Air mass (astronomy)

The air mass (English Airmass, briefly AM) is a relative measure of the length of the path traveled by the light of a celestial body through the atmosphere to the ground or to the observational astronomical observatory in astronomy.

This path influences the scattering and absorption of starlight and its spectral composition.

Geometry

The air mass is defined as the ratio of each path based on the minimum length at normal incidence:

At a zenith angle ζ 0, the light is perpendicular to the earth's surface, and the path through the atmosphere is the shortest. The light passes through an air mass AM = 1

To estimate the angle-dependent air mass is expected with an atmosphere of constant density. For the Earth, it has a layer thickness, the scale height, H = 8.4 km. Then ( R: radius of the Earth ):

For zenith angle ζ <60 ° is considered the approximation:

Due to the longer path through the atmosphere, the light is attenuated by scattering and absorption both as changes in its spectral composition.

Abbreviations:

• AM0: radiation without attenuation by atmosphere
• AM1: vertical incidence on the Earth's surface
• AM1.5: angle of incidence of 48 ° relative to vertical

At 60 ° crosses the light 2 AM at 80 ° almost 6 AM. At 90 °, the path extended geometrically to almost 40 AM.

Astronomy

The air mass is in observational astronomy is a measure of the effect of the zenith angle and allows a quick assessment of the recoverable observation quality. When air masses greater than two is generally observed only rarely; Most large telescopes have safety switches that prevent observations at values ​​above 2.5 and 3.

The attenuation of light ( absorbance ) plays a smaller role than the increasing differential refraction between the blue and red light: near the horizon is a white star on a patch of color, which is blue on top and red on the bottom.

With the air mass and the air turbulence increases significantly - both the brightness fluctuation ( scintillation ) and the focus ( seeing ). Zenith angles in excess of 80 ° (that is, angles of elevation less than 10 ° ) which extends the optical path refraction Astronomical addition, since the light beam is more strongly curved in the atmosphere.

Furthermore, a slight redness of the star light occurs when the air contains a lot of aerosol. On the dust particles, blue light is scattered more strongly than red - the red sunset a similar effect.

Solar Physics

The spectrum of the solar radiation is dependent on the path length of light through the atmosphere, and the corresponding length dimension, corresponding spectra and associated radiation power.

An oblique incidence of the sunlight is an attenuation of the radiation power and a change of the spectrum. For comparative measurements of different spectra and radiation powers were defined. AM = 0 is defined as the range outside the atmosphere ( extraterrestrial spectrum) in space, where the radiation output is 1367 W/m2 ( solar constant ). AM = 1 is the spectrum of vertically incident on the earth's surface solar radiation, ie, the sun must it be exactly in the zenith; the rays then travel the shortest way back to the earth's surface. For AM = 1.5 results in a zenith angle of about 48.2 °. In this range, the global radiation power 1000 W / m²; For this reason, AM = 1.5 was introduced as the default value for the measurement of solar modules. The spectrum AM = 1.5 is set out in the standard IEC 904-3 (1989 ) Part III. Assuming a typical ray of sunshine duration of 1000 hours of sunshine per year, is the mean energy flux density in Germany 115 W / m².

For Berlin is the winter solstice at noon zenith angle of 76 °; and thus applies here AM = 4.13. For the summer solstice and at high sun zenith angle is about 29 °, corresponding to AM = 1.14.

If we define AM on the elevation angle of the sun ( solar elevation with as the angle between the horizontal and at the observation of the solar radiation), we obtain for h = 90 ° - ζ to the air mass