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Questions tagged [space-geometry]

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3 votes
1 answer
109 views

visible solid angle of the moon

I'm reading an astronomy textbook for high schoolers, it contains an exercise to calculate the visible angle ratio of the moon in the sky (the textbook is not in english so my terminology is probably ...
2 votes
0 answers
153 views

What happens at the edge of a finite and "bounded" universe?

So far, we know only about two types of universes: Infinite and finite universes (Unbounded edition). In an infinite universe, there is no edge. Because well, an infinite space doesn't have an edge. ...
2 votes
2 answers
352 views

κ₀ for Mercury—Formula

I refer here to Ptolemy’s epicycle-and-deferent model of the Solar System, specifically that of Mercury (see drawing). In this model, Mercury (not shown) revolves on an epicycle of center C, which ...
2 votes
1 answer
180 views

By analytical geometry, how to calculate the fraction of the visible area of the Sun at a point during an eclipse?

Assume a perfectly spherical Sun and Moon, and ignore atmospheric effects for simplicity. I'm an observer in the origin of my coordinate system, and I know the positions of the centers of the Sun and ...
1 vote
0 answers
27 views

Spacetimes where symmetries vary from place to place?

Are there spacetimes or metrics where symmetries (like Poincaré, Lorentz, diffeomorphism, translational... invariances) are only local and the symmetries of one local neighbourhood are not, a priori, ...
2 votes
1 answer
80 views

Are there non-smooth metrics for spacetime (that don't involve singularities)?

I found this statement in a discussion about the application of local Lorentz symmetry in spacetime metrics: Lorentz invariance holds locally in GR, but you're right that it no longer applies ...
0 votes
1 answer
142 views

Can the curvature of a closed universe be measured?

Suppose we were in a closed universe likened with a spherical surface such as that of the Earth's surface. Assume also that the limits, represented by the limits of the observable universe, ...
4 votes
1 answer
288 views

Is it possible to overcome the problem of blind spot(s) of current gravitational wave detectors?

If gravitational waves (GWs) pass through specific points (which are known as blind spots), current GW detectors aren't able to detect the passing waves. In the future, will we be able to completely ...
6 votes
1 answer
299 views

Can we consider the Universe to be some kind of 3-sphere?

This is probably a naive question. I'm learning a bit of cosmology and I've recently covered the so called angular size-redshift relation, which states that in an expanding Universe the angular size ...
7 votes
0 answers
255 views

Get the expression of probed volume between 2 redshifts

1) I can't manage to find/justify the relation (1) below, from the common relation (2) of a volume. 2) It seems the variable r is actually the comoving distance and not comoving coordinates (with ...
4 votes
3 answers
1k views

How to calculate the nodal precession range of the Moon with respect to the celestial equator?

In the span of 18.6 years there is nodal precession of the Moon with respect to the ecliptic of 360 deg. But if we consider this precession with respect to the equator, we will find (unless I'm hugely ...
2 votes
1 answer
409 views

What is the thickness of black holes event horizon?

I would premit that I am just an enthusiast, so don't expect a good background or a deep mathematical knowledge about the topic. I just read an article about how light is reflected by a black hole (...
30 votes
6 answers
6k views

Is the universe considered to be flat?

I've read various articles and books (like this one) stating that we are not certain about the geometry of the universe, but there were experiments on-going or planned that would help us find out. ...
3 votes
1 answer
166 views

What do falsifiability and verifiability actually mean as a precondition of a scientific hypothesis?

What do falsifiability and verifiability actually mean as a precondition of a scientific hypothesis? I have been hearing from science guys that unlike philosophical hypothesis or theory, to be a ...
1 vote
0 answers
49 views

Determining observer's position based on azimuth and elevation of observed object

It is possible to generate Ephemeris through the web-interface of JPL's Horizon system based on observer's location. Unfortunately I don't know the formula that is used to do that with a high ...
5 votes
1 answer
4k views

Does the sky get darker faster during the winter?

When one lives away from the equator, say at around 37° latitude, does the sky get darker faster after the sun sets in the winter as compared to the summer? Why is this the case? I believe that if ...
5 votes
2 answers
419 views

Data to "check" Kepler’s first law

I want to "check" Kepler’s first law by using real data of Mars. From the equation of the ellipse, I derived $$\frac{1}{r}=\frac{a}{b^2}+\frac{a}{b^2}\cdot\epsilon\cdot\cos(\varphi),$$ where ...
2 votes
1 answer
442 views

Why both sunrise and sunset use the position of the sun's upper limb?

It is my understanding, that astronomical moments of both rise and set use the relative position of background object's upper limb to the foreground object's horizon. When were these definitions ...
5 votes
1 answer
480 views

What decides the shape of a galaxy?

As I know, most galaxies are spiral, some elliptical, and a rare type is ring-shaped. What causes a galaxy to become a specific shape (i.e. ring vs spiral)?
3 votes
2 answers
427 views

How is the size of a distant planet determined?

How is an object's size determined from other solar systems than our own? How are new large bodies being found in our solar system currently?
10 votes
2 answers
251 views

How did we first determine the Earth's departure from sphericity?

I'm quite interested to know the history of the study of the Earth's departure from spherical symmetry. What were the first methods used to model the non-spherical geometry of the Earth? For example, ...
1 vote
1 answer
81 views

What prevents consequent eclipses? [duplicate]

I know there are a number of parameters that determine the occurrence of a solar or lunar eclipse. The number should be 18; 6 degrees of freedom for 3 rigid bodies. But can't figure out why eclipses ...
-1 votes
1 answer
295 views

Why is the volume of Saturn is 764 times bigger than Earth, but its surface area only 83 times bigger? [closed]

If Saturn is 83x bigger that Earth by surface area why isn't the volume also 83x bigger than earth by volume? Is there a geometric or astronomical reason?
3 votes
1 answer
2k views

How to project galaxy data in x y z coordinates?

For a data visualization project, I want to extract galaxy data (in x y z coordinates) from galaxy catalogue of within 150 mpc and project each of those galaxies as dots and visualize them in a 2D ...
3 votes
1 answer
114 views

How certain are we about the universe's flatness?

The universe is thought to be flat: $\Omega = 1 \pm 1\%$. As I understand it we can determine this by measuring triangles against the CMB. Yet during inflation dark energy made the universe grew ...