88
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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 ...
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60
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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.
...
- 5,932
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 ...
- 126k
53
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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 ...
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43
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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 ...
- 126k
40
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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 ...
- 8,135
35
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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 ...
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31
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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 ...
- 156k
29
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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 ...
- 501
29
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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 $$a^3 = G(M_1+M_2) \left(\frac T {2\pi}\right)^2$$...
- 34.1k
29
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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 ...
- 1,830
28
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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. ...
- 38.9k
27
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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 ...
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24
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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
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 ...
- 126k
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 ...
- 7,650
21
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}...
- 7,549
21
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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 ...
- 4,763
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 ...
- 156k
20
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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 ...
- 1,519
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 ...
- 30.7k
20
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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 ...
- 15k
20
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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 ...
- 156k
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 ...
- 156k
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 ...
- 38.9k
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 ...
- 8,452
17
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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 ...
- 38.9k
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 ...
- 4,763
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 ...
- 37.3k
16
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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 ...
- 15k
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