The Solar System and all its planets are in constant
motion. Each of the planets revolves around the Sun. The planets also
rotate, or spin, around an internal axis. One manifestation of rotation is
the cycle of night and day. Day after day, month after month, year after
year, the alternation of night and day continues. Sometimes it is sunny
outside, and other times it is dark. In addition to rotation, day and night
occur because the Earth is spherical. When a portion of the Earth faces the
Sun, it is daytime. When the same are rotates away from the Sun, it is
nighttime. The cycle of light and dark is continuous except near the North
and South Pole. During the Northern Hemisphere summer, the North Pole always
faces the Sun, so daytime is continuous for several weeks. At the same time,
the South Pole faces away from the Sun (Southern Hemisphere winter) and is
in continuous night.
The origin of night and day is very difficult for
children to understand. The idea that the dark side is really a shadow of
the planet as it rotates away from the Sunís rays is difficult to imagine.
The rotation of the Earth on its axis, is also not easy to understand,
because we cannot feel this motion.
Rotation and revolution also cause the seasons.
The Earthís axis is tilted 23.5o from vertical. This means that
solar energy strikes the Earth unevenly. It is summer in the Northern
Hemisphere when the North Pole tilts toward the Sun. This puts the Sun more
overhead, so solar energy strikes the Earth most directly. In half of a
revolution the North Pole tilts away from the Sun, making solar energy least
direct, so it is winter.
- Read What makes Day and Night to the class.
All of the students are of course familiar with day and night, but they
may have never wondered why this cycle takes place. This book helps
students understand that the rotation of our Earth on its axis causes
night and day. Review the pictures and models in the book, so that they
can begin understanding night and day. In the Lab, the students will do
some of the experiments discussed in the book.
- To illustrate rotation and revolution, draw the
following pictures on the board. Explain that the Earth rotates on its
axis, but revolves around the Sun at the same time. Explain that night
and day is caused by rotation, and that the seasons are caused by the
tilt of the axis and the revolution around the Sun.