The line that separates day and night is called the terminator.
It is also referred to as the "grey line" and the "twilight zone."
It is a fuzzy line due to our atmosphere bending sunlight. In fact,
the atmosphere bends sunlight by half a degree, which is about 37
miles (60 km). It is commonly thought that while half of the Earth
is covered in darkness, the other half is covered in sunlight. This
is actually not true because of the bending of the sunlight results
in the land covered by sunlight having greater area than the land
covered by darkness.
The shape of the terminator curve changes with the seasons. This
difference is especially noticeable when the terminator curve from an
equinox is compared to the terminator curve from a solstice. There are three different datasets that show the terminator during 2007. In 2007, the spring equinox was
March 21, the fall equinox was September 23, the summer solstice was
June 21, and the winter solstice was December 22. During the equinox,
the sun can be observed directly over the equator. This means that day
and night are approximately the same length. The equinox is also
thought of as the start of spring and fall. Because at equinox there is
no tilt of the Earth with respect to the sun, the terminator line is
parallel to the axis of the Earth and to lines of longitude. The
solstice occurs when the Earth's axis tilts most toward or away from the
sun, causing the sun to be further north or south of the equator than
any other time. The shortest day of the year is winter solstice and the
longest day is summer solstice. When the Earth is tilted away from the
sun, the sun appears south of the equator and when the Earth is titled
toward the sun, the sun appears north of the equator. During solstice,
the terminator line is at its greatest angle with respect to the axis of
the Earth, which is approximately 23.5 degrees.
This dataset runs from July 22 - 28, 2009. During July, the diurnal cycle is especially apparent over the central United States, where heating through the day causes thunderstorms to form and strengthen and cooling through the evening weakens the storms.