'The illumination of the sky after sunset is due to the reflection of the sun's rays to' the observer from minute tides of cosmic dust, water, air, and Various subtile gases diffused in the upper regions of the atmosphere. Sunrise and sunset are calculated in astronomy when the centre of the sun's disc is on the visible horizon at sea-level, and com. puted that the horizontal refraction is BY. Others, however, calculate it when the upper limb, ar edge of the sun is on ake visible horizon. In laws and regulations, sunset -is calculated in local time.
Refraction, or the bending of the rays of light as they pass through different Jayers of the atmosphere, makes us see the sun when in reality it is below the horizon, The mean diameter of the sun being about 4 a degree, we see its lower wage just touching the horizon, though in reality the whole disc is below it. This phenomenon, is due to the law of wefraetion,
Text read from the printed page and cleaned; old type leaves the occasional error.