The Earth is a very special planet in many ways. It's revolution
around the Sun and it's rotation work together to create the
seasons.
Seasons are partly due to the tilt of the Earth. The Earth is
rotating around an axis (called the rotational axis). Some objects
rotate about a horizontal axis, like a rolling log. Some objects, such
as a skater, rotate about a vertical axis. The Earth's axis is tipped
over about 23.5° from vertical.
In space, you may wonder what is vertical? How do we define up and
down? In the solar system, we can think of vertical as straight up
and down with respect to the plane in which the Earth orbits the Sun
(called the ecliptic).
Earth's rotational axis always points in the same direction,
so that the North Pole points towards the star Polaris. Think of the
Earth as a spinning top, tipped over to one side. It remains tipped
in the same direction as it travels around the Sun. This means that
sometimes the norhtern half of the Earth is pointing towards the Sun
(summer), and sometimes it is pointing away.
The Earth's orbit is in the shape of an ellipse, so that sometimes
the Earth is a little bit closer to the Sun than at other times. But
this does not affect the seasons! The Earth's orbit is nearly
perfectly circular, and the difference between it's closest point and
it's furthest point is very small. When it is furthest away, people
in the northern hemisphere of the Earth are having summer!
The source of this material is Windows to the Universe, at http://www.windows.ucar.edu/ at the University Corporation for Atmospheric Research (UCAR). ©1995-1999, 2000 The Regents of the University of Michigan; ©2000-01 University Corporation for Atmospheric Research. All Rights Reserved. Site policies and disclaimer