|MadSci Network: Astronomy|
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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