SUNSET: 
Should you set your digital camera to a low or high ISO value in twilight?
When I’m shooting a planetary grouping in twilight, should I set my digital camera to a low or a high ISO value? The easiest way to answer this question is to make test exposures of a skyline during twilight, with or without planets. Try all the available ISO values (analogous…
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…
Can you use a replica of an 18th century New England sundial in Washington?
I have a museum’s pewter replica of an 18th-century New England sundial. The inscription says it was designed for latitude 42°. Can I use this sundial in Seattle, Washington? The iconic garden sundial, with triangular shadow-casting gnomon and horizontal plate, gives accurate readings at the latitude for which its hour…
What's the most distant object I can see with my telescope?
Is there a more distant object than NGC 4889 (Caldwell 35) that I can see with my 5-inch reflector? Yes, by a factor of 7! Think quasars. NGC 4889 is a member of the Coma Galaxy Cluster, which lies about 300 million light-years away. But the quasar 3C 273 is…
How can you determine a sunsport's size compared to that of Earth?
Turn off the telescope's motor drive (if any). Count the number of seconds it takes for the sunspot to drift past crosshairs or any speck of dust that is visible in your eyepiece. The number of seconds equals the spot's breadth in Earth diameters. The method is approximate. If…
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…
How could an astronomer living on the far side of the Moon verify Earth existed?
How could an amateur astronomer who lived her whole life on the far side of the Moon verify that Earth existed? Well, not by tuning in episodes of reality TV. Since the Moon has virtually no atmosphere, there is no mechanism (like ionospheric skip) by which radio signals from Earth…
What was the cloud spotted near the western horizon August 31, 2004?
Around 9 p.m. on August 31, 2004, I saw a bright patch of light about half the size of the Moon near the western horizon. It moved slowly upward and fluctuated somewhat in brightness. Through an 8-inch Schmidt-Cassegrain telescope here in New Bloomfield, Pennsylvania, there were two points of light…
Is it possible to have no full Moon in February during a leap year?
Everyone knows there can be two full Moons in a month. Is it possible to have no full Moon in a February containing 28 days? Yes, but only about four times in a century. This happened in 1999 and will next occur in 2018, assuming the phases are expressed in…
How did early astronomers calculate accurate solar system positions?
In the pre-computer age, say 50 years ago and back, how did astronomers calculate accurate positions of the Sun, Moon, and planets for predicting an eclipse or a transit of Venus? They did it by hand, with the help of numerical tables. These weren’t the trigonometric and logarithmic tables you…
How do astronomers accurately determine wobbles in a star's motion?
The discovery of planets around other stars is based on wobbles in the stars' motions. Don't you have to measure from a stationary point in space to accurately determine this/ Isn't there an added wobble because Earth rotates on its axis and travels around the Sun? Yes, Earth's rotation and…
How can I spot satellite triads?
While binocular observing from Northern California on August 19, 2004, at 6:33 Universal Time, I picked up a trio of satellites moving from the northwest to overhead and passing through Cygnus. I’m sure they were satellites, as all three turned reddish at the same time before disappearing into Earth’s shadow.…
How can an astrophoto shot through a refractor have diffraction spikes on bright stars?
In your Gallery department (S&TSeptember 2004, page 144), you had a nice image of the star Pollux showing diffraction spikes. That suggests it was taken with a Newtonian reflector, but the accompanying note says a Takahashi refractor was used. Where did the spikes come from? Many imaging enthusiasts like the…
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.…
A Field Guide to Supernova Spectra
With this kit, you're ready to identify stars that explode.
Reporting and Validating a Nearby Supernova
The next time a massive star explodes in the Milky Way, it will cause a mad scramble among amateur and professional astronomers.
You May Already Have a Supernova Detector
Whether you scan the heavens with your eyes, a film camera, or a CCD chip, you've probably got what it takes to find the next galactic supernova.
The Revival of Amateur Spectroscopy
It has never been easier to explore the fascinating world of astronomical spectroscopy with backyard telescopes.
What Are Celestial Coordinates?
"Right ascension" and "declination" tell you where your telescope is pointed in the sky. But what do they really mean?
