# Tag Info

## Hot answers tagged distances

84

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 finding the distance from the Earth to the sun; for example, measuring the transit of Venus across the surface of the sun. These methods are not as super ...

55

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 the apparent size also doesn't change when viewed from anywhere. So everywhere is close to the centre of the moon's orbit. Thus the moon must be much further ...

42

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 are closer, at about 3.6 light years from Alpha Centauri. Brown dwarfs are not true stars, but they do glow from their own heat. They were only discovered in ...

39

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 logarithmic, with brighter objects being lower and dimmer objects being higher. Astronomers have determined that the Sun's absolute magnitude is 4.83. Knowing ...

35

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 degree, and an arcsecond is 1/60th of an arcminute. That's simple enough, and when talking about small angular distances, it's often much handier to refer to ...

31

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 years. Think about that. The whole nebula is so big, it takes light 14 years to cross it. Any motion therein must necessarily be much, much slower. No wonder you'...

29

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 assumed the Sun was infinitely distant, since he apparently also estimated the distance to the Sun himself. In any case, his work came after that of Aristarchus who ...

27

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 million seconds) unless otherwise specified. Wikipedia gives the length as $31 557 600 s \times 299 792 458 m/s = 9 460 730 472 580 800 m$ (exactly) The ...

27

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. redshift and extinction). We don't know how a particular star or galaxy has evolved since it emitted the light we observe, except in a statistical sense, and that &...

25

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 circumference, to calculate the Earth-Sun distance. Aristarchus of Samos was the first to seriously calculate the distance to the Sun, using geometry. When the ...

24

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 exactly what that is in meters, I know what it means and it is a convenient scale for the mind. Likewise when we talk about stellar distances, what use is the ...

22

The easiest explanation for why the maximum distance one can see is not simply the product of the speed of light with the age of the universe is because the universe is non-static. Different things (i.e. matter vs. dark energy) have different effects on the coordinates of the universe, and their influence can change with time. A good starting point in ...

22

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 Hubble images suggests something like a hundred billion galaxies in the observable Universe. Put these together and you get about $10^{22}$ to $10^{24}$ stars in ...

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, they are 612.5 million km apart. Miki Sudo wins by 323 million km, given the assumed date for her birthday. If we don't trust famousbirthdays.com for Sudo's ...

22

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 able to magnify the spectrum more so as to be able to see finer details. This also allows us to see fainter lines (not all spectral lines are equally intense.) ...

20

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 over that time, something of that size must have either changed composition dramatically (to give a different colour or opacity) or it must have moved by a ...

20

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 quasars) changes because our viewing point changes as the Earth moves around the Sun in its orbit. The amount by which the position changes is inversely ...

18

According to https://arxiv.org/pdf/1808.01973.pdf, the magnitude of Neptune follows the relationship (formula 17, page 25): $V = 5 \log_{10} (rd) - 7.00 + 7.944 \times 10^{-3} α + 9.617 \times 10^{-5} α^2$ Where r is the distance of Neptune to the Sun, d is the distance of Neptune to the observer, and α is “the arc between the Sun and the sensor with its ...

17

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 the reprocessed data from the Hipparcos satellite, described in this paper. Prior to the launch of the Hipparcos satellite by ESA in 1989, parallaxes were very ...

16

Here is the very study you are looking for by Bailer-Jones (2014). Using the re-reduction of the Hipparcos astrometry, he has integrated orbits for 50,000 stars to look for objects that might come or might have come close to the Sun. The K-dwarf Hip 85605 is the winner on that timescale, with a "90% probability of coming between 0.04 and 0.20pc between 240,...

15

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 of its basic properties, as reported by the authors of the paper and known as of August 2016: Mass: At least 1.3 Earth masses Semi-major axis: 0.05 AU Maximum ...

15

The Moon has an orbital eccentricity of 0.0549, so its path around the Earth is not perfectly circular and the distance between the Earth and the Moon will vary from the Earth's frame of reference (Perigee at 363,295 km and apogee at 405,503 km), see for example second animation explaining Lunar librations in this answer. But its orbit can be said, in an ...

15

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 referring to $\frac1{60}$ of a degree, but it's not essential if there's no ambiguity, eg, in a static geometry problem where there's no mention of time. The ...

15

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, there are meaningful implications for space exploration. A Mars rover, for example, cannot be driven in real-time like a remote controlled car, since light takes ...

14

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 2100 in Observer mode it calculates apparent magnitude using all the bells and whistles (albedo model, phase angle, illumination, etc.) and gives the following ...

13

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 put together a spreadsheet. Here's a screen capture: I only entered 48 of the closest stars so it's by no mean an exhaustive list. It looks like your graphic ...

12

In short: things can not move faster that light by theirselves, but they can move faster than light due to universal expansion. The more far away, the faster they go away.

12

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 solar mass to approximately 0.35 (not a well-established number) for the more numerous M-dwarfs. Let's take a compromise value, say 0.5. The separation ...

12

OSIRIS-Rex has been spotted on its approach to Earth, at a distance of approximately 7 million miles (12 million kilometers) away, with a brightness of approximately 25th magnitude. The Large Binocular Telescope was used for this observation, this has a pair of 8.4-meter mirrors.

12

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 birthday, Europa was about 300 million kilometers closer to the Earth than on Joey Chestnut's birthday. images taken from the Star Walk iOS app

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