Archaeoastronomy: compendium of free sources of data and software

[This post is part of a series of posts on archaeoastronomy using open source software]

Satellite Imagery

I use the free Google Earth software program to obtain satellite images of archaeological sites. If you download the Google Earth virtual globe program to your computer and start it up, in the search bar on the upper left hand side you can search for locations.  In this module I describe how I use Google Earth to obtain satellite imagery of a site.

Placing Datum Points on Key Features of an Archaeological Site

Xfig is a free and open-source downloadable program for vector graphics.

In Xfig, I upload a satellite figure of an archaeological site, and then overlay circles over features in the site, trying to match as closely as possible their apparent diameter and position.  In this module, I describe how I do that.

Calculating the Changing Positions of the Stars in the Sky over Time

The pyephem ephemeris calculation library written in the python programming language is much more applicable for archaeoastronomy. I created a script stars_print_out_ra_and_dec.py that calculates the equatorial coordinates for stars going back in 50 year increments from present to 5000BC (I describe all of this in this module).  The script also does this calculation for the Pleiades and Hyades clusters (which aren’t one star, but are tight clusters of stars), and the supernovae in 1006 and 1054 (which, obviously, have coordinates that only make sense on those dates because the supernovae rapidly faded in brightness). To run the script on Unix/Linux/MacOS, download and install the pyephem library in python, and type

python ./stars_print_out_ra_and_dec.py

The output with the star name, date, right ascension and declination coordinates is in the file stars_print_out_ra_and_dec.out (note that if you aren’t interested in modifying the python script, all you need is that output file… the equatorial coordinates for the stars given in the file work for analysis of any archaeological location in the world as long as it wasn’t built before 5000BC).

Horizon Profile

In order to calculate where stars rise and set at a particular location, you need to know the horizon profile, as I describe in this module.

HeyWhatsThat website is a free online tool to calculate the horizon profile for any point on Earth.  I describe how to use it in this module.  I’ve noticed that in mountainous terrain the HeyWhatsThat calculation doesn’t capture some far away mountains that are obviously visible on the horizon when you are actually at the site.  So, for locations in mountains, like Machu Picchu for instance, use HeyWhatsThat with caution.

There is another free software tool for horizon calculations called Horizon, but it is only available for Windows (which I don’t use), thus I cannot vouch for how easy to use it is, or as to what data formats it inputs and outputs, and whether or not is does reliable calculations for hilly terrain.

Calculating the Rise/Set Azimuths of Stars on the Horizon

The rise/set “azimuth” of a star is the bearing from North on the horizon that the star rises/sets.  The US Naval Observatory has a nice website with an online calculator that provides the rise/set azimuth and times of rise/sets for the brightest stars, the Sun, and the Moon for any location on Earth and date (within recent past or future) that you provide.  The website assumes that the horizon is flat.  Because the website only can do the calculation for recent past or future, it isn’t ultimately that useful for research in archaeoastronomy.  Still it is fun to use.

You can also use pyephem to get the rise/set azimuths for stars at a certain date at a location with a flat horizon.

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