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 a 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 lengths.
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 the summer solstice. When the Earth has 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 the solstice, the terminator line is at its greatest angle with respect to the axis of the Earth, which is approximately 23.5 degrees.