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# Tag Info

## Hot answers tagged algorithm

18 votes
Accepted

### Why do people choose 0.2 as the value of linking length in the friends-of-friends algorithm?

The friends-of-friends (FOF) algorithm (Huchra & Geller 1982; Press & Davis 1982, Davis et al. 1985) for finding groups of particles can be used for various numerical problems, not just ...
• 38.7k
18 votes

### How (the heck) does Astrometry.net work?

I've been trying to figure out the technical details of astrometry.net for quite some time. As others already pointed out, the main input to the whole process is a list of stars. I will not go into ...
10 votes

### How (the heck) does Astrometry.net work?

Finding an astrometric solution from an image with Astrometry.net is usually called plate solving. As mentioned in the comments, it is based on pattern matching, using a large set of databases that ...
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10 votes
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### What is the application of pseudorandomness in astrophysics?

The primary advantage of pseudorandom numbers is speed (as Cristiano mentioned) and convenience: they're what are available in almost any software package or library, whereas true random number ...
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7 votes

### What is the application of pseudorandomness in astrophysics?

What are the advantages of using pseudorandom numbers in Monte Carlo simulation instead of random numbers? Where are you going to get those true random numbers? Even reading from ...
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7 votes
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### Algorithm to fit galaxies

The two most general, publicly available packages for galaxy image fitting (other than GALFIT) are probably Imfit and ProFit. (Note that I am the developer of Imfit.) There is also Lenstronomy, which ...
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5 votes
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### How to calculate the moon's illuminated fraction tilt?

Your update is correct. PA is the position angle of the bright limb (measured eastward from celestial north). q is the parallactic angle between the zenith and celestial north. Then the angle of the ...
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5 votes

### Computing orbit positions of Jovian Satellites / Moons using JPL data

An excellent book by Jean Meeus, "Astronomical Algorithms", provides the calculations for planetary positions and Jupiter's Galilean Moons (Io, Europa, Ganymede, and Callisto). Meeus does not really ...
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### Why do people choose 0.2 as the value of linking length in the friends-of-friends algorithm?

A quick search brought me to a book by Houjun Mo, et al. Galaxy Formation and Evolution which says For example, the frequently used friends-of-friends (FOF) algorithm defines halos as structures ...
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5 votes

### Calculate the general appearance of the night sky over +/-15,000 years

This is not a full answer but more a sketch how I would try to address the issue (but too long for a comment): If you use (or transform) all data you consider to the same datum, then you can consider ...
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4 votes

### Can automatic algorithms completely eliminate the impact of Starlink and other satellites?

No, automated algorithms won't ever be able to completely eliminate the effect of passing satellites. Some of the light reflected from the satellites into the atmosphere will then scatter and cause ...
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4 votes
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### Constellation a celestial body is in

If the coordinates are precessed to epoch B1875.0, the constellation boundaries lie neatly along the equatorial coordinate grid, allowing simpler tests against boundary segments. Roman 1987 describes ...
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4 votes
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### Computing orbit positions of Jovian Satellites / Moons using JPL data

Thank you to those people that have tried to help me with this, both back when I first posted the question, and later. With the solid advice of @barrycarter in the comments, I looked into using the ...
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4 votes
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### Sunset and sunrise formulas adopted by IAU

The IAU does provide a C library called "Standards of Fundamental Astronomy" at http://www.iausofa.org/ which states "The principal function of the SOFA Astronomy Library is to provide ...
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4 votes

### How to find the first Julian day with the same Earth-Sun constellation

Most formulas accept a Julian Day as input, but internally usually use the number of days (or years, or centuries) since Jan 1, 2000 12:00pm. Take the function below, which accepts JD as input, but ...
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4 votes
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### Calculation of Julian day is off for negative dates

This answer offers in two parts, (1) a first thought that might possibly help you to get debugged (but no promises), and (2) some ways to set up reliable date-conversion without having to 're-invent ...
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4 votes
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### Wrong results of calculating moon's altitude and azimuth

You need to normalize the sidereal time $theta0$ into the range 0...360 because the second term is hundreds or thousands of days ahead of or behind 2000 Jan 1.5 (JD 2451545.0) multiplied by 360.98.. ...
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3 votes
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### Why is the boundary of friends-of-friends (FOF) halo corresponding to iso-density contour?

If you have enough particles to sample your field, then a given (local) density corresponds to the same mean distance between your particles. The FOF algorithm keeps linking particles to a halo until ...
• 38.7k
3 votes

### Algorithm to stack astronomical images

Have a look at SCAMP for astrometry and SWarp for stacking. Like the software mentioned in the other answer, both are open source, so you can check what algorithms they use. SCAMP documentation is ...
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3 votes

### Algorithm to stack astronomical images

This page about a commercial product goes into some detail about their algorithm. It does the triangle matching you describe, with something like simulated annealing to get a more optimal solution. ...
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3 votes

### What image analysis approaches/algorithms are used to detect near-Earth objects?

Near-Earth Objects (NEO’s) are detected from imagery frames. The methodology is to ‘look’ for a tiny bright spot from several images, that has its pattern of brightness, shape and movement. Currently ...
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3 votes

### Modern equivalent of Astronomical Algorithms by Jean Meeus?

The methods described in Jean Meeus's book are still valid, and accurate enough for the needs of most amateur and many professional astronomers. The science of positional astronomy doesn't change so ...
2 votes
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### What are the reasons for referring to equinox J2000 or the equinox of the date?

Maybe you are interested in watching the occultation of a star by a solar system object. In which case you will need to compare their positions. Catalogues of stellar positions are most commonly given ...
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2 votes

### Mapping illumination to moon phase (font) icon

For LaTeX there are the following packages using fonts but they are limited: wasysym (full, new, and quarters) mathabx (same) stix (math astro symbols) china2e (artistic full, new, and quarters) ...
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2 votes

### Mapping illumination to moon phase (font) icon

While you have "illumination" it would seem to be easier just to map "moon age" (in days) to a character The average synodic month is 29.53 days, and you have 28 characters, so ...
• 125k
2 votes

### Jean Meeus - calculating moon rise, transit and set times - help isolating error in logic

You might like to try looking at the (hopefully well documented) JavaScript code of "Astron", https://friendsofthevigilance.org.uk/Astron/Astron.html Whilst primarily for sextant users, the ...
2 votes
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### Converting equatorial coordinates to Cartesian coordinates for extragalactic distances

I find this discussion of cosmological distances by David Hogg to be very useful in answering questions like this. In Section 4 he says: The line-of-sight comoving distance between two nearby events ...
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2 votes

### Calculation of Julian day is off for negative dates

Sorry, I don't know Rust, so I won't try to debug your code, but it looks like there's some issue with the Julian calendar vs the Gregorian calendar. However, there's an off-by-one error too. I see ...
• 15k
1 vote

### How does one use SPICE to calculate sunrise and sunset?

I'm no SPICE expert but here are some potential solutions (unless of course you want to try Skyfield's Almanac methods!) Firsrt possibility, but all these may not work this answer links to this ...
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