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Questions tagged [observational-astronomy]

Questions about the techniques and practice of observing the night sky.

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77 votes
4 answers
10k views

How does the Earth move in the sky as seen from the Moon?

I just want to be sure I am visualizing this correctly, because it seems odd. The Moon is tidally locked to the Earth but there are wobbles to its motion due to libration. So from a point on the ...
kim holder's user avatar
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62 votes
6 answers
8k views

Why hasn't the "9th Planet" been detected already?

In the comments to this question, there was considerable doubt placed as to the subject of if the so called "9th planet" really exists. That wasn't really the intent of the previous question, so I ask ...
PearsonArtPhoto's user avatar
53 votes
4 answers
7k views

Is there any role today that would justify building a large single dish radio telescope to replace Arecibo?

It seems that most of the modern radio astronomy instruments and observations that make the news are interferometers or phased array systems of one kind of another. Is there any application left for ...
Steve Linton's user avatar
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48 votes
3 answers
16k views

If Earth is tilted, why is Polaris always above the same spot?

Why is Polaris, the North Star, always above (or near) the North Pole? If Earth is tilted, Polaris' path should be in winter 23 degrees away from its path in summer, or not?
Guest55's user avatar
  • 379
39 votes
3 answers
993 views

If you lived on the far side of the Moon, how could you infer the existence of Earth?

Suppose that you deposit an astronomer, armed with our current knowledge of orbital mechanics, on a dome on the far side of the Moon, so that the Earth is perpetually hidden from them. (And, of ...
Emilio Pisanty's user avatar
37 votes
3 answers
2k views

If Alpha Centauri A's solar system exactly mirrored our own, what would we be able to detect?

Suppose there was an exact replica of our solar system 4.4 ly away (people included). What would we be able to detect and with what telescope(s)? Which planets? Could we detect radio transmissions and/...
Nick T's user avatar
  • 471
36 votes
2 answers
6k views

How was the mass of Venus determined?

The mass of Venus seems rather complicated to determine to me: Venus doesn't have any satellites, so you can't just apply Kepler's third law (like you would with Jupiter or Saturn for instance) to ...
usernumber's user avatar
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35 votes
3 answers
6k views

Can Pluto be seen with the naked eye from Neptune when Pluto and Neptune are closest?

When Neptune and Pluto are closest, about 100 million mi (160 million km) from each other, would an observer on Neptune (or rather on one of its moons, since Neptune is gaseous) be able to see Pluto, ...
Plutos Loyer's user avatar
34 votes
1 answer
2k views

Why is there a gap in this image of supernova discoveries?

I came across this gif showing supernova discoveries from the late 19th century to 2010. Here's the data in 2010: Notice that there's a prominent region shaped like an inverted U in which there are ...
HDE 226868's user avatar
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33 votes
1 answer
6k views

Why did it take so long to invent telescopes given glass was used 4000 years ago in Mesopotamia?

Is a telescope difficult to make? Does glass have to be polished and shaped very precisely? Or is a device using two or more lenses to magnify things just not obvious?
user avatar
31 votes
2 answers
3k views

Which of the planets would be detected if they were exoplanets?

An (almost-)omnipotent being selects the solar system, presses Ctrl+C, and then Ctrl+V several times, creating copies at distances of 5, 500, 50.000, and 5.000.000 light-years away. All in a direction ...
Ink blot's user avatar
  • 421
31 votes
3 answers
4k views

Why can't we observe the Oort cloud with a telescope?

The Oort cloud is a hypothetical structure based on our observation of long-period comets. There are currently proposals to design probes to confirm the existence of the Oort cloud. Now, sending a ...
called2voyage's user avatar
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28 votes
1 answer
5k views

How is it possible that we haven't discovered anything in the Oort cloud yet?

The Oort cloud is a hypothetical cloud of small icy bodies surrounding the Sun at more than 1000 AU. It is thought to be a vast reservoir of comets that occasionally get disrupted, sending comets ...
usernumber's user avatar
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27 votes
4 answers
3k views

Why did they decide to hit Dimorphos in the retrograde direction rather than prograde; was it a "coin-toss" or were there implications for observing?

CNN Space and Defense Correspondent Kristin Fisher does a really good job of summarizing the current state of the Double Asteroid Redirection Test early observational results for general public ...
uhoh's user avatar
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27 votes
1 answer
5k views

Why is the summer solstice night shorter than the winter solstice day?

I was looking at the sunrise and sunset times where I live (Aberdeen, Scotland) and I noticed something odd: the time between sunrise and sunset in winter is longer than the time between sunset and ...
Nierninwa's user avatar
  • 373
27 votes
3 answers
3k views

Is it possible to detect gravitational lensing of stars behind the Moon?

Eddington waited for a total solar eclipse to happen to be able to observe gravitational lensing of the stars behind the Sun. And nowadays, amateurs can do the same thing. Of course, the Moon is much ...
usernumber's user avatar
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25 votes
5 answers
3k views

Is it possible to do planet observation during the day?

Keep in mind I'm just a newbie. So.... I have a new Newtonian telescope. It has 150mm opening and 1400mm Focal Distance on top of an equatorial mount. I have 25 and 10 mm eyepieces... and a 2x Barlow. ...
eftshift0's user avatar
  • 413
25 votes
4 answers
4k views

Are there any double stars that I can actually see orbit each other?

If I had a nice amateur telescope†, are there any multiple star systems that I could observe over a few years or a few decades and actually see the movement of one or both of them over time? My short ...
uhoh's user avatar
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25 votes
2 answers
4k views

Do these results mean that I have found this exoplanet?

Yesterday I was going through the TESS mission data on mast portal and after applying some filters I found some data. I started analyzing that data using the lightkurve library. plot without using the ...
Param Kasana's user avatar
24 votes
8 answers
14k views

Why can't we see distant galaxies with the naked eye?

If light keeps travelling in a straight line, why can't we see distant galaxies with the naked eye? Surely if you stared long enough, the light from them would eventually hit your eye? I apologize if ...
Guit4eva's user avatar
  • 341
24 votes
2 answers
4k views

At what distance from Earth would our Sun be the same apparent magnitude as the next brightest star in the sky?

When I stand outside looking at the night sky, to my untrained eye, everything except the moon looks like a star. I know intellectually that some are planets circling our sun, and some are entire ...
James Jenkins's user avatar
23 votes
2 answers
4k views

What ground-based observational evidence is there that the Moon doesn't have an atmosphere?

The Moon doesn't have any significant atmosphere (surface pressure is $3\times10^{-15}$ bar). Can the absence of atmosphere on the Moon be determined with ground-based observations? When was it first ...
usernumber's user avatar
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23 votes
3 answers
3k views

How can I measure the angle between two stars?

This is a backyard astronomy question. My middle-school-aged son and I would like to measure the apparent angle between two easily-visible stars more precisely than measuring with our outstretched ...
Wyck's user avatar
  • 431
23 votes
2 answers
3k views

What observations can be expected on LIGO if any when Betelgeuse goes supernova?

What observations can be expected from the LIGO† gravitational wave observatory if and when Betelgeuse goes supernova? Would we know that Betelgeuse has gone supernova before we see it light up our ...
Lord Loh.'s user avatar
  • 339
23 votes
1 answer
6k views

Why can't observatories just stop capturing for a few seconds when Starlink satellites pass though their field of view?

Given that the positions and trajectories of the Starlink satellites are public, why can't the telescopes just ignore the photons they receive when the satellites pass through the field of view? It ...
Alexis Delrieu's user avatar
22 votes
6 answers
8k views

Puzzling quotes from astronauts about earth size

I have no doubt that we have been to the moon. This question has nothing to do with a moon landing hoax. But, there are two quotes from two different astronauts regarding the size of the earth as ...
Lambda's user avatar
  • 514
22 votes
2 answers
4k views

Please check my Mars photo

Hi built my own newtonian telescope for the first time. Up until that point I had never used a telescope. Can you please look at the photos attached and tell me if this is what Mars should look like ...
Edd's user avatar
  • 229
22 votes
2 answers
2k views

Why are wavelengths shorter than visible light neglected by new telescopes?

The diagram below, which I stole from this post by @HDE226868, shows that angular resolution as a function of wavelength suddenly drops by three orders of magnitudes from visible to UV-light. The ...
LocalFluff's user avatar
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22 votes
4 answers
15k views

How Would a Neutron Star Actually Appear?

Having seen many pictures produced by artists of neutron stars and planets that orbit some of them, I was wondering how a pulsar would appear to a human being, in visible light (assuming the intense ...
user avatar
21 votes
4 answers
6k views

Where is it possible to observe the Moon for 24 hours?

I wrote an astronomy olympiad yesterday and there was one task I couldn't figure out an answer to: Estimate at what latitudes it is possible to observe the Moon for at least 24 hours. Under what ...
ALiCe P.'s user avatar
  • 1,017
21 votes
5 answers
5k views

How was Trappist-1 discovered?

I was going through all the questions in this community related to TRAPPIST-1 in order to know how planets TRAPPIST-1b to TRAPPIST-1h were discovered, but there aren't any. How were they discovered?
Hammad Nasir's user avatar
21 votes
1 answer
3k views

Why doesn't dark matter clump strongly in the center of galaxies, since it doesn't feel either radiation pressure or the Pauli exclusion effect?

Dark matter is described as being spread not only throughout a galaxy, but also around it in a halo of some sort that extends far beyond the visible parts of the galaxy... In fact, dark matter haloes ...
Kurt Hikes's user avatar
  • 5,327
21 votes
2 answers
3k views

Does seeing a gravitationally lensed/magnified galaxy imply that they could also see us as well?

Does an observer in that galaxy see our galaxy magnified as well?
2080's user avatar
  • 1,800
21 votes
2 answers
2k views

Are there any galaxies which fell out of sight horizon due to cosmic expansion?

If farthest galaxies run away from us with acceleration making them exceed speed of light, we should expect them to disappear from sky among time with increasing quantity. Did we observe this? Can we ...
Waldemar Gałęzinowski's user avatar
21 votes
2 answers
1k views

How will Starlink affect observational astronomy?

I have recently found that people around the world have been posting observations of a line of moving objects in the sky which is very likely to be a part of Starlink satellite constellation, such ...
Leo Liu's user avatar
  • 381
20 votes
2 answers
9k views

How many pixels could an image of Proxima b taken by James Webb have?

I know it's very difficult for the James Webb Space Telescope to image the exoplanet Proxima b without an external coronograph (I have been told by NASA scientists that they don't know yet whether ...
James's user avatar
  • 301
20 votes
2 answers
5k views

Why will HD 84406 be chosen as the first target for testing JWST?

HD 84406, is a star approximately 241 light-years away in the constellation of Ursa Major. HD 84406 will be the first star to be imaged by the James Webb Space Telescope in order to test the focus of ...
NeutronCat's user avatar
20 votes
1 answer
2k views

"Next Generation Arecibo Telescope (NGAT)... would combine a 314-metre-wide platform with a swarm of 9-metre dishes on top" What would that look like?

In Nature News' August 11, 2023 Closing down an icon: will Arecibo Observatory ever do science again? found in The Observatory there is discussion of the Next Generation Arecibo Telescope (NGAT): ...
uhoh's user avatar
  • 30.3k
20 votes
2 answers
2k views

When was the martian dichotomy first observed?

The North and South hemispheres of Mars are very different one from another. They have different elevations, different crust thickness, different surface ages. This is known as the martian dichotomy. ...
usernumber's user avatar
  • 17.6k
20 votes
1 answer
462 views

How often do comets survive passage by the Sun?

I had heard that comet ISON might not survive a close pass by our Sun, and I was curious about the odds of how other comets had fared. So, how often do other comets survive a close pass by our sun? ...
Sarah Szabo's user avatar
  • 2,043
19 votes
6 answers
6k views

Could we detect the light of cities at night in other solar systems?

Would it be possible to see electric lights on distant planets using current telescopes?
user avatar
19 votes
6 answers
6k views

Why do small mirror imperfections matter with modern computers

Modern telescopes go to great lengths to have perfectly shaped parabolic mirrors. My question is, why go to the trouble of having a perfect mirror? Why not take a mirror roughly the right shape, and ...
DanielOfTaebl's user avatar
19 votes
3 answers
3k views

Why can space telescopes see through a planetary nebula?

I recently read the book "An Introduction to Planetary Nebulae" by Jason J. Nishiyama. Although I'm not an astronomy student, I could at least understand the written texts and less the ...
kiaat's user avatar
  • 193
19 votes
4 answers
5k views

What's the difference between minutes and arcminutes?

I read in trigonometry class that 1 minute is equal to 1/60 degrees. So, 'minute' is an angular unit. But also 'arcminutes' are used to measure seperation between celestial objects and also equals to ...
Tanya's user avatar
  • 301
18 votes
2 answers
2k views

How can astronomers pinpoint the location of the source of a neutrino?

In the popular press, in recent months, we have heard a lot about high-energy neutrinos from far outside our solar system reaching our detectors.... But I wonder... If a single neutrino from a great ...
Kurt Hikes's user avatar
  • 5,327
18 votes
2 answers
873 views

Did nobody in the Astronomy community think 12,000 new satellites in LEO might be a problem?

This answer to Can / should the Starlink satellites be blackened? mentions Space News' SpaceX working on fix for Starlink satellites so they don’t disrupt astronomy where SpaceX chief operating ...
uhoh's user avatar
  • 30.3k
18 votes
1 answer
461 views

Have we ever observed a body, such as a large asteroid, "hitting" the Sun?

Some other SE questions about launching ICBM's into the sun got me wondering whether we have ever observed an object on a path that intersected with the Sun? How close did it get?
rolfedh's user avatar
  • 283
18 votes
1 answer
4k views

Why does the moon sometimes appear giant and a orange red color near the horizon?

I've read various ideas about why the moon looks larger on the horizon. The most reasonable one in my opinion is that it is due to how our brain calculates (perceives) distance, with objects high ...
Larian LeQuella's user avatar
18 votes
2 answers
651 views

How long should it take for us to observationally determine if Caltech's Planet Nine is really there?

Caltech just released a report that says there is possibly a large (10 earth mass) planet in a remote orbit (10 - 20 thousand year) that explains a lot of observations of Kuiper Belt objects. How long ...
joseph.hainline's user avatar
17 votes
5 answers
7k views

Why are all quasars so far away?

Why are all quasars so far away? If the universe is homogeneous, we should expect to have a homogeneous distribution of quasars, but all of then seem to be far away from Earth. Why is that?
Carlos's user avatar
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