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Python implementation of Jean Meeus astronomical routines

Project description

Library of astronomical algorithms in Python.

PyMeeus is a Python implementation of the astronomical algorithms described in the classical book ‘Astronomical Algorithms, 2nd Edition, Willmann-Bell Inc. (1998)’ by Jean Meeus.

There are great astronomical libraries out there. For instance, if you’re looking for high precision and speed you should take a look at libnova. For a set of python modules aimed at professional astronomers, you should look at Astropy. On the other hand, the advantages of PyMeeus are its simplicity, ease of use, ease of reading, ease of installation (it has the minimum amount of dependencies) and abundant documentation.


The easiest way of installing PyMeeus is using pip:

pip install pymeeus

Or, for a per-user installation:

pip install --user pymeeus

If you prefer Python3, you can use:

pip3 install --user pymeeus

If you have PyMeeus already installed, but want to upgrade to the latest version:

pip3 install -U pymeeus

Properly Using PyMeeus

It is very common to try to run PyMeeus like this:

import pymeeus

mydate = pymeeus.Epoch(1992, 10, 13.0)

But if you do that, you’ll get an error like this:

Traceback (most recent call last):
  File "/home/user/test/", line 3, in <module>
    epoch = pymeeus.Epoch(1992, 10, 13.0)
AttributeError: module 'pymeeus' has no attribute 'Epoch'

This issue points to a misunderstanding that is very common in the Python world. The keyword import is used to import MODULES… but PyMeeus is NOT a module: It is a LIBRARY composed of MULTIPLE modules (Angle, Epoch, Coordinates, etc). As of today, the library Pymeeus has 19 different modules (if you look into the directory where pip stores the library, you’ll find one “.py” file per module).

Therefore if you want to use, for example, the module Angle you should use:

import pymeeus.Angle

I.e., your module is pymeeus.Angle, and not just Angle.

But there is more! When you use import to fetch a module, you must then use the dot notation to access the components of the module (classes, functions, etc). For instance:

import pymeeus.Angle

i = pymeeus.Angle.Angle(11.94524)

In this case, you are telling the Python interpreter that you want to use the class Angle (with parameter ‘11.94524’) from the module Angle belonging to the library pymeeus.

There is, however, a more practical (and common) way to handle modules using the statement from <MODULE> import <COMPONENT>. For instance:

from pymeeus.Angle import Angle
from pymeeus.Epoch import Epoch, JDE2000
from math import sin, cos, tan, acos, atan2, sqrt, radians, log10

This way is preferred because, among other reasons, only the required components are loaded into memory instead of the whole module. Also, now the component is directly added to your execution environment, which means that you no longer need to use the dot notation.

Therefore, the script at the beginning would become:

from pymeeus.Epoch import Epoch

mydate = Epoch(1992, 10, 13.0)


Author: Dagoberto Salazar

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



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’.


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

  1. Fork it (

  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 PyMeeus follows the PEP8 style guide for Python code (PEP8). We suggest you install and use a linter like Flake8 before contributing.

Additionally, PyMeeus 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!!!.


  • Neil Freeman - Fixed undefined variable in Epoch.tt2ut

  • molsen234 - Fixed bug when using fractional seconds, minutes, hours or days

  • Sebastian Veigl - Added functionality for Jupiter’s moons

  • Sophie Scholz - Added functionality for Jupiter’s moons

  • Vittorio Serra - Added functionality for Jupiter’s moons

  • Michael Lutz - Added functionality for Jupiter’s moons

  • Ben Dilday - Added __hash__() method to class Epoch

  • Zivoslav - Bug report of winter solstice

  • Devid, Hugo van Kemenade - Test suggestions

What’s new

  • 0.5.12

    • Fixed a bug in the computation of the winter solstice. Added new tests and information about proper use of the library.

  • 0.5.11

    • Added parameter local to the Epoch class constructor and the methods get_date() and get_full_date().

  • 0.5.10

    • Added methods moon_librations() and moon_position_angle_axis().

  • 0.5.9

    • Added method moon_maximum_declination().

  • 0.5.8

    • Fixed several bugs in Epoch class, and added method doy().

  • 0.5.7

    • Added method moon_passage_nodes().

  • 0.5.6

    • Added method moon_perigee_apogee().

  • 0.5.5

    • Added method moon_phase().

  • 0.5.4

    • Added methods illuminated_fraction_disk() and position_bright_limb() to Moon class.

  • 0.5.3

    • Fixed error in the return type of method Sun.equation_of_time().

  • 0.5.2

    • Added methods to compute the Moon’s longitude of ascending node and perigee.

  • 0.5.1

    • Changes in the organization of the documentation.

  • 0.5.0

    • Added Moon class and position() methods.

  • 0.4.3

    • Added method ring_parameters() to Saturn class.

  • 0.4.2

    • Added method __hash__() to Epoch. Now Epoch objects can be used as keys in a dictionary.

  • 0.4.1

    • Added funtionality to compute the positions of Jupiter’s Galilean moons.

  • 0.4.0

    • Added methods to compute Saturn’s ring inclination and longitude of ascending node.

  • 0.3.13

    • Additional encoding changes.

  • 0.3.12

    • Deleted encoding keyword from, which was giving problems.

  • 0.3.11

    • Added encoding specification to

  • 0.3.10

    • Fixed characters with the wrong encoding.

  • 0.3.9

    • Relaxed requirements, added contributor molsen234, and fixed format problems showed by flake8.

  • 0.3.8

    • Fixed undefined variable in Epoch.tt2ut.

  • 0.3.7

    • Fix bug when using fractional seconds, minutes, hours or days, plus documentation improvements.

  • 0.3.6

    • Add method to compute rising and setting times of the Sun.

  • 0.3.5

    • Add method magnitude() to planet classes.

  • 0.3.4

    • Add method to compute the parallax correction to Earth class.

  • 0.3.3

    • Add methods to compute the passage through the nodes.

  • 0.3.2

    • Add methods to compute the perihelion and aphelion of all planets.

  • 0.3.1

    • Fix errors in the elongation computation, add tests and examples of use of methods geocentric_position(), and tests and examples for Pluto class.

  • 0.3.0

    • Added Pluto class.

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