The earth is FURTHEST FROM THE
SUN when it is Summer in the Northern Hemisphere... how can this
be? And why does Summer in the USA coincide with Winter in
Australia?
The seasons are due to the tilt
of the earth's axis of rotation. In Northern Hemisphere summer
the Northern Hemisphere is tilted toward the sun, so that days
are at their longest. In Northern Hemisphere Winter, it is
tilted away from the sun so days are at their shortest.
Animation
The moon's elliptical orbit has
a focus at a point inside the earth. Because the time the moon
takes to complete an orbit is almost the same as the time it
takes to turn once on its axis, the moon presents approximately
the same face to the earth at any time. Since its orbit is
elliptical, its visible size and precise orientation relative to
the earth change in a complex way in the course of a month.
Planetary motion applet!
Both the earth and moon orbit the
sun while turning around a common center. The sun actually
exerts considerably more gravitational force on the moon than
the earth does!
Eclipses of sun and moon are
relatively rare because the orbit of the moon is not in the same
plane as the orbit of the earth. Only if a new or full moon
occurs where the planes cross can you have a total solar
eclipse, or a total lunar eclipse.
TIDES!
