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Questions tagged [astronomy]

The science dealing with objects and phenomena located beyond Earth. In particular, this applies to observations and data. At its core, astronomy is the physically informed cataloging and classifying of the contents of the universe in order to better understand what is out there.

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Why is it necessary to keep UTC within 0.9 seconds from UT1?

Why is it necessary to keep UTC within 0.9 seconds from UT1? I thought that UTC in fact more accurate than UT1, so why should it be based on UT1 within that range? Another question would be, why isn'...
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Could light and heat reflecting off of surface ice on Earth have a noticeable impact on the sun?

If so, what could be the impact of these reflections on the sun during an ice age on Earth?
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Why is there so much iron?

We all know where iron comes from. However, as I am reading up on supernovas, I started to wonder why there is as much iron as there is in the universe. Neither brown dwarfs nor white dwarfs deposit ...
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What percentage of stars are giants/subgiants/main sequence etc

I am trying to find the percentage of main-sequence stars along with red-dwarf percentage for my research paper. I did find one article regarding, however it is very old and i require recent data ...
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What is novel about the analysis showing that Mercury is “closer” to Earth than Venus?

Why is the recent analysis showing that Mercury and not Venus is on average the closest planet considered new? This should surely come as no surprise to anyone who has ever looked at a plot of the ...
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1answer
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Spectroscopy of Stars

Stars are dense balls of gases that have emission and absorption spectrum due to their thin outside layers. But why do stars have such different spectra even though they have essentially the same ...
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Holographic planetarium

Discussing the possibility of building a planetary with some colleagues, we started wondering about the theoretical idea of a planetarium dome based on a multichannel CGH able to show the sky of ...
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Could dark matter be normal matter that is beyond the observational horizon?

As I understand it, matter that is too distant for its light to reach us would not be observable, and the CMB is the limit of what is observable from our location within the universe. Could what is ...
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Can solar mirrors be used for amateur telescopes?

Recently I have been fascinated by astronomy and telescopes. I have a small refactor telescope, but it cannot see very far, so I was interested in buying a larger telescope. Unfortunately, I cannot ...
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37 views

Redshift z values and galactic age

Redshift for astronomical objects is often discussed in terms of z-value which I understand to be $\dfrac{\lambda_{obsv} - \lambda_{emit}}{\lambda_{emit}}$, where $\lambda$ is the wavelength. Is this ...
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How is it possible to see stars-light, from stars millions of light years apart with the naked eye? [closed]

How is it possible for us to see stars that are millions of light years 'APART' from one another with the naked eye when we look at in the night sky from horizon to horizon? (i.e., pick any stars ...
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1answer
101 views

How to calculate initial conditions to integrate a null geodesic

Suppose, this is the line element of a FLRW metric, $$ ds^2 = -[1 + 2ψ(t,x_i)]dt^2 + a^2(t) [1 - 2ϕ(t,x_i)]dx_i^2 $$ and the geodesic equation is, $$ \frac{d^2x^α}{dλ^2} = - Γ_{βγ}^α \frac{dx^β}{dλ} \...
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What orbit does a planet have to have to be a planet? [closed]

Other questions have dealt with where you could find a planet-like object, and what a planet has to consist of to count as a planet. But this question is directed as what orbit or path an object would ...
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How do we quantify and measure the net entropy of today's universe?

We know that the Universe evolved from a very low entropy state in the early universe to very high entropy state of today's universe. What is the quantitative definition of the net entropy of the ...
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1answer
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Declination as a function of orbital position

Reading this Wikipedia article and following the derivations (https://en.wikipedia.org/wiki/Solar_irradiance#Milankovitch_cycles), I have found an equation I can't see where it comes from. Perhaps it'...
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If I held up a glass lens and looked at a star through it, could I see the spectrum of the light?

Given that I can see light from stars with my naked eye. And also some lenses suffer chromatic aberration. I would think that with enough training I could simply hold up a lens to a star and look at ...
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1answer
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What aberration causes the common “cross pattern” in astronomy images? [duplicate]

I am trying to understand the origin of the plus/cross-sign aberrations commonly seen for point sources in astronomical images, even ones created with professional-grade, high-quality optics. The one ...
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Observed Data for Asteroid Lightcurve

I am looking to find out more about writing statistical analysis programs in python for astronomy problems and was recommended that I could analyse asteroid light curves for an introduction into the ...
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1answer
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What does the image from Eddington on gravitational light bending really show?

There is a famous image that one often sees taken by Eddington during the 1919 solar eclipse illustrating gravitational light bending as predicted by Einstein. Can someone explain what exactly is ...
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1answer
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Why multiply by $\frac{\pi}{4}$ when converting a light source's angular diameter to its solid angle?

Why does the formula $$\Omega =( \text{angular diameter} * \frac{\pi}{180^{\circ}})^2 * \frac{\pi}{4}$$ give a more accurate calculation of the solid angle of a light source from its angular diameter ...
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1answer
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Why does the red light from Sun bend towards the moon during the total lunar eclipse?

During the total lunar eclipse the earth, moon and the sun are in perfect alignment so that the moon is within the umbral shadow of the earth. However, instead of turning invisible the moon actually ...
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1answer
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How did Edwin Hubble estimate the velocity of distant stars?

I didn't know how to be succinct with my actual question in the title. My question is, how did he separate the components of the actual velocity of stars from that of the velocity due to recession (...
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What in optics prohibits telescopes on jets travelling at night?

What in optics prohibits telescopes on jets travelling at night? It would be a nice addition to our sky sweep to have telescopes on at least long distant night flights. Anything in optics preventing ...
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What is angle between ray to center of the sun and its perimeter?

If I took a ray from a point on the earth P to the center of the sun. And another ray from the same point on the earth to the perimeter of the sun. What would the ...
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1answer
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Is there a bound on the number of sterile neutrinos from cosmological observations?

The right-handed sterile neutrinos $\nu_{R}$ are electroweak singlets. They do not contribute to the electroweak anomaly, and therefore, their number is not fixed by the requirement of the anomaly ...
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1answer
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What is the least energy trajectory in Astronomy?

Hi I wanted to know what the least energy trajectory is in relation to astronomy as I can't seem to find a good answer on Google. I want to know because I am trying to answer this question: A ...
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3answers
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Thought experiment. Earth orbital motion if we stopped Earth spinning

This is a thought experiment that puzzles me. Imagine we could magically stop Earth’s rotation around its own axis (and locally verify it by disappearance of inertial forces generated by the spin ...
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1answer
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Doubts about the formation of the bullet cluster

The bullet cluster is formed by the collision of two clusters of galaxies. After the collision, the stars and galaxies in those two clusters passed through each other. But the intergalactic gas clouds ...
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0answers
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What is the most measurable electromagnetic wave coming from a small, dark, and not necessarily hot object in sky? [closed]

Optical fibers are recently used to generate some spectroscopic surveys of the universe map based on some projects such as DESI, MOONS, SDSS-V, etc. The current fibers are often sensitive to visible ...
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1answer
97 views

Did Eratosthenes make his measurements on the same day?

Erasthosenes measured the circumference of the earth on the summer solstice by observing the sun at Alexandria and Syene. Was this done on the same day in one year or on the same day in two ...
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What are the starting units to perform the Gauss method?

To calculate the Gauss method, we need : the respective geodetic latitude the equatorial radius of the body (here earth) the respective local sidereal time the respective declination the respective ...
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1answer
46 views

What is Earth's current true anomaly from j2000?

I enjoy amateur astronomical calculations. I calculate earth true anomaly and noticed a bug in my software just as earth passing through j2000 point and instead of going to 0 degrees it subtracting ...
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1answer
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'Harmony of the Spheres' or 'Harmony of the World' - is it debunked? [closed]

I sincerely apologise for this vague question but I'm writing an essay for my music class on musical harmony and it's historical origins. I came across the Pythagorean notion of Harmony of the Spheres ...
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Show me how the sky is like in Jupiter (but not mars)

Assuming someone is "standing" on the "surface" of Jupiter (ok, in a stable position deep inside the gas giant's gravity field analog to standing on the surface of a rock planet) where the moons can ...
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1answer
39 views

Does cosmological homogeneity imply that density is also the same?

So I know that at sufficient scales, the universe looks the same (Same average amount of galaxies, clusters, stars, etc). Does this mean that at these scales the density of the universe is also the ...
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0answers
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Strength of core-envelope coupling in stars

For a star with a given Zero-Age Main Sequence (ZAMS) mass, as a function of metallicity how strongly is the star's core coupled to its envelope? I understand that the core-envelope boundary is only (...
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1answer
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What do “broad” and “narrow” mean in the context of spectral lines?

For example, at this page: http://pages.astronomy.ua.edu/keel/agn/spectra.html it talks about some lines being broader than others. But they all look the same width to me. I feel like this is ...
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Stellar aberration: distal or local? Experimental proof?

Stokes put forward the theory (I am simplifying here.) that stellar aberration takes place out in space and that light from stars is already fixed in direction with respect to the Earth's surface by ...
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Where did the energy from the CMBR go? [duplicate]

Basically the title. The wavelength of light from the CMBR has expanded as the universe expanded and so it has lost energy since it is inversely proportional to the wavelength. Where did this energy ...
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2answers
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Does the Sun cause star to appear in the sky on two different positions?

So, I was reading A brief history of time and it was talking about the sun bending the light of a star causing it to appear on a different place from where it actually was located. I was wondering if ...
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1answer
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What is the solar wind speed as function of distance from the Sun?

Someone must have calculated at least a "zero-order approximation" of the behavior of solar wind speed, density, and pressure as a function of distance from the Sun, but the heck if I can find it. ...
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Why do we say the three-dimensional space is flat (in Physics)? [closed]

This is quote from Hawking's book: The surface of the Earth is what is called a two-dimensional space. That is, you can move on the surface of the Earth in two directions at right angles to each ...
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Introduction to Astronomy books

I am looking for a book that covers an introduction to astronomy. Currently, I am reading Introductory Astronomy and Astrophysics, by Zeilik and Gregory. However, I quickly noticed that it is much too ...
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1answer
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Optics of an all-sky camera

I'm curious about the optics of an all sky camera like this one or this one. My suspicion is that the sensor has an inherently wide field of view - the former has a lens that's 1.55, which doesn't ...
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1answer
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Milky Way Density

It seems to be a simple question, but I wasn't really able to find an appropriate answer: How dense is the Milky Way? I am certain that there are reliable statistic, maybe even new ones from the GAIA ...
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1answer
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Difference between Airy disk and seeing disk

In the diffraction pattern for a circular aperture, instead of just having a circle, we have a central disk and several faint rings around the circle. Does the Airy disk consist of only the central ...
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1answer
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What will be the expression between conformal time and scale factor for $\Lambda$ CDM model (for $k=0$, $\Omega_M + \Omega_\Lambda = 1$)? [duplicate]

An approximation of the relationship between the scale factor and time for our universe is given by the following expression: $\displaystyle a \approx \left (\frac{\Omega_{M_0}}{\Omega_{\Lambda_0}}\...
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Why are there so many different “colors” in the line spectra of simple elements?

I understand that atoms emit photons of wavelength $\lambda$ when electrons transition to lower orbitals according to the equation $E = \frac{hc}{\lambda}$. Based on my understanding, those orbitals ...
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How massive is the Sun compared to other stars?

I was just reading "Rare Earth" by Peter Ward and Donald Brownlee and they claim that $95\%$ of stars are less massive than the Sun. But, I was always told the Sun is an average star. Which is it?
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How is red colour from from rainbow hotter than blue (W. Hershel) if it has lower frequency than the latter?

We know that blue light has a smaller wavelength and higher frequency than red light, which is a consequence of higher energy in the former, then how is it true that, when scattering light from the ...