Questions tagged [distances]

Questions regarding the amount of space between two objects, the units, or measurement thereof.

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

In astronomy distances are generally expressed in non-metric units like: light-years, astronomical units (AU), parsecs, etc. Why don't they use meters (or multiples thereof) to measure distances, as ...
Arne's user avatar
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43 votes
6 answers
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In this image taken by Voyager 1, which is closer: the earth or the moon?

NASA reports this photo was taken 19 Sep 1977 by Voyager 1: It doesn't strike me as obvious how to determine which celestial body is closer to Voyager 1 as the photo was taken. My (not very ...
Rebecca J. Stones's user avatar
42 votes
1 answer
5k views

Closest star system to Alpha Centauri?

The closest star system to our Solar System is Alpha Centauri. But is our Solar System the closest star system to Alpha Centuari? If not, which star system is?
badroit's user avatar
  • 523
37 votes
8 answers
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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?

Since the speed of light is the speed of causality, the "now" state of the a faraway object for an observer should be the exact point at which its light reaches the observer, for literally ...
DoltishNeuron's user avatar
32 votes
3 answers
6k views

Is the Sun visible from Proxima Centauri to human eyes?

I know that the light coming from Proxima Centauri is not bright enough to make it naked-eye visible from the Earth. Is the Sun naked-eye visible from Proxima Centauri?
Patrick's user avatar
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31 votes
2 answers
9k views

How did Eratosthenes know that the sun is far away?

The famous measurements and calculations done by Eratosthenes around 300 BC are very widely known. He concluded correctly that the circumference of the Earth is about $252\,000$ times the length of an ...
Michael Hardy's user avatar
30 votes
5 answers
12k views

Why is the observable Universe larger than its age would suggest?

The age of the Universe is estimated at 13.8 billion years, and current theory states nothing can exceed the speed of light, which can lead to the incorrect conclusion that the universe can't have a ...
GDVS's user avatar
  • 411
29 votes
3 answers
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What is a parsec and how is it measured?

Astronomical units of measurement are mostly pretty straight-forward: Astronomical Units refer to the mean Earth-Sun distance (~150 million km or 93 million miles) Light years are the distance light ...
user avatar
26 votes
6 answers
28k views

Is the light we see from stars extremely old?

Our nearest star Proxima Centauri is 4.243 light years away from Earth. Does that mean we are seeing light that is 4.243 years old everyday?
PrivateUser's user avatar
26 votes
2 answers
3k views

How often do stars pass within 1 light year of the Sun?

Interstellar exchange of massive objects is difficult across several light years. But as the stars orbit the galaxy the distances between them change. I don't find data for neighbor star distances ...
LocalFluff's user avatar
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25 votes
3 answers
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Why is the Eagle Nebula so "static"?

This, of course, is a newbie question, as I am nothing more than a hobbyist. But I was quite surprised to see the recently-released "before and after" pictures of the so-called "Pillars of Creation" ...
kmote's user avatar
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23 votes
1 answer
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How distances are measured in the Solar System, in the Galaxy, and in the Universe?

Distances in our universe can be calculated in several ways: Stellar Parallax Standard Candles Redshift How are these methods linked, and how they are actually used to calculate distances are ...
leonard vertighel's user avatar
21 votes
2 answers
4k views

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

We know that hubble expansion increases the distance between points in space, and that the cosmological event horizon represents the distance from the observer at which objects are receding faster ...
user52978's user avatar
  • 313
21 votes
2 answers
1k views

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

xkcd #2003: gives a humorous line of succession to the United States presidency. The comic's title text, found by hovering over the comic, states that a tie is broken by whoever was closest to the ...
BeseechLoveVeil's user avatar
20 votes
2 answers
38k views

What is the distance that the Moon travels during one orbit around the Earth?

Also, does it always take the same amount of time, or does it fractionally differ on each revolution?
Asadullah Ali's user avatar
19 votes
4 answers
5k views

What's the difference between minutes and arcminutes?

I read in trigonometry class that 1 minute is equal to 1/60 degrees. So, 'minute' is an angular unit. But also 'arcminutes' are used to measure seperation between celestial objects and also equals to ...
Tanya's user avatar
  • 301
18 votes
2 answers
8k views

Understanding The Turnover Point of Angular Diameter Distance

I am trying to get a better understanding of cosmological distances, in particular the angular diameter distance which I have also seen referred to as angular size distance. What I am looking for is ...
Michael's user avatar
  • 183
16 votes
4 answers
3k views

Is parallax a "technique to measure the star distances" or a "tiny shift in star's position"?

I am not completely sure whether the Parallax is a "technique to measure the star distances" or a "tiny shift in star's position"? The a book says two points about Parallax: Astronomers have ...
Ahmed's user avatar
  • 263
16 votes
3 answers
2k views

Calculate Distance To Stars

I was just watching a lecture from Carl Sagan. He talked about figuring out the distance to the stars; it got me interested in learning more about the subject. As far as I know, the Inverse square ...
apxcode's user avatar
  • 459
15 votes
1 answer
1k views

If we count Avogadro's Number of stars that are closest to Earth, how big that space would be?

I think that the answer to this question will be useful as a scale to appreciate the size of the universe. So, If we count Avogadro's Number of stars that are closest to Earth, how big that space ...
sampathsris's user avatar
14 votes
1 answer
28k views

How was the Earth-Sun distance originally calculated?

AFAIK it was possible long before the first interplanetary probes. Who did it?
peterh's user avatar
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13 votes
4 answers
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Could Neptune be viewed with the naked eye from Uranus?

Which star or planet in our night sky can match what Neptune would look like when viewed from Uranus, or one of its moons? The answer would be for the most favourable condition, which is when Neptune ...
Constantthin's user avatar
13 votes
1 answer
2k views

How do we know if an object is redshifted?

I have some questions about redshift. I think that the first two hang on the last two, but feel free to point out anything you think I should know. How do we know if an object is actually redshifted,...
Gimcrack's user avatar
  • 179
13 votes
2 answers
865 views

Radius to which all hydrogen-burning stars are known?

For the purposes of this question, I wish to consider active, hydrogen-burning stars, not deuterium-burning brown dwarfs, or stellar remnants like black holes or neutron stars. (Though including ...
notovny's user avatar
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13 votes
2 answers
2k views

How does angular resolution of a telescope translate to its parallax precision?

We can often read in the scientific and also more casual reader literature and articles about the angular resolution of various telescopes and other optical equipment, be it ground based or onboard ...
TildalWave's user avatar
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12 votes
3 answers
6k views

What's the farthest object as determined only by parallax?

I'm wondering what the farthest object we know of, as measured only by parallax methods, from either earth or satellite based telescopes, and at either visible light or radio wavelengths. Basically, ...
Nathaniel Bubis's user avatar
12 votes
1 answer
442 views

How would parallax affect an object at 200-1000 AU (for example, the 9th planet)?

We know from this question that an object is hypothesized to be at between 200-1000 AU that is large enough to detect, known now as the 9th planet. It should move about 40 arcseconds per year. It ...
PearsonArtPhoto's user avatar
12 votes
1 answer
4k views

How do people measure the distance between the Earth and The Moon?

How do people measure the distance between the Earth and The Moon? What methods were used? Where can I find the official data?
user3052801's user avatar
11 votes
2 answers
851 views

Which is the closest exoplanet to Earth?

The closest star to Earth (after the Sun) is well-known: Alpha Centauri, 4.3 light years away. But what about the closest exoplanet to Earth, outside of the Solar System?
Zoltán Schmidt's user avatar
11 votes
3 answers
1k views

Beta Andromedae (Mirach) and distances mentioned in original Cosmos series from 1980

In one of the episodes from Carl Sagan's show Cosmos he explains that Beta Andromedae is the second brightest star in the constellation Andromeda, and is 75 light years away. The link to the video is ...
Zebrafish's user avatar
  • 213
11 votes
1 answer
1k views

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

On closer inspection of the absolute visual magnitude and the apparent visual magnitude of the sun, I noticed that their magnitude range (4.83 + 26.74 = 31.57) exactly matches the human eye. The ...
SmilySunMike's user avatar
11 votes
1 answer
1k views

How exactly was Giovanni Cassini able to measure the distance to Mars?

Recently learned that Cassini was able to calculate the distance to Mars quite well using parallax in 1672. I was surprised, since even at opposition of Mars, the parallax (with respect to the Earth's ...
d_e's user avatar
  • 1,667
11 votes
1 answer
225 views

Farthest distance two objects are "gravitationally bounded", considering expansion of the Universe

The narrative is: "on greater scales the expansion of the Universe dominates, but on smaller scales gravitationally bounded objects still stay bounded". But how small is meant by "...
Heopps's user avatar
  • 647
10 votes
2 answers
2k views

Exact measurement of a light year

What is the exact measurement of a light year? I searched google for the answer in meters and came up with $9.461\cdot 10^{15}$ meters. When I calculated the answer considering $299\,792\,458 \;\text{...
CipherBot's user avatar
  • 1,301
10 votes
5 answers
2k views

Whats the farthest distance a spacecraft has been detected (except by radio)

As far as I understand it, one can not hide spacecraft, because the heat from the engine would be very visible. Now I tried searching for the farthest distance an actual spacecraft has been detected ...
Angelo Fuchs's user avatar
10 votes
2 answers
404 views

Present distances between planet. How can I find them?

It's "commonly known" how distant are our solar system planets from Sun. But we can't easily say that about planets, which distances can differ greatly, without some observations (or simulations, ...
Kusavil's user avatar
  • 203
10 votes
2 answers
1k views

Find distance from star to star?

How do you find the distance from a star/planet/black hole to another? I know people can calculate the distance from Earth to a star, but what about from one to another?
Jont's user avatar
  • 205
10 votes
1 answer
186 views

Do we make predictions in our time, or local time?

I was scrolling idly through the Wikipedia article on Orion when I read: Orion will still be recognizable long after most of the other constellations—composed of relatively nearby stars—have ...
hairboat's user avatar
  • 200
10 votes
1 answer
372 views

What is the scale of things you can see with gravitational lensing?

I'm trying to understand the examples of gravity lensing (using the general relativity property of large masses to bend light like a lens). Most of the examples I see are of some galaxy (presumably ...
Mitch Harris's user avatar
10 votes
1 answer
112 views

How do astronomers distinguish between Cepheids and other types of variable stars?

So my understanding of how Cepheid is used as standard candle to determine distance of galaxies is like this: Astronomers observe a galaxy, notice an object within that galaxy whose brightness ...
mudiwii's user avatar
  • 101
9 votes
2 answers
4k views

Am I miscalculating distance to Moon?

The Moon's orbital period is generally given as 27.3 days, and the radius of Geostationary orbit as 26,200 miles. However, using Kepler's Third Law, if I raise 27.3 to the power of 2/3, and multiply ...
Mike Stone's user avatar
9 votes
4 answers
2k views

Does stellar parallax only occur parallel to the ecliptic?

Does stellar parallax only occur parallel to the ecliptic, or has it been observed in other directions, too?
Geremia's user avatar
  • 219
9 votes
2 answers
3k views

What would Betelgeuse look like from Earth if it was at the edge of the Solar System

If we parked Betelgeuse just outside the Solar System, how big would it look from Earth?
Jordan's user avatar
  • 388
9 votes
3 answers
5k views

Why are stars so far apart?

So, this is mere musing, but it seems that stars are quite extremely far apart. I tried to determine the mean distance between nearest neighbors for stars (in just our galaxy) but I'm not sure what it ...
pixels's user avatar
  • 243
9 votes
1 answer
4k views

Are planets moving away from the sun?

I saw on the tv show Through the Wormhole (hosted by Morgan Freeman) that the planets in our solar system have been continuously moving away from the sun for millions of years. however, when I try to ...
Yvonne Liew's user avatar
9 votes
1 answer
246 views

How to find the distance between two quasars

I want to calculate the distance between two quasars of which I know the angular position and the red shift. Let $Q_1=(\alpha_1,\delta_1, z_1)$ and $Q_2=(\alpha 2,\delta 2, z_2)$ and suppose $z_2 > ...
Emilio Novati's user avatar
9 votes
1 answer
444 views

Why has Moving Cluster Method been successful only for Hyades?

At all places where I have read of Moving Cluster Method for measuring distance to a cluster and once applied myself on data, I have been told that the method has been successfully applied only to ...
Cheeku's user avatar
  • 2,013
8 votes
1 answer
2k views

Is Webb or any near-future telescopes like ELT capable of observing redshift changes to confirm General Relativity?

The (Davis and Lineweaver (2003)) "Expanding Confusion" paper states that "the expected change in redshift due to cosmological acceleration or deceleration is only ∆z ∼ 10^(−8) over 100 ...
Glycoversi's user avatar
8 votes
5 answers
2k views

Are the stars in constellation located in a plane or they are in different distances

Are the stars in constellation located in a plane or they are in different distances from each other and when we observe them from the Earth we think they are all in a plane?
MJ.Shoori's user avatar
8 votes
1 answer
7k views

What is the angle between the equator of the sun, and the plane of the Earth's orbit

I want to know the angle between the plane that passes through the centre of sun and is perpendicular to its spin axis, and the plane that contains the orbit of earth around sun? Is this a fixed ...
Fereydoon Shekofte's user avatar

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