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88 votes
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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 ...
Sean Lake's user avatar
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60 votes
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In this image taken by Voyager 1, which is closer: the earth or the moon?

I used JPL Horizons to get the position vectors of each relative to the SSB on Sep 19, 1977. ...
Greg Miller's user avatar
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58 votes
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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 ...
James K's user avatar
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53 votes

In this image taken by Voyager 1, which is closer: the earth or the moon?

Another way to look at it is by linear measurements. Using Photoshop, and averaging three measures each of the Moon and the Earth (with the “Ruler” tool), I get 12.56 pixels for the Moon and 54.67 ...
Pierre Paquette's user avatar
43 votes
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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 ...
James K's user avatar
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40 votes
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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 ...
Sir Cumference's user avatar
35 votes
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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 ...
HDE 226868's user avatar
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31 votes

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 ...
ProfRob's user avatar
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29 votes
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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 (31.5576 ...
Toby Speight's user avatar
29 votes
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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 $$a^3 = G(M_1+M_2) \left(\frac T {2\pi}\right)^2$$...
David Hammen's user avatar
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29 votes
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Distance of the Crab nebula and the speed of light

You are absolutely right. The convention is that we date astronomical events by when we see them - which is verifiable, useful and absolutely certain. It wouldn’t be useful to know what year BC it ...
Martin Kochanski's user avatar
27 votes

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 ...
StephenG - Help Ukraine's user avatar
27 votes
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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. ...
pela's user avatar
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24 votes

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, ...
lwr's user avatar
  • 351
24 votes

Why was the standard distance set to 10 parsec and not simply to 1 parsec?

The various units used in astronomy are, essentially, practical units. The first distances to stars were measured in the 19th century. As only the nearest stars could be measured in this way, by 1895 ...
James K's user avatar
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22 votes
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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 ...
Mark Olson's user avatar
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21 votes
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What equation tells you how far in space you can go from a point and return?

Preparation Let $a(t)$ be the cosmic expansion factor. Let $x$ be a comoving coordinate, so two objects (like galaxies) that are separating solely due to the expansion of the universe have a constant ...
Sten's user avatar
  • 4,753
20 votes
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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 ...
ProfRob's user avatar
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20 votes
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Could Neptune be viewed with the naked eye from Uranus?

According to https://arxiv.org/pdf/1808.01973.pdf, the magnitude of Neptune follows the relationship (formula 17, page 25): $ V = 5 \log_{10} (rd) - 7.00 + 7.944 \times 10^{-3} α + 9.617 \times 10^{-5}...
Pierre Paquette's user avatar
20 votes

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 ...
Ralf Kleberhoff's user avatar
20 votes

In this image taken by Voyager 1, which is closer: the earth or the moon?

note: this confirms @GregMiller's answer I started typing it last night and fell asleep; woke up and finished it and clicked "post" then to discovered a bunch of new answers. For those who ...
uhoh's user avatar
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20 votes

In this image taken by Voyager 1, which is closer: the earth or the moon?

The Earth was closer. I can't find the time when that picture was taken. That NASA press release is dated Sep 19, 1977, but the consensus seems to be that the photo was taken around the 18th of ...
PM 2Ring's user avatar
  • 15k
20 votes

Distance of the Crab nebula and the speed of light

Just to reiterate my comment. The approach is correct but the accuracy is spurious - the number of significant figures is not warranted by the quality of the distance information (the date of ...
ProfRob's user avatar
  • 155k
18 votes
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What would Betelgeuse look like from Earth if it was at the edge of the Solar System

The distance to Betelegeuse is not precisely known for reasons you can read about here and here. But let's assume a likely distance of 200 pc. The angular diameter of the star has been measured with ...
ProfRob's user avatar
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17 votes
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Understanding The Turnover Point of Angular Diameter Distance

On the one hand an object spans a smaller angle the farther away it is, as expected. On the other hand, due to the expansion of the Universe and the finite speed of light, very distant objects were ...
pela's user avatar
  • 38.7k
17 votes
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Beta Andromedae (Mirach) and distances mentioned in original Cosmos series from 1980

Looking at the SIMBAD data page for Beta Andromeda shows the source of both the parallax (distance) and the magnitudes. In this case, as it will be for many bright stars, the source of the parallax is ...
astrosnapper's user avatar
  • 8,442
17 votes

What equation tells you how far in space you can go from a point and return?

sten's answer is excellent and beautifully analytical; I just wanted to illustrate the journey and show that neglecting matter (and radiation) is a good approximation. Spacetime diagram To do this, I ...
pela's user avatar
  • 38.7k
17 votes

How do we work out the light travel time on a cosmic scale?

Let's first be clear that there is no unique way to identify the time or distance between two events. This is true in every relativistic context; just think about relativistic time dilation and length ...
Sten's user avatar
  • 4,753
16 votes

Which is the closest exoplanet to Earth?

Earlier this year (2016), scientists used the radial velocity method to discover a planet orbiting Proxima Centauri: Proxima Centauri b. It was announced in Anglada-Escudé et al. (2016). Here are some ...
HDE 226868's user avatar
  • 37.2k
16 votes

What's the difference between minutes and arcminutes?

Your trigonometry book isn't wrong: both "minute" and "arcminute" can refer to $\frac1{60}$ of a degree. It's certainly a very good idea to use the term "arcminute" when ...
PM 2Ring's user avatar
  • 15k

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