Monday, June 27, 2022

How many stars are visible to the naked eye?

It is a question that probably everyone has asked himself at least once in his life and whose answer theoretically seems simple: just take the number of stars brighter than the sixth magnitude (the limit of visibility to the naked eye) and divide it by two (the number of hemispheres).

If you look at the catalogues, you'll find there are about 5,000 stars brighter than the sixth magnitude, so of a hemisphere, you could see half of them, 2,500.

In fact, the answer to this question is much more complex than that, since we have to take into account several other factors, including the place from which we are observing, air transparency and light pollution.

The numbers found above are in fact related to an observer who sees a horizon without obstacles (such as mountains in the distance).

The number of visible stars then varies slightly depending on the location from which we are observing. In the mountains, for instance, the transparency of the atmosphere will be greater and will enable us to see weaker stars than those seen from sea level. 

We must also consider the direction in which we are looking. At zenith you can see stars up to the sixth magnitude, while moving towards the horizon the limit magnitude changes. The light of the stars must in fact cross a higher atmospheric layer: for example, it is not possible to see beyond the fifth magnitude.

The final factor to consider is light pollution, which is the most limiting of any considered to date. Light pollution is highly variable from place to place, for example from the city center only the brightest stars are visible. 

Therefore, given all the considerations made so far it is almost impossible to give a precise number of stars visible to the naked eye without knowing the position of the observer. The number varies from the few stars visible from the city centre to the thousands of stars visible beneath a black sky in perfect condition.

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