# Tag Info

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### Why don't astronomers use meters to measure astronomical distances?

In addition to the answer provided by @HDE226868, there are historical reasons. Before the advent of using radar ranging to find distances in the solar system, we had to use other clever methods for ...
• 2,846
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### How did Eratosthenes know that the sun is far away?

The sun and the moon go around the observer once a day, Eratosthenes knew that the apparent size of moon doesn't change. This must mean that Alexandria is near the centre of the moon's orbit. But ...
• 88.9k
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### Closest star system to Alpha Centauri?

The sun is the nearest star to Alpha Centauri (unless you count Proxima Centauri, which is really part of the same system). There is a very small and dim pair of brown dwarfs, called Luhman 16 that ...
• 88.9k
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### Is the Sun visible from Proxima Centauri to human eyes?

Well, there's two things we'll need for this: apparent magnitude (the brightness that an object appears to have) and absolute magnitude (the actual brightness an object has). Both of these scales are ...
• 7,690
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### What's the difference between minutes and arcminutes?

This can get a bit confusing, because "arcminute" and "minute" are both sometimes used in celestial coordinate systems but mean two different things. An arcminute is 1/60th of a ...
• 33.8k
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### Why is the Eagle Nebula so "static"?

It appears to be static because it's huge beyond your imagination. The distance to the nebula is 7,000 light years. Its apparent size is 7 arc minutes. Therefore its linear size is about 14 light ...
• 17.5k

### How did Eratosthenes know that the sun is far away?

Exactly how Eratosthenes calculated the radius of the Earth has been lost. What is presently taught as his method is a simplified version described by Cleomedes. It is unlikely that Eratosthenes ...
• 116k
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### Am I miscalculating distance to Moon?

In addition to the issues raised in Ralf Kleberhoff's answer, you need to account for the mass of the Moon. The correct form for Kepler's third law is $$a^3 = G(M_1+M_2) \left(\frac T {2\pi}\right)^2$$...
• 28.1k
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### Exact measurement of a light year

By convention, astronomy uses the Julian Year for the computation of a light year: Although there are several different kinds of year, the IAU regards a year as a Julian year of 365.25 days (31....
• 427
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### Is saying that what we see of an object 1 light-year away happened 1 year "ago" in any a way useful view of the universe?

You're right that astronomers don't really care what's going on in a some galaxy right now; we care about how they evolve through time, and how its light has been altered during its journey (e.g. ...
• 32.3k
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### How was the Earth-Sun distance originally calculated?

The book The Transits of Venus, by Sheehan and Westfall, describes how Aristarchus used Hipparchus' calculation of the Earth-Moon distance, who in turn used Eratosthenes' calculation of the Earth's ...
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### Why don't astronomers use meters to measure astronomical distances?

I would suggest it also makes the material more reachable for the human mind. I just can't work with insanely large or small numbers. They convey no meaning. But 1 AU is easy, even if I don;t know ...

### Was Earth closer to Europa on 1983-11-25 or 1985-07-22?

tl;dr Miki Sudo Using JPL's SPICE toolkit, I computed the positions of Earth and Europa for the times in question. On 1983-Nov-25, Earth and Europa are 935.2 million km apart, while 1985-Jul-22, ...
• 351
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### If we count Avogadro's Number of stars that are closest to Earth, how big that space would be?

It's going to be all (or perhaps nearly all) of the observable Universe. Roughly speaking, there are several hundred billion stars in the Milky Way. And extrapolating the number of galaxies in deep ...
• 2,692
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### How do we know if an object is redshifted?

There are two methods, one more reliable than the other (though both are pretty good.) Key point: The brighter a star is, the more detail we can see in its spectrum -- you can think of it as being ...
• 7,390

### Why is the Eagle Nebula so "static"?

To add to Florin Andrei's answer, with an image height of 7,000 pixels for 14 light years, that's 17.5 light hours per pixel. That's 20 billion kilometres per pixel. To make a change in a single pixel ...
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### Is parallax a "technique to measure the star distances" or a "tiny shift in star's position"?

It is both - a small shift of the position of a star on the sky as we see it, and a means of estimating the distance to the star. The apparent position (with respect to very distant objects like ...
• 116k

### Am I miscalculating distance to Moon?

You use rounded values for orbital period and geostationary radius with only three significant digits. And your result differs from the actual value just by 1 in the third significant digit. As a rule ...
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• 9,407

### Is saying that what we see of an object 1 light-year away happened 1 year "ago" in any a way useful view of the universe?

I agree that for very distant objects which are completely out of humanity's reach, the time delay of how we see the rest of the universe may not have much practical impact. But for nearer objects, ...
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### Could Neptune be viewed with the naked eye from Uranus?

Supplementary answer supporting @PierrePaquette thorough and well-source answer: I tried the nice new JPL Horizons interface and fired up Excel which I haven't used in a long time. For years 1800 to ...
• 31.6k
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### Compute Distance Between Stars

If you know the right ascension and declension of the stars, then you know the angle between them (ie the A-Sun-B angle). Working this out is an exercise in spherical trigonometry. The cosine of ...
• 88.9k

### How often do stars pass close (~1ly) to the Sun?

I was curious about the same things. I believe it was in the astronomy stack exchange I was referred to an online data base that gives position and velocity vectors for neighboring stars. From those I ...
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### Was Earth closer to Europa on 1983-11-25 or 1985-07-22?

Any online planetarium or equivalent mobile app will tell you that on 1983-11-25 Jupiter was near to its conjunction with the Sun: while on 1985-07-22 it was close to opposition: So on Miki Sudo's ...
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### Why are stars so far apart?

Most stars are of a solar-mass or below. The average number of companions that each stars has (in the sense of being part of binary or higher multiple systems) systems ranges from 0.75 for stars of a ...
• 116k