SUNSET: 
Why are the stars so far away?
Why are the stars so far away? That question has a real answer: Because if the stars were much closer, Earth wouldn’t exist, and we wouldn’t be here to ask.
Should I wear eyeglasses while observing?
Should I wear eyeglasses while observing? If you are near-or farsighted, with no other eye problems, definitely take them off. You’ll have to tweak the focus, but you’ll see objects in the telescope just as clearly as if you had 20:20 vision. By taking off your glasses you’ll avoid the…
What’s the difference 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…
How can I get my German equatorial mount to hold its tilt?
My old German equatorial mount won't hold the proper tilt for my latitude. What can I do? Believe it or not, grease the threads and washer of the large bolt that clamps the polar-axis housing to the tripod head. Then tighten it. — Dennis di Cicco
Four Galilean moons shadows
Can all four Galilean moons cast their shadows on Jupiter ’s disk at the same time?No. A triple shadow transit is as good as it gets (S&T: July 2004, page 94). The motions of the three inner moons are gravitationally locked so that, as English astronomer George B. Airy wrote…
Will Mercury and Venus ever transit the Sun simultaneously?
Will Mercury and Venus ever transit the Sun simultaneously? Yes, but this will not happen until AD 69,163. A transit can take place only when Mercury or Venus is near an ascending or descending node, meaning a crossing point of its orbit on the plane of the ecliptic. For this…
What's the difference between two Plössl eyepieces?
I bought exactly the same Plössl eyepiece that a buddy has, but mine doesn’t work as well as his. Did I get a lemon? That’s possible, but very unlikely. Our guess is that your telescope has a short focal ratio, perhaps f/5 or less, while your friend may have an…
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…
What size first telescope mirror should I make?
What size is best for someone’s first attempt at making a telescope mirror? Most books on telescope making recommend starting with either a 6- or an 8-inch mirror, and that’s good advice. Either size is suitable for a first-timer, but a 6-inch will probably take a little less time to…
When's the next blue Moon?
When's the next blue Moon? To read the full story with a schedule that shows all blue Moons through 2020, see our article "What is a Blue Moon?" Here's the abbreviated version: In 1999 Sky & Telescope admitted to its “blue Moon blooper,” an error that had crept onto the…
How can eyepieces offer a telescope's widest true field?
Some eyepieces are being advertised as offering the widest possible true field in any telescope. How is this determined? The amount of sky shown by an eyepiece is governed by the diameter of its field stop, the ring that defines the edge of the visible field. On a given telescope,…
When is the earliest sunrise of the year?
The earliest sunrise of the year doesn't always occur on June 20th (the solstice), which has the longest day. Why? The earliest sunrise actually depends partly on latitude. In 2004, for latitude 40° north, it was June 13th. For 50° north, the earliest sunrise was on June 16th. At 30°…
Focal Ratio vs. Aperture: What Makes an Object Look Brighter Through a Telescope?
How can you make a star or deep-sky object, such as galaxies or nebulae, look brighter? The total brightness of a star or other object is determined by the telescope’s aperture, also called light grasp. The focal ratio (also called f/number, that is, the focal length divided by the aperture)…
What's the distance to Polaris?
I have three astronomy programs on my computer that give three different distances for Polaris. I just obtained a fourth program, and it says the distance is unknown. What’s the problem? Those new to astronomy expect the distances of bright stars to be very accurately known, but many still aren’t.…
If stars appear as points, why are some stars big and some small in different images?
If stars are unresolved points, even in the largest telescopes, why do bright ones look bigger than faint ones? Our eyes are amazing instruments, but they’re limited by physics and biology. When starlight enters an optical system with a circular aperture, such as a telescope or a human eye, the…
Jennifer Barlow: Dark-sky Devotee
One person can make a difference, as demonstrated by a high-school student's single-handed effort to help us all appreciate the night sky.
Home Lighting Guide
Here's how to find exterior residential lighting that's dark-sky friendly.
Astronomy Education Links
Organizations Astronomical League Youth Observing Program Astronomical Society of the Pacific (ASP) Astronomy Education Review DarkSky International Challenger Center for Space Science Education Challenger Space Science Center of Arizona European Association for Astronomy Education Euro Space Center (Belgium) International Association for Astronomical Studies (IAAS) National Space Society (NSS) Planetary Society…
Good Lighting Fixtures: Where to Get Them
Here's a selection of manufacturers and suppliers of good lighting equipment.
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…
