SUNSET: 
How can a telescope have an f/ratio of f/42?
I was amazed at Jim Melka’s beautiful picture of Mars on page 136 of the January 2006 issue but puzzled by the caption, which said that he used a 12-inch reflector at f/42. How is this possible? Knowing that a telescope’s f/ratio is its focal length divided by its aperture,…
Will all eight planets ever line up on the same side of the Sun?
Will there ever be a moment when all eight major planets are in a straight line on the same side of the Sun? Jean Meeus addresses this in Mathematical Astronomy Morsels (Willmann-Bell, 1997). He points out that you have to start by defining the question precisely. Let’s reduce the problem…
Is it true that Andromeda Galaxy is moving toward us?
Is it true that the Andromeda Galaxy is blueshifted and moving toward us? How can this happen in an expanding universe? The Andromeda Galaxy (M31) is indeed approaching us, by about 300 kilometers (190 miles) per second measured with respect to the Sun. If you subtract the Sun’s orbital motion…
Why do the best roof-prism binoculars need a phase-correction coating?
Why do the best roof-prism binoculars need a phase-correction coating? How does it work, and why is it not used in Porro-prism binoculars? A roof prism splits the light cone from an objective lens into two separate pathways, involving different internal reflections, before recombining them into an erect image at…
Why do photos of lunar or Martian craters sometimes appear inside-out?
Frequently when looking at photos of lunar or Martian landscapes, I see craters as blisters. This can usually be rectified by turning the picture upside down; then craters reveal themselves as indentations rather than bulges. Do I need spectacles? No; this common illusion is all in the brain. Whether you…
Can Venus ever be far enough from the Sun to appear east of the meridian?
During the Northern Hemisphere winter solstice, when the Sun is farthest south and sets in the southwest, can Venus ever be far enough from the Sun to appear east of the meridian? Yes, but only when viewed from near-arctic latitudes. In late 2008, for example, Venus is the Evening Star…
Why are Kuiper Belt discoveries made at aphelion?
Why is it that so many of the recent Kuiper Belt discoveries — including the “10th planet,” 2003 UB313 — have been made while the objects are at aphelion? Timing and luck. Objects that orbit the Sun spend most of their time near the farthest point of their orbits, where…
How many stars are in the Milky Way?
What is the best educated guess for the number of stars in the Milky Way? I’ve seen figures ranging from “some 20 billion” to “just under 600 billion.” The first seems too low, whereas the latter too high. The short answer is, nobody knows. But your reasoning is sound: 20…
How many digits are satisfactory in the measurement of pi?
In the 3rd century BC, Archimedes proved that the ratio of a circle’s circumference to its diameter is less than 3 1/7 but larger than 3 10/71. That’s about 3.141. Later mathematicians have computed what we now call p (pi) to greater and greater accuracy — but how many digits…
What does "lignes" mean for aperture of antique telescope?
Paging through a reproduction of the 1909 Sears, Roebuck and Co. catalog, I found a telescope with an aperture of 25 lignes. Are you familiar with that term? In short, no. We’re also surprised to learn that such an archaic measurement was attached to a “modern” telescope. A ligne is…
When will (or did) the galactic equator cross the ecliptic very close to the latter's northern and southern extremes?
As a teenager, when examining an equinox-1950 star atlas, I noticed that the galactic equator crossed the ecliptic very close to the latter’s northern and southern extremes (that is, the solstices at right ascension 6h and 18h). On equinox-2000 charts they are even closer. I’d love to know when the…
How well defined is a meteor-shower radiant?
How well defined is a meteor-shower radiant? Is it a point on the sky or a several-degree-wide spot? Radiants are spots, not points. A meteor shower’s radiant is the location on the sky where all the meteors would appear to come from if we could see them approaching in the…
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…
What does “true color” mean in a deep-space photograph?
What does “true color” mean in a deep-space photograph? How would a galaxy or nebula actually look to the naked eye from close up? “True color” is how an object would look if you were nearby or were viewing it in a telescope under a dark sky, and if its…
Is there a good test for optical quality?
In late October 2005 I hoped to see major surface markings on Mars, but my 10-inch scope showed only a uniformly yellow ball. Do I have an inferior mirror? The optics were well collimated. Is there a good test for optical quality? Starting in mid-October, dust storms on Mars spread…
Can an OIII nebula filter be called “oh-three"?
I have an ongoing dispute with everybody. I say an OIII nebula filter cannot be “oh-three,” since OIII stands for doubly ionized oxygen atoms. I call it an “oh-two” filter. Who’s right? Sorry Philip, you lose. Not only is “oh-three” the universal usage; it makes sense. Well, sort of. A…
When will the last total solar eclipse occur?
With the Moon slowly receding from Earth, there will come a time when it no longer has sufficient angular size to cover the Sun completely. When will the last total solar eclipse occur? When the Moon’s at its closest and biggest and it can no longer cover the Sun at…
Why do you need a hydrogen-alpha filter to see solar prominences?
If I can see solar prominences with the naked eye during a total eclipse, why do I need a hydrogen-alpha filter to see them at other times? During a total eclipse the Moon completely blocks the Sun’s photosphere, or visible “surface,” allowing prominences (and the corona) to shine in all…
If asteroid 99942 Apophis ever strikes Earth, how big would the crater be?
If the near-Earth asteroid 99942 Apophis ever strikes Earth, how big would the crater be? Apophis (formerly 2004 MN4) ranks at the top of the worrisome-asteroid list. On April 13, 2029 (yes, that will be a Friday the 13th), it will pass only 30,000 kilometers (18,600 miles) from Earth’s surface…
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 -…
