Hobby-based Q&A

Browse the stargazing questions and answers below to delve into the hobby of astronomy. Learn about different types of equipment and what they can do, and discover the limits (read: challenges) of observing the night sky with instruments and the unaided eye.

The Q&As presented here cover a wide range of questions posed over the years by our readers, and we’ve responded with detailed and well-researched answers. What’s the difference between a 6-inch and a 12-inch scope? Can we see the shadows of Saturn’s moons as we can Jupiter’s? And how can you turn your stargazing hobby into an astronomy career? Read on to learn the answers, and if there’s a question that hasn’t been asked, ask it yourself by sending a note to info@skyandtelescope.com.

Crescent Moon and Venus at sunset

Could the lunar crescent be seen in a telescope at new Moon?

Is the Moon’s orbit inclined sufficiently that, when it misses the Sun by the greatest amount north (or south), the lunar crescent could be seen in a telescope at new Moon? Probably not. The inclination of the Moon’s orbit to the ecliptic varies from 5.0° to 5.3°. French astronomer André Danjon (1890–1967) showed that no…

Rising Moon

How is the time of new (or full) Moon defined?

How is the time of new (or full) Moon defined? Astronomically, the Moon is new when it and the Sun have the same celestial longitude. The Moon is at first-quarter phase when its longitude is 90° greater than that of the Sun. The Moon is full when its longitude is 180° greater, and at last…

Dione

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 Saturn with 6 1/2-inch instruments.…

Tim Hunter and his telescope

How Can Amateurs Find Asteroids?

In a 1996 article titled “Hunting Asteroids,” you said a dedicated amateur could discover an asteroid on almost any night using a CCD-equipped 8-inch telescope. Is that still true today? It was easier for backyard observers to discover asteroids a few years ago. Today, massive professional surveys such as LINEAR, NEAT,and LONEOS are sweeping the…

Why doesn’t a GPS receiver read longitude 0° 00′ 00″ while standing on the prime meridian at the Greenwich Observatory?

Why didn’t my GPS receiver read longitude 0° 00' 00" when I visited Greenwich Observatory and stood on the prime meridian? The NAVSTAR satellites of the Global Positioning System can provide your location to an accuracy of 3 meters (10 feet) or better. The coordinates you get are based on a particular geodetic datum, such…

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 the ecliptic plane (the plane…

SLRs: film (left) and digital (right)

Why do people doing CCD imaging often stack five 1-minute exposures instead of taking just one 5-minute exposure?

Why do people doing CCD imaging often stack, say, five 1-minute exposures instead of taking just one 5-minute exposure? Modern digital cameras capture faint astronomical objects with much shorter exposures than their film-based counterparts did, but it still takes an exposure of many minutes to produce a good picture. So-called image stacking is the easiest…

Why Do Bright Stars Look Bigger?

If stars appear as mere points, as we’re always told, why are some stars big and some small in every image I’ve ever seen? Photography does strange things to stars. In fact, the sky on photographs looks rather different from the sky we see visually (S&T: June 2004, page 128). A star in the sky…

Saturn and Antares

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 got home I looked at…

Were stars artistically depicted with diffraction spikes before the invention of the Newtonian reflector?

Were stars artistically depicted with diffraction spikes before the invention of the Newtonian reflector? If so, why? Stars were being drawn with points or spikes long before Isaac Newton announced his reflecting telescope in 1671. Just look at early works of art, flags, ancient coins, and the charts of the Pleiades and Praesepe star clusters…

FIsh's Mouth or Dementor?

Why aren’t Earth’s night skies more colorful?

Why are Earth's skies so boring? You see pictures of galaxies, nebulae, clusters, etc., but they're always far away. Why couldn't Earth have been in the middle of a colorful nebula or some other non-boring zone of space? Celestial photos show what things would look like if your eyes were as powerful, sensitive, and versatile…

Moon's skyglow

How does the Moon’s phase affect the skyglow of any given location, and how many days before or after a new Moon is a dark site not compromised?

How does the Moon's phase affect the skyglow of any given location? How many days before or after a new Moon is a dark site not compromised? The answer is complicated because the Moon's glow is even more directional than light pollution. Skyglow is several times brighter near the Moon than on the opposite side…

Perseid fireball

How much space debris falls into Earth’s atmosphere every year?

Has anyone ever calculated the combined tonnage of meteroids and space debris falling into our atmosphere yearly? Yes, but it's hard because different methods are needed for different particle sizes. Ground radar and exposed spacecraft surfaces are best for detecting the very smallest bits, whereas photographic surveys have been used for particles large enough to…