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26 votes
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If dark matter bends light, how do we know the stuff in the sky is where we think it is?

The local dark matter density is actually quite tiny, on the order of $\rho\sim10^{-19}\text{ g/cm}^3$ (see e.g. Bovy & Tremaine (2012)). This means that there is roughly $0.001$-$0.01M_{\odot}$ ...
HDE 226868's user avatar
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24 votes
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How far do we have to go to leave the galaxy?

The sun is within a few parsec (15-25 pc) of the galactic plane, slightly above. The thin disk of the milky way (containing ~85% of the stars and gas) has a density going roughly like $\rho_0 \exp(-|z|...
Anders Sandberg's user avatar
23 votes
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How do planetarium apps and software calculate positions?

There are quite a few different types of methods of computing the position of celestial objects, and the method used to compute the position generally depends on the type of object and how accurate ...
Greg Miller's user avatar
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18 votes
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What positional accuracy (ie, arc seconds) is necessary to view Saturn, Uranus, beyond?

The size of the field of view you will see depends on the eyepiece used and the telescope. For example, that telescope comes with a 20mm and a 4mm eyepiece, and has a focal length of 700mm. The ...
Greg Miller's user avatar
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17 votes
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What is the Galactic Latitude and Galactic Longitude? How is different from the latitude and longitude we use on Earth?

The Galactic Coordinate system is a longitude-latitude coordinate system that is used to define the positions of objects in space, most commonly objects within our own galaxy. It uses the center of ...
zephyr's user avatar
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16 votes
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JPL Horizons - "highly accurate measurements of planetary positions" - how do they do it?

The JPL ephemerides are good, but they certainly are not precise to the fraction of a millimetre! From the Horizons docs: Statement of Ephemeris Limitations To produce an ephemeris, observational ...
PM 2Ring's user avatar
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14 votes
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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 ...
James K's user avatar
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13 votes
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How the position of this black hole known?

The article is referring to the black hole known as V4641 Sagittarii. It was originally thought to be the closest known black hole to Earth, at 1,600 light-years (0.5 kiloparsecs). It's a binary ...
HDE 226868's user avatar
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13 votes
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Straightforward derivation of the sunrise equation

We can start by converting between equatorial coordinates (right ascension $\alpha$ and declination $\delta$) to horizontal coordinates (azimuth $\text{Az}$ and elevation/altitude $a$). If you want to ...
HDE 226868's user avatar
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12 votes
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How was the First Point of Aries measured in ancient times?

The trick, as explained in the Almagest (which I have translated to French; see https://ecliptiqc.ca/Almagest.php) is to measure the Sun–Moon angular distance, then wait for it to be dark enough, then ...
Pierre Paquette's user avatar
12 votes

How do planetarium apps and software calculate positions?

How do planetarium apps and software calculate positions? The generic answer is that they use some form of ephemeris. The VSOP87 is an example of an ephemeris. So is JPL Horizons, which uses ...
David Hammen's user avatar
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11 votes

JPL Horizons - "highly accurate measurements of planetary positions" - how do they do it?

@PM2Ring's answer does a great job of explaining that folks who do careful numerical integration keep a large number of digits beyond the final accuracy because if they didn't, after tens of thousands ...
uhoh's user avatar
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10 votes
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Is it possible to figure out your location in the Milky Way if you are suddenly wormholed to a random, distant location in the galaxy?

Assuming you had access to the relevant astronomical catalogues and data, then yes it would. Looking at constellations would of course be hopeless and most stellar catalogues only contain star ...
ProfRob's user avatar
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9 votes
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Tips of Crescent

They don't. In this image (from 28th Dec 2021) the blue grid represents lines on an equatorial grid. The lines that run from top right to bottom left are North-South lines. The tips of the crescent ...
James K's user avatar
  • 123k
9 votes

Why don't we use the Voyager/Pioneer etc space probes to measure stellar parallax?

The basic reason is that Voyager and friends have cameras optimised for imaging planets, not for doing very precise astronometry. So why not send a telescope, like that on Gaia, into interplanetary ...
James K's user avatar
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8 votes

Generate a uniform distribution on the sky

Random points on the surface of a sphere can be generated by allowing the azimuthal angle $\phi$ to take a uniformly distributed random value between 0 and $2\pi$. To convert this to RA in degrees you ...
ProfRob's user avatar
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8 votes

Where can I find the positions of the planets, stars, moons, artificial satellites, etc. and visualize them?

To add to the excellent answer by barrycarter, there are 2 planetarium-like codes, that I know of, that run on a mac and would make excellent tools for viewing certain astronomical objects. The codes ...
Natsfan's user avatar
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8 votes
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What are the correct positions or alignment of planets in relation to each other and the Sun, is it vertical center, baseline or completely random

It is impossible to have the planets line up like in your image. This is done for illustration purposes only. If such an image was to scale, either the bodies would be extremely small and impossible ...
Pierre Paquette's user avatar
8 votes
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Measurement unit of coordinate systems in astronomy

In science, there are cases in which some weird units are used for historical reasons only and we would all be better off without them. One could argue that the magnitude systems is one of those cases,...
Prallax's user avatar
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7 votes
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Is this a real picture of the moon?

I don't know whether this picture is a fake, but you could get this sort of image without cheating, if it were taken a long way away from Rio, and the magnification cranked up. In other words, get ...
Dr Chuck's user avatar
  • 4,349
7 votes

How was the First Point of Aries measured in ancient times?

In the Last part of Astronomia Nova (Ch. 69) Kepler describes a way the ancient astronomers might did it. Kepler suspects this way Ptolemy was using but he is not sure. The process can be generally ...
d_e's user avatar
  • 1,667
7 votes

What positional accuracy (ie, arc seconds) is necessary to view Saturn, Uranus, beyond?

Eyepieces are different, the specifications for your telescope don't give the field of view, but you might expect about 0.5 degrees, or 30 arc-minutes. And a star near the equator will take a couple ...
James K's user avatar
  • 123k
7 votes

Difference in results between JPL Horizons and cspice (rust-spice)

The issue is with how you have configured HORIZONS to show the output. You have apparent RA, Dec selected as output columns which includes the precession and nutation, in addition to the light time ...
astrosnapper's user avatar
  • 8,357
6 votes

Is this a real picture of the moon?

I like @DrChuck's answer and this Astronomy Picture of the Day shows how this has some plausibility: ...but you could get this sort of image without cheating, if it were taken a long way away... ...
uhoh's user avatar
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6 votes
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Generate a uniform distribution on the sky

Heres more python than you can shake a telescope at. I just used @ProfRob's algorithm. This is just a python script, the real answer to the question is @ProfRob's answer and I've just scripted it. The ...
uhoh's user avatar
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6 votes
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how to calculate distance between galaxies in the distant past

I'm assuming you're talking about physical distances (as opposed to any of the other distance measures in cosmology). The comoving distance to a galaxy at redshift $z$ is $$ d_C(z) = \frac{c}{H_0}\...
pela's user avatar
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6 votes
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Rising and setting time of planets in relation to the sun

It's geomety: The day has 24 hours. The circle has 360°. So the sky "moves" 15° per hour due to the Earth's rotation of 360° in 24 hours once around its axis. Assuming that an object is ...
planetmaker's user avatar
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6 votes
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How can I convert my sky coordinate system (RA, Dec) into galactic coordinate system (l, b)?

Applying spherical trigonometry, it follows that the conversion from equatorial coordinates $\alpha$ and $\delta$ to galactic coordinates $l$ and $b$ is: $$\left. \begin{aligned} \sin b &=\sin \...
Albert's user avatar
  • 2,102

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