SUNSET: 
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…
If you lived on Saturn, would its rings be visible from the equator or the poles?
If I lived on Saturn, would its rings be visible from the equator or the poles? Above the cloudtops and neglecting refraction, geometry dictates that you’d see nothing from the poles. As you moved toward the equator, the bright A ring would start coming into view at latitude 66° (roughly…
What defines a planet's north pole?
Uranus is often said to have a retrograde rotation with its axis tilted 98°. Why don’t we say it has a direct spin with the axis tilted 82°? Since 1982, the International Astronomical Union has defined the north pole of a planet to be the pole that lies north of…
How can we find the Sun's place among the constellations?
How can I find out how our Sun would look among the constellations, as seen from a nearby star? What made me curious was a painting by space artist David Hardy that pictured the Sun as an extra star in Cassiopeia. Fancy planetarium projectors produce such scenes, but you can…
Can you adjust a f/7.7 spherical mirror to act like a f/8.0 parabolidal mirror by racking the focus farther out?
My 114-millimeter (4.5-inch) Newtonian reflector came with an f/7.7 spherical mirror. I can purchase a 114-mm f/8.0 paraboloidal mirror. Can I simply adjust for the 34 mm of added focal length by racking the focuser farther out, or should I extend the main tube? Either way, is the upgrade worth…
What was that flashing light in the sky?
I'm new to astronomy (1½ months) and I live in New Jersey. Last night, July 31st, I saw a bright planet (I assume Jupiter) in the southwestern sky, and just below it what looked like an airplane with a flashing red tail marker — but it never moved. When I…
Would a perfectly reflecting full Moon be just as bright as the Sun?
I've read that the full Moon has a magnitude of about -13 and an albedo of about 3%. That implies that if the lunar disk reflected 100% of sunlight it would appear more than 30 times as bright, or about magnitude -17. But since it has the same angular diameter…
Will asteroid 99942 Apophis eclipse the Moon when it passes by?
When I heard that a 300-meter asteroid will approach Earth at a distance of only 18,000 miles in 2029, I figured it was time to break out the old slide rule. Wouldn't an object that big and close be able to eclipse the Moon, if it should pass in front…
Where did the 1 1/4-inch standard size for eyepiece barrels come from?
Where did the 1 1/4-inch standard size for eyepiece barrels come from? This barrel size was adopted shortly before 1890 by John A. Brashear in Pittsburgh, Pennsylvania, says Bart Fried of the Antique Telescope Society. Then W. & D. Mogey of Plainfield, New Jersey, and other firms followed suit.…
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…
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 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…
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…
What’s the phase of the Moon during the Perseids?
What’s the phase of the Moon during the Perseids? Use this simple calculation to figure it out.
What's my naked-eye magnitude limit?
How can I find out my naked-eye magnitude limit? Count stars inside the Great Square of Pegasus, which is well placed on November evenings. If you can see only two stars within the square, you are reaching visual magnitude 4.6 and your skies are probably light polluted. If you spot…
Can telescopes increase an object’s surface brightness?
I normally can’t see the Triangulum Galaxy (M33) with the naked eye, but it’s easy in a rich-field telescope. So how can you claim (June issue, page 128) that a telescope never increases an object’s surface brightness? The explanation has nothing to do with optics and everything to do with…
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…
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°…
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.…
