There are many other interesting optical phenomena connected with sunrise and sunset, including, first of all, the morning and evening twilight. When the sun, or any other heavenly body, is only a little below a clear horizon, it is still visible, on account of the bending of its rays by the atmosphere. This lifting effect, known as _astronomical refraction_, amounts to about half a degree (at the horizon), which is about equivalent to the apparent diameter of the sun or moon. As the sun sinks farther below the horizon, in the evening, the only daylight that comes to us is that reflected from the upper levels of the atmosphere, which are still illuminated. This is called twilight, and it lasts until the sun is about 18 degrees below the horizon, when total darkness sets in. The period as a whole is sometimes called _astronomical twilight_, in distinction from the briefer period known as _civil twilight_, during which there is light enough for outdoor occupations; the latter lasts from sunset until the sun is about 6 degrees below the horizon. Morning twilight is more commonly called “dawn.” An interesting succession of light and color effects is observed before and after sunset and, in inverse position and order, about sunrise. Considering sunset only: After the sun has sunk out of sight, a broad band of golden light, called the _bright segment_, is seen along the western horizon. Above this, in the western sky, appears a more or less circular expanse of rosy glow, known as the _purple light_. In the eastern heavens, after sunset, there rises steadily from the horizon the so-called _dark segment_, which is the blue or ashy shadow of the earth on the sky. This is bordered above by the pink or purplish _antitwilight arch_. As time goes on, the purple light in the west, after increasing in brightness for a while, finally sinks behind the bright segment; while in the east the rising dark segment encroaches upon and finally obliterates the antitwilight arch. Sometimes, in clear weather, there is a fainter repetition of these lights and colors (_second purple light_, etc.).