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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19
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3answers
10k views

How long does it take to travel 36 light years with tolerable acceleration and deceleration?

The recent discovery of HD85512b only 36 light years from Earth has promising attributes to harbor life. Assuming we want to travel there, we cannot instantaneously jump to light speed, (StarTrek ...
18
votes
6answers
4k views

Why are the orbits of planets in the Solar System nearly circular?

Except for Mercury, the planets in the Solar System have very small eccentricities. Is this property special to the Solar System? Wikipedia states: Most exoplanets with orbital periods of 20 ...
13
votes
1answer
528 views

What percentage of the sky is occluded by stars?

If you drew rays from the center of the earth out to infinity at every angle, what percentage of them would intersect a star? Extra details: Assume the rays are mathematical rays, or that they ...
6
votes
0answers
312 views

Why is my approach to the equation of time off by a constant?

I'm trying to better understand the causes for the equation of time by deriving an approximation from first principles. My naive approach, $EOT_{NAIVE}$, is to take the difference between the right ...
17
votes
1answer
662 views

How fast is the Earth-Sun distance changing

This is inspired by Evidence that the Solar System is expanding like the Universe?, which referenced an article by G. A. Krasinsky and V. A. Brumberg, "Secular Increase of Astronomical Unit from ...
13
votes
10answers
237 views

In astronomy what phenomena have theory predicted before observations?

As far as I know, astronomy is generally an observational science. We see something and then try to explain why it is happening. The one exception that I know of is black holes: first it was thought ...
13
votes
4answers
746 views

Is there a limit to the resolving power of a mirror telescope?

Like, if you hammered out the asteroid 16 Psyche into a 1 mm thick iron foil disc telescope mirror with 2.4x the radius of the Sun, could you resolve details on the surface of an exoplanet? At what ...
11
votes
5answers
2k views

Anti-Matter Black Holes

Assuming for a second that there were a pocket of anti matter somewhere sufficiently large to form all the type of object we can see forming from normal matter - then one of these objects would be a ...
7
votes
1answer
364 views

Are the distances we measure to objects in the universe incorrect because of their relative motion?

For example the agreed distance to the Andromeda Galaxy is 2.5 million light years, and consequentially, we see the galaxy as it was 2.5 million years ago! (A time interval in which the galaxy must ...
6
votes
2answers
605 views

How did pre-Copernican astronomers accurately predict planetary position?

Copernican elements (circular orbital elements) are not very accurate. But Copernicus simplified our understanding a great deal by placing the Sun at the center of the system. Im astonished by the ...
5
votes
6answers
278 views

Are there free data available online from cosmology (or astrophysics) experiments that anyone can analyse?

One can understand a subject better in physics by trying to solve as many problems as one can from a textbook say. When it comes to experimental physics and data analysis, no book on experimental ...
2
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1answer
746 views

How did Copernicus establish the relative distance to the superior planets?

I understand that the relative distances to the planets had been calculated using various methods since ancient times, and, in particular, that the assumptions of the Copernican model of the Solar ...
1
vote
4answers
7k views

How does the moon reflect light?

We can see the moon in the night because it reflects sunlight. But the light is incident on the opposite side of moon with respect to the observer in the night. In this case, how does the moon ...
14
votes
3answers
7k views

Do all the individual stars that we can see in the night sky belong to Milky Way?

I was wondering if all the stars that we can see with the unaided eye as distinct point sources are from our own galaxy? In other words, can we see stars from the Andromeda Galaxy or other galaxies ...
10
votes
2answers
2k views

What is the farthest-away star visible to 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 concept that might have ...
10
votes
1answer
214 views

How many earth-sized planets have been discovered outside the solar system?

How many earth-sized planets have been discovered outside our solar system? Is there a combined registry of them anywhere? Where might I look for more information?
8
votes
4answers
2k views

How can I stabilize an unstable telescope?

I have an 80 mm refractor telescope on a tripod, but it shakes on every touch. It's very hard to see via 6 mm (x120) ocular. Even a little wind causes the image to become too unsteady. How ...
5
votes
1answer
43 views

What fraction of baryonic matter is in stars?

We know from big bang nucleosynthesis that baryonic matter accounts for about 5% of the universe's total mass-energy density. What is the current best estimate of how much of this is in the form of ...
5
votes
2answers
111 views

How do we estimate $10^{23}$ stars in the observable universe?

Now, I read somewhere, that there are $10^{23}$ stars in the observable universe. How did scientists estimate this?
5
votes
1answer
173 views

About the hump on galaxy rotation curves

The past days I have been studying the rotation curves of disk galaxies and I am currently trying to understand how we can extract information about the dark matter of a galaxy by looking its rotation ...
5
votes
1answer
72 views

Is there an established standard for naming exoplanets?

I understand that exoplanets are named by adding a lowercase letter to the a designation of the planet's parent star or stellar system, beginning with 'b' (the star itself is 'a') in order of ...
3
votes
2answers
1k views

When is the right ascension of the mean sun 0?

I understand that the right ascension of the mean sun changes (at least over a specified period) by a constant rate, but where is it zero? I had naively assumed that it would be zero at the most ...
2
votes
1answer
237 views

Stable and Unstable Orbital Resonance

I was wondering if anyone can show me why some orbital resonances are unstable. For example in the asteroid belt there is a depleted distribution at 3:1 resonance with jupiter. What is the cause of ...
2
votes
1answer
175 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 ...
2
votes
2answers
431 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 ...
2
votes
2answers
585 views

How can I visualize a gas giant?

From wikipedia Gas giants are commonly said to lack solid surfaces, but it is closer to the truth to say that they lack surfaces altogether since the gases that make them up simply become ...
-2
votes
5answers
275 views

Does the rotational speed of a planet consistently become faster and faster given that there are no conflicting events? [closed]

Does the rotational speed of a planet consistently become faster and faster given that there are no conflicting events?
-3
votes
1answer
561 views

What is the difference between UT0, UT1 and GMT time?

Every reference I find says that they are "essentially" the same, which we all know really means that they are not the same, but different only by a some small amount that someone else other than me ...
19
votes
2answers
334 views

Why don't stars in globular clusters all orbit in the same plane?

Globular clusters like Omega Centauri certainly don't seem to be very coplanar at all. In other words, why doesn't the explanation at Why are our planets in the solar system all on the same ...
15
votes
5answers
2k views

What do the colors in false color images represent?

Every kid who first looks into a telescope is shocked to see that everything's black and white. The pretty colors, like those in this picture of the Sleeping Beauty Galaxy (M64), are missing: The ...
11
votes
3answers
636 views

Why is this radio telescope's reflector spherical and not parabolic?

This is the Arecibo Observatory in Arecibo, Puerto Rico. Its reflector is spherical, measuring 1,001 ft. in diameter. It is considered the most sensitive radio telescope on Earth, but the fact that ...
10
votes
4answers
884 views

How big should a lens of a telescope be so we can see the American flag on the Moon from the Earth's surface? [duplicate]

How big in diameter should the lens of the imaginary telescope to be so we can see the American flag from e.g. some observatory in Hawaii?
10
votes
1answer
406 views

How to explain the Moon halo phenomenon?

Today, here in Brazil, I have observed (and is still observing) an interesting phenomenon. The Moon is near to a big star in the sky, but this is normal. The interesting part is what's around them. ...
8
votes
1answer
378 views

Could the earth have another moon?

First, to clarify: I'm not asking if perhaps there's a moon that we haven't found yet. The question is, theoretically, would the earth be able to have another stable moon in addition to the current ...
7
votes
2answers
540 views

How would one navigate interstellar space?

Headed out from Earth within the Solar System, Sol and Earth both may be used as reference. When traveling in interstellar space with stellar systems themselves traveling at varying velocities even ...
7
votes
3answers
5k views

How fast will the sun become a red giant?

I've read many accounts of our sun's distant fate, but what I've never heard is on what time scale these events occur. For instance, when the sun runs out of hydrogen, I presume it doesn't just WHAM! ...
7
votes
6answers
1k views

Can any telescope be used for solar observing?

Can any telescope, such as a 8" reflector, that is normally used at night to look at planets be used or adapted for solar observing? What kind of adapters or filters are required or is it better to ...
7
votes
2answers
1k views

What does the sky look like to human eyes from orbit?

There are numerous pictures, obviously, of the blackness of space from the shuttle, the space station, and even the moon. But they all suffer from being from the perspective of a camera, which is not ...
6
votes
2answers
514 views

What is the frequency of occurrence of stellar classifications off the HR main-sequence?

An alternative version of this question would be: "if was to pick a star from the $10^{11}$ or so in our galaxy at random, what are the probabilities of it being various kinds of star?" (and I do mean ...
6
votes
3answers
1k views

How long was a day at the creation of earth?

Since the earth is slowing its rotation, and as far as I know, each day is 1 second longer every about 1.5 years, how long was an earth day near the formation of earth (4.5 billion years ago)? I ...
6
votes
1answer
190 views

Axial Tilt and precession rate of exoplanets

The Earth's axis is 23.5 degrees away from othogonality to the ecliptic, and it takes about 26 000 years for it to precess fully. I have neither an intuitive sense nor the formula for precession ...
6
votes
1answer
3k views

How do we know the masses of single stars?

I have recently read that we can only know the masses of stars in binary systems, because we use Kepler's third law to indirectly measure the mass. However, it is not hard to find measurements for the ...
6
votes
1answer
672 views

Is there a simple yet accurate formula for where on Earth the Sun and Moon are directly overhead?

I'm trying to improve a site that shows the region of the Earth currently under daylight, and I need a formula that, given the current time, tells where (latitude/longitude) the sun and moon are ...
6
votes
2answers
222 views

What's the best way to start out with astrophotography on a tight budget?

I'm new to astronomy and I don't have a great deal of cash to spend. I currently have a 3" reflector telescope which I've had great fun with. I also have a Pentax DSLR which I've been using to take ...
5
votes
0answers
255 views

Mirrors and light beam divergence technology limits

There are many applications for orbital space mirrors in astronomy (better telescopes) and space propulsion (solar power for deep space probes), but this is limited by the minimum beam divergence ...
5
votes
3answers
1k views

Is a water world possible, and for how long could it be stable?

I have several questions regarding this topic. First, could a water world be stable for thousands of years with most of its surface remaining covered in water. What would it take for this to be ...
5
votes
1answer
3k views

How much sky do we see at any one moment?

When we look at any particular point the sky, what percentage of the celestial sphere do we see? This question arises from the notion that on average there passes one meteor per hour overhead. So ...
4
votes
1answer
245 views

Velocity distribution in Plummer's models and others mass distributions

The Plummer's sphere is an model for the mass density in a globular cluster of stars. For an $N$-body simulation I have initialized the position of $N$ masses with a Monte-Carlo technique but cannot ...
4
votes
2answers
356 views

How many stars did people think there were in the 11th century (or thereabouts)?

I hope this isn't too off-topic. Someone showed me a reference to a French, 11th century biblical commentator who implied that there were over 600,000 stars. This got me thinking, how many stars did ...
4
votes
1answer
251 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 ...