banner

Article Index

Effect of the Atmosphere

AtmosphereLayersAtmosphere profile
Image Wikipedia

The atmosphere is really very thin. Imagine that you have a globe of the Earth with a diameter of 30 cm.
The atmosphere of 480 km is then less than half a millimetre at that scale.

The Earth's atmosphere is a thin layer of gases that surrounds the Earth. It is composed of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide, and trace amounts of other gases. The Earth's atmosphere is about 480 km thick, but most of the atmosphere (about 80%) is within 16 km of the surface of the Earth. There is no exact place where the atmosphere ends; it just gets thinner and thinner, until it merges with inter-planetary space.

This thin gaseous layer insulates the Earth from extreme temperatures; it keeps heat inside the atmosphere and it also blocks the Earth from much of the Sun's incoming high-energy radiation.

 

Absorption and scattering

The nearer a star is to the horizon the fainter the star appears because of atmospheric extinction. This is caused by a combination of absorption (by water and ozone) and scattering.

Rayleigh scattering affects blue light more than red so that when we see an object lower towards the horizon, there is a corresponding reddening of the object. This scattering is also the reason why we see the sky during the day as blue. The blue component of sunlight scatters most, whereas the yellow and red colours of the sunlight are more passing straight through the atmosphere. So we see the sky as blue and interpret sunlight as being predominantly yellowish.

Atmospheric extinction is the reason that the best spot in the sky to observe astronomical objects is the zenith which is directly overhead. Star light travels through less atmosphere at the zenith than in any other direction. Extinction is worst at or near the horizon.

For example, you can look directly at a Sun set because the Sun's light has maximum atmospheric absorption at the horizon. When the Sun is high in the sky it is painful and dangerous to look at the Sun without optical protection.

400px Top of AtmosphereRayleigh scattering causes the atmosphere
to appear blue.
Image: Wikipedia
Refraction
As stellar light passes through the atmosphere, it is refracted (bent) just as through a lens. This bending results from the increase in the atmosphere’s density as the light ray travels downward toward the observer. Thus refraction makes an object appear higher in the sky than it actually is.

 refractionRefraction does make a star appear
higher in the sky than it actually is.
Image: mbmg.mtech.edu

Turbulence
The resolution of a telescope is significantly limited by atmospheric turbulence. This is caused by irregular motion of air in the atmosphere and happens in and around an observatory dome and in the free atmosphere. It depends largely on meteorological conditions.

vt photo 02 tpmVenus in
turbulent sky.
Image: eso.org

We are all familiar with the distortion of a setting Sun or Moon. These bodies average about 30 arc minutes in size. The image to the right illustrates the flattening effect by refraction as well as the reddening closer to the horizon. It also includes the effects of atmospheric turbulence.

 

sun setting w Brown PelicanImage: birdsasart.com

 

 

Because of all these atmospheric limitations, we prefer to observe objects above a minimum altitude, usually 20 or 30 degrees above the horizon. Professional observatories are built at high altitudes above sea level to minimise these atmospheric effects. Better still, we now have many astronomical observatories in space where there are no atmospheric limitations at all. But a lot more technical problems.

distantkecksKeck Observatory, Mauna Kea is at 4,200 m above sea level.
Image: noao.edu (edited)

 

 

 

 

Go to top