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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Polaris distance correction and implications for cosmological measurements

Polaris has been a guiding light of navigation for centuries. But Polaris also happens to be the closest cepheid variable to our sun. These, together with the type II-A supernovae constitute standard ...
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3answers
424 views

How much of the universe is observable at visible wavelengths?

Knowing that: The Zone of Avoidance (Looking towards the center of the Milky Way) blocks roughly 20% Each Milky Way star has an angular size, depending on proximity, that obscures a certain ...
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5answers
707 views

Why cant one see tidal effects in a glass of water?

Why cant one see the tidal effect in a glass of water like in an ocean?
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4answers
2k views

View of the Sun from Voyager 1

Today Voyager 1 is approximately 122 AU distance from the Sun. What does the sun look like from this distance? How much brighter does the sun appear to the naked eye compared to other stars in the ...
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1answer
995 views

what is the farthest-away individual star that is visible with the naked eye?

My girlfriend and I were watching Cosmos, and something Carl Sagan said got us wondering what the farthest-away visible star is. Obviously "visible to the naked eye" is a fuzzy enough concept that ...
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4answers
354 views

How accurate are Mayan astronomical “ephemerides”?

Because of the hype surrounding the "end" of the Mayan calendar (along with the usual cultural relativism and Western guilt) it is nearly impossible to find an objective quantitative assessment of the ...
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1answer
169 views

How do physicists and astronomers handle leap seconds?

I'm confused by the many contradictory descriptions I see about how UTC leap seconds are accounted for. I understand that there are various ways to handle them in common practice, and I've seen a ...
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5answers
307 views

Why relative positions of stars and other objects in a galaxy do not change over the year

Though the identifiable stars in a constellation(say andromeda) and galaxy(say andromeda galaxy) are situated light years apart, why isn't the galaxy appearing at different positions(with respect to ...
4
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1answer
188 views

What time scale is used by the JPL HORIZONS system?

I'm confused by the ust of the term "UT" in the description of time scales used by the JPL HORIZONS system. Their manual states that UT is Universal Time This can mean one of two non-uniform ...
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2answers
315 views

Exploding Earth

I have always heard that the Earth, due to its structure, cannot explode. Now, I'm quite fascinated by astronomy physics but I admit I only hold some more than basic knowledge. In any case, even ...
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0answers
68 views

Elimination of the node

Recently I have been reading a lot of astronomy papers on reductions in models of the solar system. A reoccurring concept is the elimination of the node. However, they never explain what the node is ...
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1answer
167 views

How long in advance can dates of moon phases be accurately predicted?

When researching an article on Chinese New Year I found a few websites that predict the dates of Chinese New Year (CNY) tens - or sometimes even hundreds - of years in advance. I´m wondering how many ...
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1answer
685 views

How distorted does the Andromeda Galaxy appear to us due to the speed of light?

The Andromeda Galaxy appears to us at an angle to the galactic disk, i.e. we are not in the Andromeda Galaxy disc's plane, nor are we near the direction that the galaxy's pole points. Therefore, due ...
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1answer
159 views

Is there an “authoritative” source for ephemeris data?

I find some variation in the values reported for ephemeris by the various sources I have access to. For example for 2012-11-27T03:31:55 UTC I get solar declination values of -21.1828°, -21.18296°, ...
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5answers
528 views

How is distance measured to far away stars and galaxies?

What I need is an accurate description of the methods used to determine the distance to Andromeda. The Parallax method is for nearby objects as I presume. The red shift method applies, but how do you ...
2
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1answer
161 views

Fractal Cosmology and Misner's Chaotic Cosmology

I have a question pertaining to the ideas behind the considered homogeneity and isotropic nature of the universe (at a grand scale) versus the theory of a chaotic and anisotropy structure of the ...
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3answers
5k views

What are good books for graduates/undergraduates in Astrophysics?

There are no book recommendations for Astrophysics here. I will write my own answer, but I am also interested in what are others' views on the question (I will NOT mark my own answer as the best one). ...
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1answer
104 views

If the observable universe keeps expanding would we eventualy see light 24/7… clouds permiting? [duplicate]

If we can see more observable universe could there be a time when day and night looked the same?
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2answers
162 views

Is there any dynamical reason for the winter solstice to happen close to the perihelion?

When the winter solstice arrives, the angular momentum of the Earth, its orbital angular momentum and its radius vector with the orbital focus in the Sun are in the same plane. This happens quite ...
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2answers
242 views

Why does a blue sky at dusk appear nearly black through a telescope?

Earlier this evening I was looking at the Moon through my cheap toy telescope (x150 magnification) when I noticed a (rather mundane) optical effect I couldn't explain. The Sun had just dipped below ...
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2answers
110 views

A thought about Quasars

If Quasars are "beams" of energy exiting a super-massive black hole, in order for them to get through the black-hole's event horizon, they'd have to be traveling faster than the speed of light. My ...
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2answers
250 views

Is there a “present state” of distant stars if simultaneity is relative?

Special relativity theory says simultaneity is relative, meaning that different observers will not agree on what happened first and what second. Does it then make sense to say that looking at distant ...
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2answers
453 views

What accounts for the discrepancies in my calculations of year lengths?

A common exercise in many introductory astronomy texts is to use the lengths of various kinds days to calculate the approximate length of the corresponding year. For example, ratio $k$ of the length ...
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1answer
166 views

What is the likelihood of a long-period comet impact?

I always see lots of information about asteroid impacts, but very little is said about comet impacts. As I see, a long period comet impact is much more dangerous because a) we wouldn't have time to ...
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1answer
193 views

What day/night cycles, climate and seasons would experience Alpha Centauri Bb inhabitants?

Alpha Centauri Bb is an exoplanet orbiting Alpha Centauri B. It is asserted that given the close distance to the star the planet should be tidally locked. The orbiting period of the planet is about ...
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2answers
411 views

Does the length of the sidereal day vary systematically?

I'm confused about some properties of the sidereal day, in particular whether its duration varies systematically over the course of the year.1 It seems to me that that must be the case, but the ...
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3answers
130 views

How is a star's parent galaxy recognized?

A star is probably visible/detected by it's radiation. But that star may or may not belong to our own galaxy ... yet news reports speak of detecting a star/nova in a distant galaxy. How does one ...
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2answers
592 views

What is the formal definition of a stellar day?

I'm having trouble understanding precisely what a stellar day is. Neither the USNO nor the IERS sites provide a definition. And Wikipedia's description as the "rotation period relative to the fixed ...
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1answer
2k views

Why are the orbits of the planets in our solar system along the same basic plane? [duplicate]

Possible Duplicate: Why are our planets in the solar system all on the same disc/plane/layer? After watching this video I realized that the orbits of the planets in our solar system ...
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0answers
102 views

What is the correct way of integrating in astronomy simulations? [closed]

I'm creating a simple astronomy simulator that should use Newtonian physics to simulate movement of plants in a system (or any objects, for that matter). All the bodies are circles in an Euclidean ...
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1answer
227 views

Future space-based telescope array

Radio-telescopes (e.g. the Very Large Array (VLA)) can simulate one gigantic dish by using separate smaller dishes. Q: Could such an array of optical telescopes potentially see an exoplanet at say 20 ...
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2answers
159 views

how do we know that the base of entire universe is the proton (hydrogen) and not the antiproton?

It may be that the base of a part of the world is anti-proton, We've always been on the planet Earth and the Milky Way. how do we know that the base of entire universe is proton (hydrogen atom)?
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1answer
348 views

Is there a simple, accurate formula for calculating transit times from rise and set times?

I have access to "rise" and "set" times for astronomical objects, and want to determine the corresponding times of culmination. Is there a reliable and accurate way to do this? It isn't clear to me ...
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1answer
84 views

Alpha Centauri Bb: Comparing astrometric precision vs doppler precision

How do you compare astrometric precision vs doppler spectroscopic precision? In particular will the Gaia spacecraft to be launched next year be able to confirm the existence of Alpha Centauri Bb? ...
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1answer
86 views

Are comets known to exist in other star systems?

Are comets a feature unique to our Solar System? Or, are comets/cometary clouds detected around discovered/observed extra-solar systems too? If they were detected elsewhere, how do such cometary ...
3
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1answer
130 views

What would happen in the final days of the universe?

I would like to know the stages of how the universe would end and what would happen and what the possible scenarios are. I understand that eventually all the stars would burn out and that would ...
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1answer
2k views

What is the relationship between mass, speed and distance of a planet orbiting the sun?

After reading this fascinating story about a new exoplanet, I was wondering about how mass, speed and distance determine a circular orbit of a planet around a star. Given the mass of the sun and ...
2
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1answer
78 views

closure phase measurement in astronomy for vlbi technique?

I have read this page on closure phase and could not understand the following: \begin{align} \psi_1 & = \phi_1 + e_B - e_C \\ \psi_2 & = \phi_2 - e_B \\ \psi_3 & = \phi_3 - e_C. ...
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1answer
99 views

Is “Egyptian Year” the same as a modern sidereal year?

Copernicus uses the term "Egyptian Year" throughout his discussions of the movements of the Earth, and of his and other models of the movements of the planets; but is unclear from his text, or from ...
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1answer
2k views

How do astronomers measure the distance of the planets from its star?

I know they have accurate means of measuring the distance of the planets in the Solar System from the Sun. I'm skeptical how can they use the same tools or techniques for other systems so far away as ...
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1answer
533 views

Spaghettification of humans near black holes

A few months ago I was discussing the spaghettification phenomenom with my wife, just for the fun of it. This was when the mass of the super massive black hole from M87 hit the news. The black hole ...
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1answer
87 views

Does the spectrum of Sol's emission change as it ages?

A follow-up to my earlier question How would one navigate interstellar space? that just occurred to me; albeit on a different tack. Sol is probably in a state of continuous flux. The change of state ...
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2answers
128 views

What is an extreme deep field image (XDF) and how is it captured?

NASA recently took this Extreme Deep Field image (XDF) that is the area of only small fraction of the dia of moon and contains 5,500 galaxies. Nasa says this was capture by extreme long exposures so ...
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2answers
457 views

Magnitude of New Comet C/2012 S1 (ISON)

A new comet (magnitude 18.8) has been discovered beyond the orbit of Jupiter. Comet ISON will get within 0.012 AU of the Sun by the end of November 2013 and ~0.4 AU from of Earth early in January ...
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1answer
138 views

How is retrograde motion explained in Tycho's model?

Tycho proposed a model of the solar system where all planets but Earth move around the sun while the sun and the moon move around the earth. I wonder how this model could explain the retrograde ...
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1answer
61 views

how is the reference for the star maps in google determined?

If I go to google sky - http://www.google.com/sky/ Ihere is a longitude and latitude measure at the bottom of the page. Iow are these coordinates determined and who sets it? Do those coordinates ...
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6answers
5k views

Why is the solar noon time different every day?

If you check the local time for solar noon is different every day. Why is it so? Is it because Earth doesn't make a complete rotation in exactly 24 hours?
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1answer
141 views

What are “cycles of anomaly” and “cycles of longitude”?

In several early (pre-1600) astronomical texts I read about "cycles of anomaly" and "cycles of longitude", but it us unclear to me what these terms mean. They were clearly familiar to authors at the ...
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0answers
132 views

What is the angular distance between Ptolemaic perigees of Mercury?

In his excellent treatment of the history of the science of astronomical distances and sizes, Albert van Helden says (p.29) that The complicated [Ptolemaic] model of Mercury has the curious ...
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1answer
302 views

Why are solar eclipses more common in the southern hemisphere?

I've seen the claim that solar eclipses are more common in the southern hemisphere than the northern hemisphere and would like to understand why and if that is the case? Does it relate more to the ...