25 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 $0.001$-$0.01M_{\odot}$ ...
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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|...
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16 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 ...
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15 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 ...
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  • 10.4k
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 ...
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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 ...
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11 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 ...
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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 ...
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9 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’m almost done translating to French) is to measure the Sun–Moon angular distance, then wait for it to be dark enough, then measure the angular distance ...
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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 ...
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8 votes
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What is the Northernmost Latitude of Saturn?

Salem is at 42.5° north. At the equator, the position of the sun varies by 23.5 degrees on either side of the vertical over the course of each year, due to earth's 23.5° axial tilt with respect to the ...
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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 ...
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8 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 ...
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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 ...
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  • 4,065
7 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 ...
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  • 117k
7 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 ...
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  • 117k
6 votes
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Calculating Angular Distance

You use Spherical Trigonometry Given $A_1$ and $A_2$ are the respective azimuthal coordinates of the two objects, and $a_1$, $a_2$ their respective altitudes, the angular seperation $\theta$ is ...
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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}\...
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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 ...
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5 votes

Why do stars seem not to move relative to each other?

They do move - just far too slowly for you to detect by eye even over several human lifetimes. Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean,...
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  • 1,089
5 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... ...
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5 votes
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What is the difference between ICRS and ICRS2?

Not much has changed. Each update is consistent with its predecessor. The original ICRF is defined by 212 extragalactic sources. The ICRF2 is defined by 295 sources, 97 of which are shared by ICRF1 ...
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5 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 ...
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5 votes
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Will time that moon crosses meridian always be periodic?

Your formula is a crude approximation. It's ok if you just want a rough idea of the number of orbits the Moon makes over a long period of time, but it's not useful for calculating the daily motion of ...
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  • 10.4k
5 votes

If the earth was not tilted, what would be the north star?

It turns out that 0, 0 on ecliptic coordinates is not the "winter" direction, nor the "summer" direction. And it turns out to be a good thing. Zero longitude is defined using the "spring" direction, ...
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5 votes

How do I make a custom pulsar map?

The pulsar map used by Pioneer and Voyager is a diagram of the position of the Sun relative to 14 galactic pulsars. It encodes their positions, distances, and pulse periods, which in theory makes it a ...
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  • 34.1k
5 votes
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Sky from Equator at 8,000 years Before Present?

You can equally well just type in a negative year and Stellarium will navigate you into the corresponding year B.C. Don't forget that the year 0 didn't exist. 1 A.D. follows directly after 1 B.C. A ...
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