It’s a bright night sky, littered with shiny white stars against a pitch black backdrop. As dawn approaches the sky begins to fade into blue and a new sunrise lights up the horizon in bands of orange and red. The lights blink out one by one in a wave across the sky, as the sun breaches the horizon. Now that day has come, where have the stars gone?
The array of bright spots visible in the night sky represents a collection of not just stars but also planets, man-made satellites, deep space objects and, on some occasions, comets and meteors. These range in distance from low earth orbiting satellites like the International Space Station to stars as far as 1,500 light years away. The apparent brightness of these objects is determined by a combination of factors including size, distance, luminosity and whether the object is emitting its own light or reflecting that of the sun. The objects sweep across the sky according to either their own trajectories around Earth or in relation to Earth’s orbit around the sun and its rotation. These objects, then, are always present in the sky, but not necessarily visible depending on where you are on Earth and what time of day it is.
Why, then, do these lights disappear during daylight hours?
The sun, because of its proximity, is simply so much brighter and more pervasive than all the other relatively dim celestial bodies that it overpowers them. As the part of the Earth an observer is on rotates to face the sun, the white light of the sun starts moving in waves through the atmosphere. In the atmosphere, theses light waves will collide with water droplets, dirt, debris and gas molecules. The water and debris is so big, compared to the light waves, that all wavelengths are reflected equally, and the light that results remains white. As light passes through gas molecules, however, the shorter wavelengths of blue light are absorbed and refracted in different directions through the sky, leaving the longer reds and oranges untouched. This effect, known as Rayleigh scattering, is not only the reason the sky looks blue; it also spreads light across the dome of the sky fairly evenly once the sun is high enough above the horizon. This wash of light also means that any object outside the atmosphere is generally not bright enough to be seen by the naked eye. Human eyes are generally just not sharp enough.
This effect can also be seen on any regular night. Compare the night sky of the countryside to that seen in the city. Light pollution in the city from buildings, streetlamps and other sources acts in much the same way the sunlight does but even closer to an observer’s eyes. City lights refract and reflect through the lower atmosphere, spreading a blanket of artificial light across the sky that effectively blots out all other light. As one moves out of the city into a darker suburban area or a rural area, more and more stars will become visible. With the reduction of local light pollution, stars start becoming comparatively brighter.
In the end, it’s all about light and how much there is. Whether it be from the sun or artificial sources, the closer the light, the brighter it’s going to appear in relation to any and all other light sources.
