Environmentalists remind us that it’s the only home we have, so it’s important that we
take care of it. The spherical Earth concept is essential for understanding geography,
weather, and certain phases of history, such as the great voyages of exploration in the
15th and 16th centuries. Learning about voyages to the Moon or space probes to other
planets makes little sense if students fail to understand that Earth is one planet among
several in the solar system. In other words, the spherical Earth and gravity concept is
an essential building block for many other concepts that we expect students to learn
about the world. To transform the task of teaching about Earth’s shape and gravity from a seemingly intractable challenge into a great opportunity, we turn to the Instructional Planning
Framework described in the early chapters of this book.