pyalmanac 1.0.1

Creates the daily pages of a Nautical Almanac using Ephem

This is the PyPI edition of Pyalmanac-Py3. Version numbering starts from 1.0 as the previous well-tested Pyalmanac-Py3 versions that are on github were never published.

Pyalmanac is a Python 3 script that creates the daily pages of the Nautical Almanac. These are tables that are needed for celestial navigation with a sextant. Although you are strongly advised to purchase the official Nautical Almanac, this program will reproduce the tables with no warranty or guarantee of accuracy.

Pyalmanac was developed based on the original Pyalmanac by Enno Rodegerdts. Various improvements, enhancements and bugfixes are implemented. Pyalmanac contains its own star database (similar to the database in Ephem 3.7.6 whose accuracy was sub-optimal). It has been updated with data from the Hipparcos Star Catalogue and the GHA/Dec star data now matches a sample page from a Nautical Almanac within 0°0.1’ (which is very good).

However Pyalmanac is nearing the end of its useful days. Almanacs generated after 2022 should not be used for navigational purposes. SFalmanac (or Skyalmanac with some restrictions regarding the accuracy of sunset/twilight/sunrise and moonrise/moonset) are the new norm as these are based on the more accurate algorithms employed in the NASA JPL HORIZONS System. The same algorithms are implemented in Skyfield.

Pyalmanac is implemented using Ephem (originally named PyEphem), which in turn uses XEphem that is based on the VSOP87D algorithms. XEphem is also ‘end of life’ as no further updates are planned. The major discrepancies are related to the projected speed of Earth’s rotation. Even though the future speed of the Earth’s rotation is a matter of conjecture, currently the discrepancies in GHA between Ephem and Skyfield (1.31) can be summarized thus:

• in 2020: 00.0 to 00.1 arcMINUTES GHA too high in Ephem

• in 2030: 04.0 to 04.8 arcMINUTES GHA too high in Ephem

• in 2050: 13.9 to 14.9 arcMINUTES GHA too high in Ephem

• in 2100: 38.0 to 40.2 arcMINUTES GHA too high in Ephem

• in 2200: 90.1 to 94.1 arcMINUTES GHA too high in Ephem

The GHA discrepancy above applies to the sun, moon, the First Point of Aries and to all planets.

Software Requirements

Most of the computation is done by the free Ephem library.

Typesetting is done typically by MiKTeX or TeX Live.

These need to be installed:

• Python v3.4 or higher (the latest version is recommended)

• Ephem >= 3.7.6

• MiKTeX  or  TeX Live

Installation

Install a TeX/LaTeX program on your operating system so that ‘pdflatex’ is available.

Ensure that the pip Python installer tool is installed. Then ensure that old versions of PyEphem, Ephem and Pyalmanac are not installed before installing SkyAlmanac from PyPI:

python -m pip uninstall pyephem ephem pyalmanac
python -m pip install pyalmanac

Thereafter run it with:

python -m pyalmanac

On a POSIX system (Linux or Mac OS), use python3 instead of python above.

This PyPI edition also supports installing and running in a venv virtual environment. Finally check or change the settings in config.py. It’s location is printed immediately whenever Pyalmanac runs.

Guidelines for Linux & Mac OS

Quote from Chris Johnson:

It’s best to not use the system-provided Python directly. Leave that one alone since the OS can change it in undesired ways.

The best practice is to configure your own Python version(s) and manage them on a per-project basis using venv (for Python 3). This eliminates all dependency on the system-provided Python version, and also isolates each project from other projects on the machine.

Each project can have a different Python point version if needed, and gets its own site_packages directory so pip-installed libraries can also have different versions by project. This approach is a major problem-avoider.

Troubleshooting

If using MiKTeX 21 or higher, executing ‘option 5’ (Increments and Corrections) it will probably fail with:

! TeX capacity exceeded, sorry [main memory size=3000000].

To resolve this problem (assuming MiKTeX has been installed for all users), open a Command Prompt as Administrator and enter:

initexmf --admin --edit-config-file=pdflatex

This opens pdflatex.ini in Notepad. Add the following line:

extra_mem_top = 1000000

and save the file. Problem solved. For more details look here.