<p>Sky and Telescope's <a href="https://skyandtelescope.org/astronomy-news/how-bright-will-comet-leonard-get/" rel="nofollow noreferrer">How Bright will Comet Leonard Get?</a> includes the skymap and projected (possible) light curve for comet Leonard C/2021 A1 that will potentially reach +4 magnitude this month. It has an interesting trajectory in that it's passed Earth (at least) once before but this time will leave the solar system forever.</p>
<p><strong>Question:</strong> How to make a guide map to comet comet Leonard C/2021 A1 using Skyfield?</p>
<p>I know that Skyfield has a simple propagator; it can extract parameters from the minor planet data center automatically then propagate them in a <em>heliocentric</em> orbit only, and for a finder chart for binoculars that's going to be sufficiently accurate.</p>
<p>I also know that Skyfield has the ability to read in star catalogs which will make it possible to plot stars with their sizes indicating visual magnitudes.</p>
<p>But I've never really done these, instead I just concentrate on the planet data that Skyfield gets from the JPL Development Ephemerides.</p>
<p><a href="https://i.sstatic.net/GKweG.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/GKweGm.png" alt="Skymap for Comet Leonard (C/2021 A1) from Sky and Telescope" /></a> <a href="https://i.sstatic.net/e8dGk.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/e8dGkm.png" alt="possible light curve prediction for Comet Leonard (C/2021 A1) from Sky and Telescope" /></a></p>
<p>We can't promise to add bounties ahead of time (there's no commit to future bounty function) but I'm a pretty heavy bountier and so I certainly hope to be able to engage in some bountification if a working script is posted.</p>
<p>It should be pretty flexible, so one should be able to add the desired date, field of view (e.g. 7 for binoculars) and dimmest magnitude to plot at the top of the script and running it will show a bunch of black dots for stars and a green dot for the comet (since comets are often green).</p>
<p>If one is ambitious, one could draw the trajectory for say +/- 1 or 3 days and add dots or tick marks at intervals, but that seems overly ambitious.</p>

CC BY-SA 4.0
2021-12-04T11:21:40.760Z
There is a complete example in the skyfield documentation with comet neowise, including daily tickmarks: https://rhodesmill.org/skyfield/example-plots.html (top example is Venus, below that Neowise). What is missing there (except adapting it to read the data of the comet in question)?
planetmaker
CC BY-SA 4.0
2021-12-04T11:28:22.977Z
@planetmaker Oh, the examples are (relatively) new! I hadn't seen them. Yes, what's needed is to read the data of the comet in question. It's possible someone may appreciate the opportunity to refine their scripting skills, so I'll leave this here as-is. *Thanks!*
uhoh
CC BY-SA 4.0
2021-12-04T22:45:06.410Z
I've not used Skyfield before, but it's quite easy to do in Cartes du Ciel: https://i.sstatic.net/vB00B.png If using a different program is acceptable for you then I can put it in an answer.
Aaron F
CC BY-SA 4.0
2021-12-04T23:01:31.683Z
@AaronF [What are the ways we can quickly generate custom star charts to find moving asteroids or comets in binoculars and telescopes?](https://astronomy.stackexchange.com/q/47702/7982) go for it, *thanks!*
uhoh
CC BY-SA 4.0
2021-12-04T23:34:33.627Z
[done](https://astronomy.stackexchange.com/a/47703/28758)
Aaron F
CC BY-SA 4.0
2021-12-04T23:36:33.090Z
(and I still haven't forgotten about the exit pupil question - I keep meaning to answer that, but it'll take more time to do it properly) :-)
Aaron F
CC BY-SA 4.0
2021-12-04T23:42:46.973Z
@AaronF I really wish SE included a "to do" list option. I have several random "do today in SE" text files in my laptop, but if there was just button to push to see "personally flagged for further action" I would use it all the time.
uhoh
<amateur-observing><comets><skyfield><prediction>
Make a guide map to comet comet Leonard C/2021 A1 using Skyfield
uhoh
# Make a guide map to comet comet Leonard C/2021 A1 using Skyfield

(Asked by: username_0 on 2021-12-04)

Sky and Telescope's [How Bright will Comet Leonard Get?](https://skyandtelescope.org/astronomy-news/how-bright-will-comet-leonard-get/) includes the skymap and projected (possible) light curve for comet Leonard C/2021 A1 that will potentially reach +4 magnitude this month. It has an interesting trajectory in that it's passed Earth (at least) once before but this time will leave the solar system forever.

**Question:** How to make a guide map to comet comet Leonard C/2021 A1 using Skyfield?

I know that Skyfield has a simple propagator; it can extract parameters from the minor planet data center automatically then propagate them in a *heliocentric* orbit only, and for a finder chart for binoculars that's going to be sufficiently accurate.

I also know that Skyfield has the ability to read in star catalogs which will make it possible to plot stars with their sizes indicating visual magnitudes.

But I've never really done these, instead I just concentrate on the planet data that Skyfield gets from the JPL Development Ephemerides.

[![Skymap for Comet Leonard (C/2021 A1) from Sky and Telescope](https://i.sstatic.net/GKweGm.png)](https://i.sstatic.net/GKweG.png) [![possible light curve prediction for Comet Leonard (C/2021 A1) from Sky and Telescope](https://i.sstatic.net/e8dGkm.png)](https://i.sstatic.net/e8dGk.png)

We can't promise to add bounties ahead of time (there's no commit to future bounty function) but I'm a pretty heavy bountier and so I certainly hope to be able to engage in some bountification if a working script is posted.

It should be pretty flexible, so one should be able to add the desired date, field of view (e.g. 7 for binoculars) and dimmest magnitude to plot at the top of the script and running it will show a bunch of black dots for stars and a green dot for the comet (since comets are often green).

If one is ambitious, one could draw the trajectory for say +/- 1 or 3 days and add dots or tick marks at intervals, but that seems overly ambitious.

**Tags:** amateur-observing, comets, skyfield, prediction
> **Comments:**
> **username_1**: There is a complete example in the skyfield documentation with comet neowise, including daily tickmarks: https://rhodesmill.org/skyfield/example-plots.html (top example is Venus, below that Neowise). What is missing there (except adapting it to read the data of the comet in question)?  
> **username_0**: @planetmaker Oh, the examples are (relatively) new! I hadn't seen them. Yes, what's needed is to read the data of the comet in question. It's possible someone may appreciate the opportunity to refine their scripting skills, so I'll leave this here as-is. *Thanks!*  
> **username_2**: I've not used Skyfield before, but it's quite easy to do in Cartes du Ciel: https://i.sstatic.net/vB00B.png If using a different program is acceptable for you then I can put it in an answer.  
> **username_0**: @AaronF [What are the ways we can quickly generate custom star charts to find moving asteroids or comets in binoculars and telescopes?](https://astronomy.stackexchange.com/q/47702/7982) go for it, *thanks!*  
> **username_2**: [done](https://astronomy.stackexchange.com/a/47703/28758)  
> **username_2**: (and I still haven't forgotten about the exit pupil question - I keep meaning to answer that, but it'll take more time to do it properly) :-)  
> **username_0**: @AaronF I really wish SE included a "to do" list option. I have several random "do today in SE" text files in my laptop, but if there was just button to push to see "personally flagged for further action" I would use it all the time.  

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## 0 answers

