How far can the naked eye see?

Star gazing

Also in this week's column:

How far can the naked eye see?

The human eye can see an almost unlimited distance. Looking up into the sky on a very clear night, the Triangulum Galaxy can sometimes be seen. This is a distance of 3.14 million light years. The Andromeda Galaxy is also sometimes visible. This is a distance of 2.5 million light years.

However, it must be pointed out that when we see these galaxies what we actually "see" is something at a somewhat lesser distance. What we "see" is light from the far away object rather than the object itself. What happens is that light coming from the object strikes our optic nerve. The optic nerve signals the brain. The brain then interprets this message and forms an image. The brighter the object, the farther away it can be from us and still be "seen".

The star, Rigel, is many times brighter than another star, Alpha Centauri, yet Alpha Centauri is many times closer.

According to Dr Brent Archinal of the Astrogeology Team of the US Geological Survey in Flagstaff, Arizona writing in the May, 1997 issue of Astronomy, the human eye can see about 2,500 stars in the clearest of night skies. But usually only about 1,500 to 2,000 stars can be seen due to weather, pollution, etc, and fewer still in night skies of cities due to the additional factor of city lights.

Five planets can be seen from earth with the naked eye under the best of circumstances: Mercury, Venus, Mars, Jupiter, and Saturn.


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