Perception Limits

M33, the Triangulum Galaxy

Can telescopes increase an object’s surface brightness?

I normally can’t see the Triangulum Galaxy (M33) with the naked eye, but it’s easy in a rich-field telescope. So how can you claim (June issue, page 128) that a telescope never increases an object’s surface brightness? The explanation has nothing to do with optics and everything to do with human vision. The telescope...

Samples of lunar soil collected by Apollo 17 astronauts
          in 1972 continue to yield new findings; they contain an isotope of beryllium
          that is providing clues to the workings of the Sun's atmosphere. Eugene
          A. Cernan and Harrison H. Schmitt (pictured here) returned to Earth
          with more than 100 kilograms of lunar material.

Could you see astronauts on the Moon?

If there were astronauts on the Moon, could we see them? I get some variant of this question almost every time I show someone his or her first view of the Moon through a telescope, especially at high magnification. The breathtaking sight of lunar craters, mountains, and other features down to the limit of...

The Pillars of Creation in Messier 16, the Eagle Nebula, may be the Hubble Space Telescope''s best-known (and most beautiful) photo.

How can I see more colors through my telescope?

Nebulae and galaxies invariably look like shapeless, colorless blobs in my 6-inch telescope, a far cry from their spectacular appearance in photographs. If I buy a 12- or 14-inch scope, will I see a dramatic improvement? Would that it were so! A larger telescope will better reveal the shapes of nebulae and galaxies, and...

Moon's skyglow

How does the Moon’s phase affect the skyglow of any given location, and how many days before or after a new Moon is a dark site not compromised?

How does the Moon's phase affect the skyglow of any given location? How many days before or after a new Moon is a dark site not compromised? The answer is complicated because the Moon's glow is even more directional than light pollution. Skyglow is several times brighter near the Moon than on the opposite...

Venus 7 days from inferior conjunction, imaged by Damian Peach.

Is it possible to see the crescent of Venus?

Is it possible, with better than normal eyesight, to see the crescent of Venus? That question has been controversial, but in fact some people can. The rough rule of thumb is that someone with excellent vision can just resolve two image elements 60 arcseconds (60") apart. At times, this is enough resolution to make...

Where to find Pegasus

What’s my naked-eye magnitude limit?

How can I find out my naked-eye magnitude limit? Count stars inside the Great Square of Pegasus, which is well placed on November evenings. If you can see only two stars within the square, you are reaching visual magnitude 4.6 and your skies are probably light polluted. If you spot eight stars you are...

FIsh's Mouth or Dementor?

Why aren’t Earth’s night skies more colorful?

Why are Earth's skies so boring? You see pictures of galaxies, nebulae, clusters, etc., but they're always far away. Why couldn't Earth have been in the middle of a colorful nebula or some other non-boring zone of space? Celestial photos show what things would look like if your eyes were as powerful, sensitive, and...