29
votes
Accepted
Does matter accumulate just outside the event horizon of a black hole?
Yes, you are absolutely right, from OUR VIEWPOINT it does.
From Kip Thorne's book "Black Holes and Time Warps: Einstein's Outrageous Legacy."
“Like a rock dropped from a rooftop, the star’s surface ...
- 462
22
votes
Are there any galaxies which fell out of sight horizon due to cosmic expansion?
No. In fact the opposite is the case.
(See the last paragraph for an intuitive explanation.)
It is a common misbelief that galaxies receding faster than the speed of light are not visible to us. ...
- 33.8k
12
votes
Does matter accumulate just outside the event horizon of a black hole?
We need to think about just where the time dilation effect occurs. By then thinking about the observations from each point of view, that is the free falling object and the external observer, we can ...
- 251
8
votes
Are there any galaxies which fell out of sight horizon due to cosmic expansion?
As time passes, there are galaxies that are currently not in the observable universe which will become observable But this is not a sudden winking on. Instead, over hundreds of millions of years we ...
- 91.4k
8
votes
Accepted
Sun path at poles
It moves in circles. At the poles there is no direction defined (east, west, north or south).
This is how the sun moves at the north pole: http://www.jaloxa.eu/resources/daylighting/docs/...
- 1,534
7
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Does matter accumulate just outside the event horizon of a black hole?
Several wonderful yet technical answers have been given, and I cannot add anything to those very nice answers that explain why it is not useful to think black holes get "frozen" at their event ...
- 5,170
6
votes
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Can Mercury be seen from the 60th parallel north on Earth? Shetland, Faroe, Norway, St Petersburg, Alaska
Yes it can be seen, but as the angle of the ecliptic is lower in more Northern Latitudes it is harder to see.
Chris Brown of the Shetland times writes in Feb 2000:
February brings a chance to try and ...
- 91.4k
5
votes
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A plot of the Moon-rise/sets on the Horizon Location
We are wondering if the moon wobbles back and forth along the horizon the same way the sun wobbles.
Yes, but over a period of 27 days compared to a full year for the Sun.
If you visualize a compass, ...
- 7,160
5
votes
Does matter accumulate just outside the event horizon of a black hole?
Thought provoking cosmologists!
I'm uber late to this discussion as I see it has been ongoing for literally years and don't know if there is still anyone monitoring this thread, but here' goes.
I ...
- 59
4
votes
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Why aren't all planets in the same plane?
Your reasoning is correct: if Mercury orbited in the same plane as Earth, we'd see it transit the Sun every 4 months or so. In fact these orbital planes are inclined 7 degrees to each other, and the ...
- 16.6k
4
votes
How much more atmosphere is the sun passing through at sunset as opposed to at its peak in the sky?
The amount of atmosphere that light from outside Earth has to pass through before it reaches the ground is given by the air mass, $X$. It is normalized to unity for a source directly overhead (at "...
- 33.8k
3
votes
How to find the moons maximum culmination at a location
Let's suppose, that moon is always located on the ecliptic. Ecliptic ranges from declination $\delta=-23.5°$ to $\delta=23.5°$. That means, that the maximum apparent height of the Moon in Calgary will ...
- 2,844
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Should the sun appear very low in the horizon if its path is always between the two tropics?
Your explanation is correct, so you are not remembering correctly. The farther north you go, the lower the sun is to the horizon at noon.
Of course it depends on how far north you go. If you only ...
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3
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Calculate viewing times at a location - when location has an obstruction
I don't know of a chart in a book or online, but I know of some other options:
Nightshift (Android app) will give you transit times and show you
graphs.
SkEye (Android app) will show you graphs.
...
- 1,498
3
votes
Accepted
Is the appearance of the twilight not constant in the interval before the sunrise?
The length of Astronomical twilight isn't constant, but the main reason is the tilt of the Earth, and not the orbital speed of the Earth.
The change from day to night is a result of the Earth spinning....
- 91.4k
3
votes
What parameters cause sunburn?
Yes, all of those contribute to the total irradiance, which is the amount of sun power falling on a particular area, measured in Watts per square meter. You can imagine a 1m$^2$ "window" perpendicular ...
- 1,890
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Is there any formulae to calculate whether the sun will depress 18 degrees or more below the horizon on a certain day / location?
An approximation of the Sun’s declination for a certain date is:
sin δ = 0.39795 ⋅ cos [ 0.98563 ⋅ ( N – 173 ) ]
where N is the number of days since January 1.
That, combined with the formula given by ...
- 4,897
3
votes
Small Angle Approximation Discrepancy in Horizon Dip Angle
This edit is a doozy. The first equation on the right side, "rcos(theta) + Dsin(theta)= h+R" is correct. It's saying that the portion of the r+h line above the yellow chord plus the portion ...
- 1,258
2
votes
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The furthest horizon in the Solar system
The distance depends on the diameter of the planet and of your height above the surface (such as on a mountain). The greater the diameter, the farther away the horizon will be.
You can see this in ...
- 3,321
2
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Does matter accumulate just outside the event horizon of a black hole?
As I understand it, the presence of an event horizon (EH) from gravitational collapse is a case in which GR violates local causality in the outer (w.r.t. EH) universe. By Birkhoff theorem the EH can ...
2
votes
Has the velocity of photons coming from black holes already been measured?
No.
Photons can't come from the singularity of a black hole, or from beyond the event horizon.
While some very good candidates for black holes exist, none have been certainly observed.
We have never ...
- 91.4k
2
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Is there any formulae to calculate whether the sun will depress 18 degrees or more below the horizon on a certain day / location?
First see the plethora of diverse answers to Where can I find the positions of the planets, stars, moons, artificial satellites, etc. and visualize them?
Then if you can use Python or would like to ...
- 31.8k
2
votes
A plot of the Moon-rise/sets on the Horizon Location
We are wondering if the moon wobbles back and forth along the horizon the same way the sun wobbles.
Yes, but over years even more than the Sun does!
Per answer(s) to Does the Honey moon have a ...
- 31.8k
2
votes
Can Mercury be seen from the 60th parallel north on Earth? Shetland, Faroe, Norway, St Petersburg, Alaska
I am the OP, and I can confirm it, I've just seen Mercury from shetland, 31st october 2021 6 am local time. Apologies for posting this on another answer, but I am so excited and happy about it, almost ...
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Small Angle Approximation Discrepancy in Horizon Dip Angle
If you want $D$ to go away so that you have a relationship between $r,h,\theta$,
the exact relationship is
$$
\frac{r}{r+h} = \cos\theta = 1 - \frac{\theta^2}{2!} + \frac{\theta^4}{4!} - \cdots
$$
In ...
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vote
Calculate viewing times at a location - when location has an obstruction
You won't find a reference that tells you the altitude of an object when it passes your meridian. That altitude depends on your latitude. Saturn for all this year is near 18 degrees south declination, ...
- 1,258
1
vote
Accepted
Solar Azimuth angle Derivation
First we set up our global coordinate system as following:
Origin: center of Earth. $\vec z$: earth's axis (North Pole is the positive direction), $\vec x$ and $\vec y$ represent the
equatorial plane ...
1
vote
Solar Azimuth angle Derivation
Have a look at https://www.pveducation.org, specifically section 2.4 on Terrestrial Solar Radiation. It describes the equations required and has some online calculators and interactive plots.
If you ...
- 41
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vote
Solar Azimuth angle Derivation
What I think you're trying to do is workable, but approximate. You first find the Sun's geographical position (GP), the point on the Earth where the Sun is in zenith at the time of the observation. ...
- 1,258
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vote
Should the sun appear very low in the horizon if its path is always between the two tropics?
I am not sure how your observations are so mistaken, but this is exactly what happens. In fact at high latitudes, there will be parts of the year when the sun never rises above the horizon. Try going ...
- 4,969
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