PyPlanets

Library of astronomical algorithms to compute planetary ephemeris written in Python.

PyPlanets at its current state is basically a refactored version of PyMeeus. PyMeeus itself is a Python implementation of the astronomical algorithms described in the classical book "Astronomical Algorithms, 2nd Edition, Willmann-Bell Inc. (1998)" by Jean Meeus. The original author of PyMeeus is Dagoberto Salazar. This fork is based on commit 9b25a79 (https://github.com/architest/pymeeus/commit/9b25a790999251afd5d68c074e144df9f74a6dfd).

Why refactor PyMeeus?

That’s a valid question of course, especially since no (or only little) functional enhancements are made. But that is also part of the answer. Looking for astronomical libraries to "play around", I stumbled across PyMeeus which really gets most of the job done. On the other hand, when trying to extend PyMeeus with some features I got myself lost a bit in the functional coding so I decided to restructure and refactor the code base to my liking (and experience) before adding new features.

Main guiding principles for refactoring have been object-orientation and a clearer separation of concerns to ease the handling of the modules and facilitate extensibility. As a side-effect it is now e.g. straight forward to compute e.g. the mars-centric co-ordinates of Earth for a given point in time. Well, you might correctly object here "Why would someone require that?", but

1. it comes at no extra cost, since it’s just a side effect of object orientation making it clearer, that the relative ecliptical position of a planet depends on the view point and therefore is not a property of a planet and
2. maybe Elon Musk will find this useful in his new home someday (@Elon: please drop me a note then).

PyPlanets vs. PyMeeus - what’s the difference?

The most obvious difference is the file (module) structure, and an increased number of modules with a lower number of lines of code in total (hopefully increasing maintainability). The higher number of modules is due to

• moved VSOP87 parameters to separate files (../parameters)
• moved usage examples from original sources to separate modules (../examples/pymeeus)
• added common base class for planets (Planet.py)
• separated class Ellipsoid and related methods from Earth.py to ellipsoid.py
• restructured (simplified) method vsop_pos(...) of module Coordinates

A lot of effort has been made by Dagoberto to create meaningful tests and documentation for the PyMeeus library in the form of test cases, usage examples and inline documentation including doctests. These are preserved to its full extent, but needed to be updated to the new module structure. Basically it boils down to three changes:

Replace calls to static methods with calls to instance methods

PyMeeus:

# Let's now compute the heliocentric position for a given epoch
epoch = Epoch(2018, 10, 27.0)
lon, lat, r = Mars.geometric_heliocentric_position(epoch)

PyPlanets:

# Let's now compute the heliocentric position for a given epoch
epoch = Epoch(2018, 10, 27.0)
mars = Mars(epoch)
lon, lat, r = mars.geometric_heliocentric_position()

Computation of geocentric coordinates is achieved by class Constellation

PyMeeus:

# Compute the geocentric position for 1992/12/20:
epoch = Epoch(1992, 12, 20.0)
ra, dec, elon = Mars.geocentric_position(epoch)

PyPlanets:

epoch = Epoch(1992, 12, 20.0)
mars = Mars(epoch)
earth = Earth(epoch)
constellation = Constellation(earth, mars)
ra, dec, elon = constellation.geocentric_position()

Especially this change seems a bit verbose, but it is planned to a) generalize the Constellation class also to be useful for viewpoints other than earth and b) introduce a convenience pattern to provide the usual geocentric coordinates.

Method perihelion_aphelion(...) split into dedicated methods

PyMeeus:

# Find the epoch of the Aphelion closer to 2032/1/1
epoch = Epoch(2032, 1, 1.0)
e = Mars.perihelion_aphelion(epoch, perihelion=False)

PyPlanets:

# Find the epoch of the Aphelion closer to 2032/1/1
epoch = Epoch(2032, 1, 1.0)
e = Mars(epoch).aphelion()

Functional improvements

For precise planetary positions it is necessary to introduce light-time corrections for computed and observed positions of planets. The calculation used in the PyMeeus library computes the correction once. While in most cases probably sufficient, the accuracy can be enhanced by recalculating the correction factor several times until the arithmetic precision of the machine is reached. Typically, about 4 - 5 iterations are required, not having too much impact on processing times.

Comparison PyMeeus / PyPlanets

To make sure refactoring didn’t break obvious things, two measures were taken:

• Usage examples of PyMeeus have been outsourced to ./examples/pymeeus and ported to the slightly new API. A script executes the examples against the “old” and “new” API and compares the output to stdout.
• Tests from PyMeeus are preserved under ./tests/pymeeus and ported to the slightly new API.

Roadmap

Amongst other things, it is planned to

• add more algorithms from Meeus’ book e.g. including ephemeris of moons
• add some visualization options, e.g. by integrating Jupyter notebooks
• have a look at Astropy and find ways to integrate with or benefit from

Installation

The easiest way of installing PyPlanets is using pip:

pip install pyplanets

Or, for a per-user installation:

pip install --user pyplanets

Meta

Author: Martin Fünffinger

Distributed under the GNU Lesser General Public License v3 (LGPLv3). See LICENSE.txt and COPYING.LESSER for more information.

Documentation: https://pyplanets.readthedocs.io/en/latest/

GitHub: https://github.com/martin5f/pyplanets

If you have Sphinx installed, you can generate your own, latest documentation going to directory ‘docs’ and issuing:

make html

Then the HTML documentation pages can be found in ‘build/html’.

Contributing

The preferred method to contribute is through forking and pull requests:

1. Fork it (https://github.com/martin5f/pyplanets/fork)
2. Create your feature branch (git checkout -b feature/fooBar)
3. Commit your changes (git commit -am 'Add some fooBar')
4. Push to the branch (git push origin feature/fooBar)
5. Create a new Pull Request

Please bear in mind that PyPlanets follows the PEP8 style guide for Python code (PEP8). We suggest you install and use a linter like Flake8 before contributing.

Additionally, PyPlanets makes heavy use of automatic tests. As a general rule, every function or method added must have a corresponding test in the proper place in tests directory.

Finally, documentation is also a big thing here. Add proper and abundant documentation to your new code. This also includes in-line comments!!!.

What’s new

• 0.4.2
  • Initial refactoring completed, basic release and documentation workflow established.
• 0.0.1 - 0.3.6 (pymeeus - previous releases)
  • See https://github.com/architest/pymeeus#whats-new