44 votes

Is the Astronomical Unit measured from the Sun's center or from the surface?

The AU is, by definition, exactly 149,597,870,700 m. This is based not on a distance to the sun (which varies) but on the shape of an ellipse that closely approximates the orbit of the Earth. An ...
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  • 87.7k
20 votes

Is there a precise definition of "Supermoon"?

No, there is no special name for the closest supermoon. Besides, the very name “supermoon” was coined by an astrologer, NOT an astronomer, and it is basically all just media hype. During a so-called “...
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19 votes

Would an analogue of the definition for planets also work for moons?

In 2006 the IAU had a trilemma. They could decide that Eris was a planet, and potentially allow for future discoveries of tens of new planets. They could be inconsistent, declare that Pluto was a ...
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17 votes
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Metallicity of Celestial Objects: Why "Metal = Non-metal"?

To first order, the relative abundances of the heavier elements to iron (for instance) are constant. So the metal content of a star is shorthand for the content of any element heavier than He. (NB: we ...
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  • 114k
11 votes
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Do astronomers generally agree that the distinction between comets and asteroids is not so clear?

If we're going to get technical, Asteroids are not really an official name anymore. In 2006, when the IAU redefined what a planet was (and thus demoted Pluto), they also decided to more formally ...
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  • 14.4k
11 votes
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What exactly is a "moon"?

Unlike "planet" the IAU hasn't attempted to precisely define "moon". General usage requires that a "moon" is a natural satellite of a planet (or dwarf planet, asteroid, or perhaps even of another moon?...
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  • 87.7k
10 votes
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Would an analogue of the definition for planets also work for moons?

Singling out one point: Moons must have "cleared the neighbourhood" of their own orbit, i.e. in their particular orbit around the host they are by far the most massive body This would mean ...
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  • 4,565
9 votes

Why isn't Eris considered a planet despite being the body of dominant mass?

You are correct that the IAU definition of "clearing the orbit" has the problem of being not explicitly quantified. And a complete clearing was obviously never the intention behind the definition. I ...
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  • 2,400
9 votes

What is the definition of a "pole" of a celestial body?

You are mixing the rotation of the body around a barycenter with its moons, and the rotation of the body around its own center. For a bound rotation like Pluto and Charon both have to have the same ...
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  • 10.8k
9 votes
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Is there a precise definition of "Supermoon"?

To directly answer your question: There is no "precise" definition of the term "supermoon." Term Origin: The origin of the term is generally attributed to the astrologer Richard ...
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8 votes
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What is an asteroid called if it is in deep space?

We have only discovered one such object so far: ʻOumuamua. (Although a second, 2I/Borisov, was discovered in 2019) The general practice has been to call it an "interstellar asteroid" or &...
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  • 87.7k
8 votes

Metallicity of Celestial Objects: Why "Metal = Non-metal"?

In 1812 Fraunhofer measured a set of absorption lines in the sun. It wasn't until later that Kirchoff and Bunsen figured out that these absorption lines matched emission features from metals they were ...
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  • 1,453
6 votes

Metallicity of Celestial Objects: Why "Metal = Non-metal"?

There is no 'history' behind it. Stellar physics is less than a 100 years old. Thus, it is not a terminology that came to be due to some anecdotal reason. This is the way it has been since the start. ...
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  • 1,993
5 votes
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So is there nothing like satellite anymore?

This is really a question of Language rather than Astronomy. The meaning of words is defined by their use. The IAU has proposed one way of using the word "planet", this group of scientists have ...
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  • 87.7k
5 votes

Official Definition of Satellite?

In fact there is no official definition of a satellite. Issac Asimov makes a good argument that the Moon is a planet since its orbit is convex around the Sun for its entire orbit, unlike the other ...
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5 votes

Would an analogue of the definition for planets also work for moons?

I'd like to expand on @James K's good answer. The question you need to answer before your question can be answered is: why do you want to have a definition of "moon"? What purpose does it serve ...
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  • 7,380
5 votes
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What was the definition of a planet before August 24, 2006?

What was the definition of a planet before August 24, 2006? I think this deserves a bit more of an answer because it's not quite so simple. Yes, it's true that there was no formal definition of what ...
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  • 22.9k
5 votes
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What is the definition of a "pole" of a celestial body?

I'll just add a supplement to @planetmaker's answer. As long as a body is distinct and not connected to anything else, it will have a center of mass. If the body is roughly spherical its center of ...
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  • 31.4k
5 votes
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How to determine the age of a star using asteroseismology?

Asteroseismology effectively measures the sound speed inside a star by finding the characteristic oscillation frequencies of a star. The sound speed depends on the composition because the pressure at ...
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  • 114k
4 votes
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Whats the name of this eye-shaped galaxy?

That is galaxy NGC 1097, a barred spiral galaxy in the constellation Fornax. It has been distorted by interactions with neighbouring galaxies and has a weakly active galactic nucleus, with jets coming ...
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  • 87.7k
4 votes

Why do we use the value 206265 in the small angle formula?

$$1^{c}=57.2958^{\circ}=57.3\times3600=206265″$$ $$ \Longrightarrow\theta_{\rm arcsec}=\left(\frac{d}{D}\right)\times206265$$
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4 votes

Why do we use the value 206265 in the small angle formula?

The number 206265 arcseconds/radian is often used in astronomy for angular conversions. It is simply derived from the product of 3600 arcseconds/degree and 57.2958 degrees/radian. Edit based on ...
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  • 1,240
4 votes
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What other definitions for a planet were proposed?

The IAU website has this draft definition of "planet" and "plutons" page, published on 16 August 2006 in Prague. It says (emphasis mine): The world's astronomers, under the ...
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4 votes
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If Jupiter and Saturn were considered stars, which of their moons would qualify as planets by 'clearing the neighbourhood'?

I've noted before that the IAU naming critera are guidelines rather than laws of nature. If applied to Jupiter's moons the four Galilean satellites would probably be "planets" (they don't ...
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  • 87.7k
4 votes

What is the completeness of an observation and how do I calculate it?

a) What is the definition of completeness? Completeness is the number of objects in a data set that are detected over the number that exist. In astronomy, completeness is often estimated for a ...
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  • 13.7k
4 votes

Why is the Milky Way called like that?

Milky way obviously is an English word with that literal meaning a way (path) covered in milk. So it's not exactly derived. But it is a literal translation from Latin 'via lactia' and the Greek ...
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  • 10.8k
3 votes
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Will NEOWISE be known as the Great Comet of 2020?

I'm inclined to say a hard no using the casual definition of a "great comet" A great comet is a comet that becomes exceptionally bright. There is no official definition; often the term is ...
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  • 22.9k
3 votes
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How variable does a star have to be, to be a variable star?

There is no lower limit, and as you say, all stars are somewhat variable. However catalogues of variable stars exist, and they can record a wide range of levels of variability. For example, the ...
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  • 87.7k
3 votes
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What is the difference between a moon and a random chunk in the rings

Your question: There also are satellites in between the rings. What distinguishes a moon from any other chunk in orbit around a planet? Wikipedia should be taken with a grain of salt, but they ...
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  • 22.9k
3 votes
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IAU 2018 Exoplanet definition

"What" is straightforward: L4 and L5 are two points on an orbit that are 60 degrees ahead and 60 degrees behind the planet, and so move as the planet orbits. If the planet is less than ...
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