Skip to main content
47 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 ...
James K's user avatar
  • 125k
25 votes

Why doesn't the IAU definition of "Planet" disqualify Mercury and Venus as planets?

You are citing a paper that has been cited only six times in the peer reviewed scientific literature since it was published in 1984, which was almost 40 years ago. One of those six citations was a ...
David Hammen's user avatar
  • 34.1k
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 “...
Pierre Paquette's user avatar
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 ...
James K's user avatar
  • 125k
14 votes

Why doesn't the IAU definition of "Planet" disqualify Mercury and Venus as planets?

I directed this question to Mike Brown, and he answered on Twitter. Mike Brown is about as authoritative as possible. The real answer here is to not get too hung up on definitions, which I admit is ...
Schroeder's user avatar
  • 511
13 votes
Accepted

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?...
James K's user avatar
  • 125k
12 votes
Accepted

What unit of year in JPL Small-Body Database Lookup?

Based on the Astrodynamic Parameters page, It appears to be the astronomical Julian year, defined as exactly 365.25 days of 86400 SI seconds.
notovny's user avatar
  • 4,786
11 votes
Accepted

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 ...
zephyr's user avatar
  • 15k
11 votes

Why doesn't the IAU definition of "Planet" disqualify Mercury and Venus as planets?

One subtle aspect of all this is that a "planet" need not be currently in hydrostatic equilibrium". From https://solarsystem.nasa.gov/planets/in-depth/: The IAU therefore resolves that ...
Oscar Lanzi's user avatar
  • 1,201
10 votes
Accepted

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 ...
Glorfindel's user avatar
  • 4,800
10 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 ...
SpaceCore's user avatar
  • 3,441
10 votes
Accepted

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 ...
Stuart Robbins's user avatar
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 ...
planetmaker's user avatar
  • 19.9k
9 votes

Why doesn't the IAU definition of "Planet" disqualify Mercury and Venus as planets?

No planet is in perfect hydrostatic equilibrium as no planet can be considered an ideal fluid but always has some finite rigidity. So their rotational flattening is always different from that of an ...
Thomas's user avatar
  • 3,504
8 votes
Accepted

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 ...
userLTK's user avatar
  • 24.1k
8 votes
Accepted

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 &...
James K's user avatar
  • 125k
8 votes
Accepted

J2000 and J2000.0 difference?

They're identical. See https://en.wikipedia.org/wiki/Epoch_(astronomy)#Julian_years_and_J2000 "J2000 = JD 2451545.0 (TT)" - PM 2Ring They are the same. It is possible for an epoch ending ...
6 votes
Accepted

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 ...
uhoh's user avatar
  • 30.7k
6 votes
Accepted

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 ...
ProfRob's user avatar
  • 155k
6 votes
Accepted

Distance from Earth to Sun averaging method in AU definition

Because the orbital period depends on the semimajor axis. Two orbits of small bodies around the sun with the same semimajor axis will have the same orbital period. So the "size" of the ...
James K's user avatar
  • 125k
5 votes
Accepted

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 ...
James K's user avatar
  • 125k
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 ...
Mark Olson's user avatar
  • 7,650
5 votes
Accepted

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 ...
James K's user avatar
  • 125k
5 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 ...
Connor Garcia's user avatar
  • 16.3k
5 votes

Earth and the Moon Question

The moon "Luna" is a moon. There is no "IAU" definition, so you have to fall back to general scientific usage. You have a lot of irrelevant stuff about "Tug of War" ...
James K's user avatar
  • 125k
4 votes
Accepted

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 ...
James K's user avatar
  • 125k
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 ...
amateurAstro's user avatar
  • 1,790
4 votes
Accepted

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 ...
Jean-Marie Prival's user avatar
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 ...
planetmaker's user avatar
  • 19.9k

Only top scored, non community-wiki answers of a minimum length are eligible