SUNSET: 
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…
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…
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…
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?…
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…
BASIC Programs from Sky & Telescope
Examples of celestial computation from the dawn of the home-computer age.
Did that earthquake off the coast of Sumatra alter Earth's rotation and tilt?
Did the earthquake off the coast of Sumatra that triggered the horrific tsunami on December 26, 2004, alter Earth’s rotation and tilt, as some news stories suggest? The Sumatra earthquake released as much energy as 475 megatons of TNT. That’s some 23,000 times the energy released by the atomic bomb…
Are machine-made telescope mirrors better than those made my hand?
Are machine-made telescope mirrors better than those made my hand? In general, no. Making a telescope mirror is a two-step procedure. First you generate and refine the curve in the glass, and then you carefully modify that curve, in the process known as figuring, to give the mirror’s polished surface…
How is the date of Easter determined?
How is the date of Easter determined? The rule most people remember is that Easter falls on the first Sunday after the first full Moon following the March equinox. In practice, Roman Catholic and Protestant churches follow a method of calculation adopted with the Gregorian calendar reform of 1582. The…
How will you know when you telescope mirrors need re-aluminizing?
I have read that telescope mirrors require periodic re-aluminizing. How will I know when mine needs this? While it’s true that the aluminized surface of a telescope mirror will deteriorate over time, there is no hard and fast rule about how long this will take. Some coatings last only a…
What should I do if my telescope mirror is dusty?
I took my new scope out last night and pointed a flashlight down the tube. My mirror looks really dusty. What should I do? The answer is simple: Don’t shine a light down the tube at night! Seriously, it only makes the dust look a lot worse than it is.…
How Can I Prevent My Finder From Dewing Over?
How do I keep my finder from dewing over when I observe? Your finder should have a light shield (“dewcap”), which you can cover between uses. Or you can purchase an anti-dewing device. This need not be expensive. You probably have a box full of them at home: A simple…
Is software available to analyze optical designs of telescopes?
Does software exist, preferably downloadable from the Internet, that I can use to analyze original optical designs of telescopes? Yes. The easiest way to find what’s out there is with a fine-tuned Google search. Right now, if you enter “optical design software” as the search terms, you get a few…
Roger W. Sinnott
Roger came to Sky & Telescope in 1971, after gaining an astronomy degree from Harvard University and making two deployments to Southeast Asia as an officer in the US Navy Seabees. From 1974 to 1998 he conducted the magazine's Telescope Making department, showcasing homemade telescopes, accessories, and cutting-edge optical designs.…
Sean Walker
Sean Walker joined the staff of Sky & Telescope in 2000. A graduate of Massachusetts College of Art, Sean brings extensive experience with digital image-processing techniques, as well as decades of visual observing. A self-described "gear head," he is constantly testing out or trading astronomy equipment. Rather than…
Alan Dyer
Alan Dyer is one of Canada's best known popularizers of astronomy, and is a veteran of numerous total and annular solar eclipses. His special interest in amateur astronomy is astrophotography of all types from nightscapes on film to deep-sky portraits on CCDs. Alan is author or co-author of several astronomy…
Image & Animation Links
Research Institutions Anglo-Australian Observatory ARC Telescope Astronomical Institutes of the University of Bonn Big Bear Solar Observatory European Southern Observatory Great Images in NASA Harvard-Smithsonian Center for Astrophysics Hubble Space Telescope Institute for Astronomy NASA's Observatorium Image Gallery NASA's Planetary Photojournal Nordic Optical Telescope Ohio State University Astronomy Dept. Stardial…
Astronomy Software Vendors
Contact information for commercial developers of astronomical software.
Good Lighting Fixtures: Where to Get Them
Here's a selection of manufacturers and suppliers of good lighting equipment.
