SUNSET: 
How large an asteroid could a person jump off?
How large an asteroid could a person jump off? This interesting question goes back to 1952 or even earlier. It turns out that the asteroid must be no bigger than 3.9 v(ρe / ρ) kilometers in diameter, where ρ is the asteroid’s density and ρe is that of Earth in…
Why do Schmidt-Cassegrain telescopes have focal ratios of f/10?
Why do most if not all of the Schmidt-Cassegrain telescope I see advertised have focal ratios of f/10? During the past 20 years or so, amateurs have favored shorter tubes for both refractors and reflectors, even if it meant paying for more costly optics. Portability is the chief motivation -…
What does "error" mean?
In your product reviews, when you state that a telescope drive has a periodic error of, say, 10 arcseconds, do you mean that it has a tracking accuracy of ±10 arcseconds or ±5 arcseconds? All gear systems have inherent mechanical errors that limit the accuracy with which a telescope drive…
Why are there two peaks in the light curve on page 97 of the October 2005 issue?
Why are there two peaks in the light curve on page 97 of the October 2005 issue? One might imagine that a planet of the star would produce only one sharp peak on the rounded dashed line. But why two? This is an insightful question, because astronomers who use the…
What is the most distant human-caused event that’s been easily verifiable with ordinary optics?
Like other amateurs, I observed the brightening of Comet Tempel 1 after it was hit by Deep Impact on July 4, 2005. Was this the most distant human-caused event that’s been easily verifiable with ordinary optics? (I used my 12-inch Dobsonian reflector.) Yes, we think so. The Deep Impact collision…
What percentage of our galaxy’s stars move in retrograde orbits?
Do astronomers have any idea what percentage of our galaxy’s stars move in retrograde orbits? The short answer is yes, a very small percentage. But the long answer is more interesting. First, let’s agree what we mean by “retrograde.” If you were to look down on the solar system from…
Why are the Large and Small Magellanic Clouds not listed among the Caldwell objects?
Why are the Large and Small Magellanic Clouds not listed among the Caldwell objects? They are also not given in Stephen James O’Meara’s book The Messier Objects in the list of 20 spectacular non-Messier objects nor among O’Meara’s 20 spectacular non-Caldwell objects in The Caldwell Objects. Are they too bright?…
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…
How can binary stars orbit each other so fast?
In a News Note (S&T: November 2004, page 16) you described WR 20a, a binary star in Carina with components of 83 and 82 solar masses and an orbital period of 3.686 days. How can two huge balls of gas move so fast? Grab any introductory astronomy textbook and turn…
How do you convert celestial coordinates for equinox 1950.0 to 2000.0?
I use Burnham’s Celestial Handbook, which gives objects’ celestial coordinates for equinox 1950.0. How do I convert these to 2000.0, the current standard? First off, the difference isn’t great. Fifty years of precession change an object’s right ascension and declination by a total of 0.7° at most (if it’s near…
Is there another planet besides Earth whose moon(s) would fit perfectly over the Sun?
It is amazing when you think about the coincidence of the Sun being 400 times bigger than our Moon, yet 400 times farther away, producing an almost perfect fit for a total solar eclipse. Is there another planet whose moon(s) would fit perfectly over the Sun, or are we just…
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…
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…
What date were the two principal stars of the Alpha Centauri system last known to be closest to each other?
On what terrestrial date were the two principal stars of the Alpha Centauri system last known to be closest to each other in space? According to the orbit recently published by Belgian astronomer Dimitri Pourbaix, α1 and α2 Centauri were closest in space (at periastron) in early August 1955, and…
Could the lunar crescent be seen in a telescope at new Moon?
Is the Moon’s orbit inclined sufficiently that, when it misses the Sun by the greatest amount north (or south), the lunar crescent could be seen in a telescope at new Moon? Probably not. The inclination of the Moon’s orbit to the ecliptic varies from 5.0° to 5.3°. French astronomer André…
How is the time of new (or full) Moon defined?
How is the time of new (or full) Moon defined? Astronomically, the Moon is new when it and the Sun have the same celestial longitude. The Moon is at first-quarter phase when its longitude is 90° greater than that of the Sun. The Moon is full when its longitude is…
Do Saturn’s moons cast observable shadows on Saturn?
I’ve seen the tiny black shadows cast by Jupiter’s moons. Do Saturn’s moons cast observable shadows on that planet? Shadow transits of Titan, Saturn’s largest moon, have been seen with a telescope as small as 2 7/8-inch (73-millimeter) aperture. In 1891, two English observers saw the shadow of Rhea on…
How Can Amateurs Find Asteroids?
In a 1996 article titled “Hunting Asteroids,” you said a dedicated amateur could discover an asteroid on almost any night using a CCD-equipped 8-inch telescope. Is that still true today? It was easier for backyard observers to discover asteroids a few years ago. Today, massive professional surveys such as LINEAR,…
Why doesn't a GPS receiver read longitude 0° 00' 00" while standing on the prime meridian at the Greenwich Observatory?
Why didn’t my GPS receiver read longitude 0° 00' 00" when I visited Greenwich Observatory and stood on the prime meridian? The NAVSTAR satellites of the Global Positioning System can provide your location to an accuracy of 3 meters (10 feet) or better. The coordinates you get are based on…
If you combine the magnitudes of all visible stars, how bright a star will you come up with?
Just wondering. . . . If you combine the magnitudes of all visible stars (down to 6th magnitude), how bright a star will you come up with? About 9,000 stars are brighter than magnitude 6.5, the traditional criterion for naked-eye visibility. Their combined magnitude is almost exactly –5, which is…
