The diminishing of light from astronomical objects due to the earth's atmosphere, in which molecules (air, dust, etc. ) of the atmosphere absorb, reflect, and refract light before it reaches the ground. Extinction becomes a severe problem for astronomers when objects are viewed close to (especially within 20 degrees of) the local horizon. There are various methods that have been developed for astronomers to try and compensate for this extinction, but it is always best to make measurements of astronomical objects when they are as high in the sky as possible (to minimize errors).