MadSci Network: Astronomy

Re: Please explain the extent of light pollution's interference of the viewing

Date: Tue Mar 9 07:58:45 1999
Posted By: Michael Martin-Smith, Other (pls. specify below), Family Physician, Fellow,BIS, amateur astronomer( BAA), British Interplanetary Society
Area of science: Astronomy
ID: 920860934.As

On a clear moonless night, the usual maximum magnitude given as 
accessible to the naked eye is about 6.0. This means that under optimum 
conditions it is for example possible to see the asteroid Vesta and the 
planet Uranus with the naked eye, if you now just where to look.
 The MIlky Way, and about 3,000 individual stars are visible, as is the 
M31 Andromeda Galaxy( mag 4.5) Such ideal conditions are obtainable in 
for example Greek Islands such as Crete, in places, or the countryside 
at 40- 50 miles from a city
 In a city, magnitude 2 stars  are sometimes the best achievable, but in 
parks or open spaces in cities one can get down occasionally to 
magnitude 4. I live in HULL, a city of 300,000 population in the North 
of England. On one or two nights per year, I can see the Milky Way- 
usually on very clear August or April nights, my best magnitude ever 
being 4.9( some stars in Lyra constellation, for estimates).
 An interesting exercise recently promoted by the BBC "Tomorrow's World" 
Programme asked people up and down the UK to look at the sky one hour  
after sunset through a spent roll of toilet paper.  A field of 9 stars 
is considered top class. Most saw considerably fewer!
 Stellar magnitudes range from -1.46(  Sirius ( the brightest) down to 
30 - infinitesimally faint, as seen by the Hubble Space Telecsope at 10 
days exposure!!! The scale is logarithmic, such thta each increase in 
magnitude by a factor of 1- eg mag 1 to mag 2 - corresponds to a fall in 
brightness by a factor of 2.5. Thus the range magnitude 1 to 6 equals a 
fall off in brightness of 100 times( 2.5 to the power 5)
 Thus , to really escape the problem of light pollution, you need either 
a park or field 10-20 miles beyond the city limits, or a lakeside 
vacation - but barring that, an inner city park will make an appreciable 
difference as you look nearer to the zenith; the horizon of course 
remains difficult without a clear skyline and a dark sky! 

Michael Martin-Smith

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