# Questions tagged [definition]

Questions targeting the exact (possibly mathematical) description of a phenomena or term.

49 questions
Filter by
Sorted by
Tagged with
104 views

### Distance from Earth to Sun averaging method in AU definition

Historically, $1~\text{AU}$ is defined as $1/2(r_{min}+r_{max})$, but the problem is that this is not the average radius if Earth orbit would be transformed into circle, with Sun at the center and ...
145 views

### Earth and the Moon Question [closed]

(For the purpose of this question, I will refer to Earth's Moon as Luna to keep it separate from the general term of 'moon') I know the IAU has been hedgey about giving an official definition of what ...
5k views

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

Here's the IAU definition of a planet (source): A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ...
• 511
619 views

### J2000 and J2000.0 difference?

I've seen the standard epoch described as both J2000 and J2000.0. Is there any difference between the two designations?
• 1,033
94 views

### Are there two mean Suns?

Are there two mean Suns? In the book I'm reading, the authors appear to refer to a mean Sun on the ecliptic moving in a circular orbit with constant angular velocity and with the same orbital period ...
• 1,033
1 vote
97 views

### Deriving the Tully-Fisher relation from the virial theorem

So I know this is a loose proof and a lot of assumptions are made, but I do not understand how these assumptions lead to the quantities we obtain so I will be asking questions at each step: start with ...
• 171
606 views

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

I'm new to astronomy and therefore probably missing the obvious here, but what type of year (tropical, sidereal?) is used for the orbital periods in the JPL Small-Body Database Lookup? For example, ...
• 1,033
124 views

### What is a "valid" coordinate system?

I was reading "Natural Coordinate System in Curved Spacetime" by Ying-Qui Gu. The first sentence of the abstract struck me (bold emphasis added by me): In this paper we establish a ...
• 16.3k
8k views

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

1 AU is defined as the distance between the Sun and the Earth, which is roughly 150 million km. But is this supposed to be from the Sun's center (Solar system's center of mass) or from the Sun's ...
• 381
1 vote
207 views

### What are Euclidean-normalized differential source counts?

I would like to know what the Euclidean-normalized and differential source counts mean in "Euclidean-normalized differential source counts". It would be great to hear a) a clear definition ...
• 61
712 views

### What are Ellerman bombs and how can we identify them?

Have you ever heard about Ellerman bombs? Spaceweather.com talks about them on their news page. You could see some of them around sunspot AR2835: Ellerman bombs are magnetic explosions about one-...
• 5,646
227 views

### Why is the Milky Way called like that? [closed]

From which language is the term milky way is derived and what is the largest star located in it?
637 views

### What is the difference between the terms collision and merger? How are they used differently in Astronomy?

We often hear of mergers of two stellar objects but we also sometimes talk about these or much smaller objects like planets or asteroids colliding. What is the actual differences between Astronomy and ...
• 30.5k
2k views

### Is there a precise definition of "Supermoon"?

Elsewhere online I keep reading that a supermoon is the situation when full moon coincides with moon perigee. But see this plot from wikipedia Lunar Distance entry The perigee distance varies wildly ...
• 153
113 views

### What is the lower limit of a coronal mass ejection?

Reading an 2005 article by Fraser Cain called Smallest Ever Coronal Mass Ejection I became interested in the exact definition of CME. It seems like a simple question - but I did not manage to find ...
• 5,646
587 views

### How to determine the age of a star using asteroseismology?

I keep reading that a standard way to determine the age of a star is asteroseismology, and I tried to learn more about it. I am wondering if somebody could help me to describe the method in simple ...
• 5,646
1 vote
338 views

### IAU 2018 Exoplanet definition

In 2018 the IAU produced a definition of an exoplanet: The current official wokring definition of an exoplanet as amended by IAU Comission F2: Exoplanets and the Solar System in August 2018, reads as ...
• 1,471
1 vote
98 views

### Defining a nautical mile

Astronomy is new to me so my question might be stupid: I read that a nautical mile is defined as one minute of latitude along any line of longitude. What if it was the other way round, i.e. one minute ...
1 vote
197 views

### Are there actual asteroids in the Oort cloud?

This answer to At what annual rate are new exoplanets being recognized? How does it compare to new asteroids? shows that the rate of new asteroid discovery is roughly two orders of magnitude higher ...
• 30.5k
33 views

### Timing based on upper or lower part of sun/moon

When you time an astronomical event, such as the moon/sun rising above the horizon, do you note the time when the the first sliver breeches the horizon or when the bottom of the moon/sun just becomes ...
• 1,559
1k views

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

Many publications write about "completeness". I have a vague idea that it has to do with the ratio of how many sources from an observation are detected and how many sources are actually in ...
• 61
106 views

### What happened to the 2017 proposal on redefining planethood? Is this information available?

In 2017, Alan Stern et al. submitted a geophysical planet definition to the IAU for review which states “A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has ...
• 397
821 views

### If Jupiter and Saturn were considered stars, which of their moons would qualify as planets by 'clearing the neighbourhood'?

Jupiter has four and Saturn seven spherical satellites. Jupiter's moon Ganymede and Saturn's Titan are larger than Mercury and they're the most massive moons at 0.0248 M♁ (Ganymede) and 0.0225 M♁ (...
• 397
481 views

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

What is the definition of a "pole" of a celestial body? Earth's pole is defined as it's rotational pole. The North and South Poles are the two points on Earth where its axis of rotation ...
• 624
151 views

### What other definitions for a planet were proposed?

This article from the IAU states The first draft proposal for the definition of a planet was debated vigorously by astronomers at the 2006 IAU General Assembly in Prague and a new version slowly took ...
• 17.7k
797 views

### What was the definition of a planet before August 24, 2006?

In 2006, the IAU produced a definition of what it is to be a planet. This definition famously excludes Pluto, to the disarray of this small body's fans. Before this decision, what was the definition ...
• 17.7k
225 views

### Is there still a possibility that Makemake is not ellipsoidal but asteroid-shaped?

In 2008, Makemake and Haumea have been named under the expectation that they would prove to be dwarf planets. Well, did they prove to be ones? Haumea obviously did since we know its shape which is a ...
• 1,110
133 views

### Will NEOWISE be known as the Great Comet of 2020?

Many or all comets visible to the naked eye are referred to as Great Comets of their year(s). Might NEOWISE too be called like this? Why or why not? Is the IAU responsible for declaration of Great ...
• 1,110
5k views

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

Let $d$ be the distance, $D$ is the linear size, and $x$ the angular size. In the formula $$D = \frac{dx}{206 265}$$ what does this number mean?
• 35
1 vote
97 views

### When and why was zero right ascension defined to be at the vernal equinox?

When (and why) was zero right ascension defined to be at the vernal equinox? Who established or defined this standard?
• 183
1 vote
540 views

### Why does the Gaia color-magnitude diagram have this shape?

I was looking at the visualization tools of Gaia, and tried plotting a color-magnitude diagram. On the right hand side of the image, photGMeanMag (mag) (the amount ...
• 17.7k
994 views

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

The 2006 definition of a planet states that a planet is a celestial body that (a) is in orbit around the Sun (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it ...
• 1,262
108 views

### How variable does a star have to be, to be a variable star?

Variable stars are stars whose apparent magnitude varies. But there are so many phenomena that can cause a star to be variable, that I would expect all stars to be variable. A rotating star has a ...
• 17.7k
1 vote
124 views

### Should only spherical satellites be considered 'moons'? [closed]

Since Titan and Ganymede fall into the same category as Deimos and Phobos or the components of planetary rings, the category of moons, it's like if all asteroids were considered planets. Tiny ...
• 1,262
613 views

### What is the difference between spectroscopy, spectrography and spectrometry?

Spectroscopy is the study of spectra, spectrography is the writing of the spectra, and spectrometry is the measure of spectra. So from an etymological perspective, there is no real difference between ...
• 17.7k
561 views

### What is the difference between a moon and a random chunk in the rings

Jupiter has 79 (known) moons, Saturn 82, Uranus 27 and Neptune 14 (numbers come from Wikipedia). These planets also all have rings. The rings are made up of chunks of rock and ice. There also are ...
• 17.7k
281 views

### Definition of the light that is projected by a celestial body/luminary [closed]

I'm creating an application which will simulate the light created by sun or moon on a 3d topographic map of Earth. Something like this: Is there a word or definition for this light/light projection?
• 121
312 views

### How far above the Earth's surface does interplanetary space begin?

The Kármán line is often cited as the beginning of outer space. The Voyager probes have identified the boundary between the heliosphere and interstellar space. Is there a agreed upon definition of ...
• 1,489
245 views

### What is an asteroid called if it is in deep space?

Pretty much every definition I've read about asteroids is something like "Asteroids are rocky worlds revolving around the sun that are too small to be called planets." What would an asteroid be ...
• 163
3k views

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

This is a follow up question to What exactly is a "moon"? The conclusions I draw from James K's answer is that the IAU should define what a moon is. They haven't done so yet, but they should. The ...
• 4,831
2k views

### What exactly is a "moon"?

Recently some new moons of Jupiter were discovered. However the moons are pretty damn small indeed - only a few kilometers wide. That makes me wonder if they should even be called moons in the first ...
• 4,831
135 views

### Unsure how to obtain galactic roll?

I am having some issues with understanding what my adviser means by galactic roll, but I have not been able to find anything online regarding this. My project ...
• 31
96 views

### Is the ratio $L_B / L_{H\alpha}$ important to determine star formation rates?

Could someone please tell me if the ratio $$\frac{L_B}{L_{H\alpha}}$$ is important in determining star formation rates? Additionally, could someone please explain the implication of the ratio to me or ...
433 views

### Whats the name of this eye-shaped galaxy?

I would like to know the name of this eye-shaped galaxy: Strange eye-shaped galaxy has black-hole iris, Betsy Mason, 23.07.2009, Wired.com:
• 123
610 views

### So is there nothing like satellite anymore?

I just read this article. I don't know how credible it is, since I couldn't find any link to the original paper. But it says that pluto will be considered a planet again, along with more than 100 ...
626 views

### Do astronomers generally agree that the distinction between comets and asteroids is not so clear?

edit: I just saw this tweet and find it incredibly relevant :) begin question: See this answer and then consider if there are known or likely solar system bodies that might be identified as both ...
• 30.5k
1 vote
67 views

### How close must a full moon be to become a supermoon?

According to this answer there is a scale of super moons depending on how close the full moon(henceforth anti-sol) is to perigee. So how close to anti-sol does the moon's perigee need to be to be ...
• 323
916 views

### Official Definition of Satellite?

So I had an argument with a friend, which was about whether the Moon is a planet or a satellite. IAU 2006 Resolution B5 gives definitions on what it is to be a planet, but there is a vagueness, as I ...
• 191