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Astronomy Questions & Answers

Four Things to Know About the December 2020 Solar Eclipse

Prepare for the 2020 solar eclipse with these basic facts and other resources.

Big Dipper photo

Hobby-based Q&A

How far away are the stars I see through my telescope?

The faintest stars I can see in my 4-inch refractor are 12th magnitude. If one of these stars is just like the Sun, how far away is it? The Sun would be magnitude 12.0 if it were 880 light-years away. That’s not very far in the grand scheme of the…

Jupiter with two moons and three shadows on its face

Astronomy Questions & Answers

Is it possible to detect Jupiter's satellites with the unaided eye?

I’ve heard it might be possible to detect Jupiter’s satellites with the unaided eye if Callisto and Ganymede appear together when Ganymede is at greatest elongation from Jupiter. Will this happen anytime soon? Jupiter is now setting soon after sunset. But three times in 2008, Texans (and North Americans generally)…

Schmidt-Cassegrain

Astronomy Questions & Answers

Is obstruction of less consequence for photography than for visual observing?

Some of the best images of planets are taken with Schmidt-Cassegrain telescopes, yet people often say that the large central obstruction makes this design ill-suited for planetary observing. Is the obstruction of less consequence for photography than for visual observing? Do CCDs and digital cameras make the central obstruction less…

Saturn and Mars hug the ecliptic

Astronomy Questions & Answers

Why doesn't S&T produce a star atlas with mirror-image charts?

Why don’t you produce a star atlas with mirror-image charts? It would be of immense value to the many observers who use a star diagonal on their telescopes. It’s a great idea in theory, but not in practice. Here’s why: · economics. Printing two versions of an atlas would double…

Hobby-based Q&A

How can a telescope have an f/ratio of f/42?

I was amazed at Jim Melka’s beautiful picture of Mars on page 136 of the January 2006 issue but puzzled by the caption, which said that he used a 12-inch reflector at f/42. How is this possible? Knowing that a telescope’s f/ratio is its focal length divided by its aperture,…

Roof-prism binoculars

Astronomy Questions & Answers

Why do the best roof-prism binoculars need a phase-correction coating?

Why do the best roof-prism binoculars need a phase-correction coating? How does it work, and why is it not used in Porro-prism binoculars? A roof prism splits the light cone from an objective lens into two separate pathways, involving different internal reflections, before recombining them into an erect image at…

Closeup of 8-mile crater

Hobby-based Q&A

Why do photos of lunar or Martian craters sometimes appear inside-out?

Frequently when looking at photos of lunar or Martian landscapes, I see craters as blisters. This can usually be rectified by turning the picture upside down; then craters reveal themselves as indentations rather than bulges. Do I need spectacles? No; this common illusion is all in the brain. Whether you…

Astronomy Questions & Answers

Can Venus ever be far enough from the Sun to appear east of the meridian?

During the Northern Hemisphere winter solstice, when the Sun is farthest south and sets in the southwest, can Venus ever be far enough from the Sun to appear east of the meridian? Yes, but only when viewed from near-arctic latitudes. In late 2008, for example, Venus is the Evening Star…

Pi

Astronomy Questions & Answers

How many digits are satisfactory in the measurement of pi?

In the 3rd century BC, Archimedes proved that the ratio of a circle’s circumference to its diameter is less than 3 1/7 but larger than 3 10/71. That’s about 3.141. Later mathematicians have computed what we now call p (pi) to greater and greater accuracy — but how many digits…

Astronomy Questions & Answers

What does "lignes" mean for aperture of antique telescope?

Paging through a reproduction of the 1909 Sears, Roebuck and Co. catalog, I found a telescope with an aperture of 25 lignes. Are you familiar with that term? In short, no. We’re also surprised to learn that such an archaic measurement was attached to a “modern” telescope. A ligne is…

Astronomy Questions & Answers

When will (or did) the galactic equator cross the ecliptic very close to the latter's northern and southern extremes?

As a teenager, when examining an equinox-1950 star atlas, I noticed that the galactic equator crossed the ecliptic very close to the latter’s northern and southern extremes (that is, the solstices at right ascension 6h and 18h). On equinox-2000 charts they are even closer. I’d love to know when the…

Astronomy Questions & Answers

How well defined is a meteor-shower radiant?

How well defined is a meteor-shower radiant? Is it a point on the sky or a several-degree-wide spot? Radiants are spots, not points. A meteor shower’s radiant is the location on the sky where all the meteors would appear to come from if we could see them approaching in the…

Laser beam from Subaru Telescope

Astronomy Questions & Answers

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…

Mars Oct. 21-22

Astronomy Questions & Answers

Is there a good test for optical quality?

In late October 2005 I hoped to see major surface markings on Mars, but my 10-inch scope showed only a uniformly yellow ball. Do I have an inferior mirror? The optics were well collimated. Is there a good test for optical quality? Starting in mid-October, dust storms on Mars spread…

Astronomy Questions & Answers

Can an OIII nebula filter be called “oh-three"?

I have an ongoing dispute with everybody. I say an OIII nebula filter cannot be “oh-three,” since OIII stands for doubly ionized oxygen atoms. I call it an “oh-two” filter. Who’s right? Sorry Philip, you lose. Not only is “oh-three” the universal usage; it makes sense. Well, sort of. A…

solar prominence

Sun

Why do you need a hydrogen-alpha filter to see solar prominences?

If I can see solar prominences with the naked eye during a total eclipse, why do I need a hydrogen-alpha filter to see them at other times? During a total eclipse the Moon completely blocks the Sun’s photosphere, or visible “surface,” allowing prominences (and the corona) to shine in all…

Schmidt-Cassegrain

Astronomy Questions & Answers

Why do Schmidt-Cassegrain telescopes have focal ratios of f/10?

Why do most if not all of the Schmidt-Cassegrain telescope I see advertised have focal ratios of f/10? During the past 20 years or so, amateurs have favored shorter tubes for both refractors and reflectors, even if it meant paying for more costly optics. Portability is the chief motivation -…

Astronomy Questions & Answers

Will we ever see a mirror image of the conjunction of Venus and crescent Moon, as depicted on the Turkish flag?

On the evening of April 19th 2007, I witnessed a spectacular conjunction of Venus and the screscent Moon with earthshine. The cusps of the crescent pointed left (south) as the Moon set in the west. Will we ever see a mirror image of this event as depicted on the Turkish…

Gaussian function

Astronomy Questions & Answers

What does "error" mean?

In your product reviews, when you state that a telescope drive has a periodic error of, say, 10 arcseconds, do you mean that it has a tracking accuracy of ±10 arcseconds or ±5 arcseconds? All gear systems have inherent mechanical errors that limit the accuracy with which a telescope drive…

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