Perception Limits

Can 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...

Differences between nebula filters and light-pollution filters

What’s the difference between nebula filters and light-pollution filters? Nebula filters block out most of the visible spectrum while transmitting the handful of finely tuned colors that emanate from the most luminous constituents of cosmic gas clouds: hydrogen atoms and oxygen ions. Light-pollution filters, by contrast, are designed to block the colors emitted by...

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...

Is it possible to detect Jupiter’s satellites with the unaided eye if Callisto and Ganymede appear when Ganymede is at greatest elongation from Jupiter?

I’ve heard it might be possible to detect Jupiter’s satellites with the unaided eye if Callisto and Ganymede appear together when Ganymede is at greatest elongation from Jupiter. Will this happen anytime soon? Jupiter is now setting soon after sunset. But three times in 2008, Texans (and North Americans generally) will have a chance...

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...

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...

Telescope never increases 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...

What happened to the “spokes” in Saturn’s rings?

In 1979 the Voyager spacecraft revealed “spokes” in Saturn’s rings. Yet recent images from Cassini have shown no spokes. What happened to them? Those spokes have kept planetary scientists scratching their heads for decades. It’s thought that the spokes are radial fingers of dark, extremely fine dust particles that become electrostatically charged by pulses...

What is the faintest object imaged by ground-based telescopes?

I know that the Hubble Ultra Deep Field imaging campaign reached a limiting magnitude of 31, but what is the faintest object imaged by ground-based telescopes? Furthermore, how is it that an amateur astronomer was able to reach magnitude 24 with a 16-inch telescope, when even Hubble has gone no deeper than 31st magnitude?...

Why are there no green stars?

There are red stars, orange stars, yellow stars, and blue stars. Why no green stars? A star (or other hot object) emits light across the entire visible spectrum, but the wavelength at which it shines brightest depends on its temperature. Hotter stars emit proportionately more blue light than red; for cooler ones it's the...