RA/Dec Twitter bot

This is the code used to run the RA/Dec Twitter Bot, @ra_dec_bot. This bot periodically tweets the right ascensions and declinations of the Sun, Moon, and planets, along with the phase and illumination of the Moon.

Each time the script is called, it will generate two tweets. One will look something like this:

Current RA/Dec of the Sun & Moon:

☉: 21h22m18s; -15°23′32″
☾: 17h06m14s; -22°50′09″

The moon is a waning crescent and is 34% illuminated.

And the other will look something like this:

Current planetary RA/Decs:

☿: 21h31m14s; -11°2′08″
♀: 20h36m25s; -19°27′05″
♂: 02h50m31s; +17°45′15″
♃: 20h55m14s; -17°55′50″
♄: 20h32m43s; -19°15′42″
⛢: 02h17m57s; +13°20′03″
♆: 23h21m54s; -05°15′59″

Installation

The purpose of this code is more to be inspected rather than to be widely distributed and in common use. I want it to be easy to see where the numbers that @ra_dec_bot tweets come from. Since this is unlikely to be widely installed, I didn't want to pollute the Python package index namespace and so the installation will take a few more steps than the typical Python package (though hopefully not too many).

1. Clone the repository.
2. cd /path/to/repo
3. Create a virtual environment and enter into it.
4. Run pip install -r requirements.txt.

Setting up the Twitter API keys

If you would like to run a Twitter bot of your own, you will need to follow the instructions to set up a Twitter developer account.

You will need to create an app associated with the account you will be tweeting from, and then generate API keys. In particular, in order to tweet you will need the following:

• API Key
• API Secret Key
• Bearer token
• Access token
• Acess token secret

These should be stored in a YAML file in $HOME/.config/radecbot/config.yaml.

Usage

To run the script and tweet, simply run

python /path/to/repo/radecbot/radecbot.py

This will generate the text for the two tweets and automatically post them (assuming you have set up the API keys correctly).

I recommend that you install and run this from within a unique virtual environment in order to keep the dependencies from changing underneath you. I personally find virtualenvwrapper to be convenient.

The first time the script is run it will need to download the ephemerides from JPL. While the ephemerides are not an especially large file (17 MB), their server is also not especially fast so it will take a little longer on your first run. Subsequently, radecbot caches the ephemerides at $HOME/.cache/radecbot. This will tide you over until 2053, at which point you will need to download a new set of ephemerides.

Running radecbot periodically

@ra_dec_bot tweets at noon and midnight UTC every day. If you are in a Unix environment, you can set up this script to do this by creating a cron job. To edit your cron jobs, run:

crontab -e

And then add a line that looks like this:

0 */12 * * * /path/to/virtual/environment/python/executable /path/to/repo/radecbot/radecbot.py

FAQs

Where do the positions come from?

This code uses Skyfield to calculate the current positions of the Sun, Moon, and planets. Behind the scenes Skyfield is using a set of ephemerides provided by JPL. In particular, radecbot uses the DE421 ephemerides, which is valid from July 29, 1899 to October 9, 2053.

These positions are different from WolframAlpha! What gives?

Some services like WolframAlpha and Stellarium provide positions that disagree from this code by up to a few arcminutes. I have not been able to reverse engineer exactly how they are computing their positions, but because these services closely agree with each other, I suspect that they're using some lightweight, lower precision algorithm.

At any rate, the positions provided by @ra_dec_bot match with the positions provided by JPL's Horizon service to within one arcsecond.