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Accepted

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 $0.001$-$0.01M_{\odot}$ ...
• 34.1k
Accepted

• 33.8k
Accepted

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 ...
• 11.6k

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,...
• 1,089

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... ...
• 32.4k
Accepted

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 ...
• 90.5k
Accepted

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 ...
• 32.4k
Accepted

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 ...
• 10.4k

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, ...