Constellation Boundaries for Sky in Google Earth

Google Sky with IAU Constellation BoundariesIn the year 1930 the International Astronomical Union (IAU) formalized the way that the night sky can be divided into various regions by defining boundaries for 88 constellations, which form a patchwork of neighboring areas covering the entire sky. To view the constellation regions while exploring stars and galaxies through Sky in Google Earth, download the KML file (below) and open it with Google Earth. The constellation names are then listed under Places where one can also find convenient links to information on the web about each constellation, such as to Wikipedia, pronunciation audio by Sky & Telescope, downloadable PDF constellation maps, and to Ian Ridpath’s Star Tales.

Although the illustrated boundaries are accurate for the most part, one should be careful not to interpret these borders as absolute truths, especially when zoomed in with great magnification. This may sometimes be a result of visual approximations used in Google Earth, but also due to the fact that I tweaked the IAU data ever so slightly to fix apparent issues. For example, the raw data files from the IAU contain a few weird coordinates that cause minor gaps and overlaps between boundaries (in particular between Cepheus and Ursa Minor near Polaris, Ophiuchus and Serpens Cauda near HD 170973, as well as Hydrus and Octans near θ Oct and γ Oct). These may be some of the same issues noted in the “constbnd.txt” file of the VI/49 catalogue. Furthermore, the original data-set created by Eugène Delporte (1930) uses B1875.0 coordinates, and the conversion process to the required J2000.0 equinox may need additional polygon points to be inserted for a better approximation of the line segments between corner points (similar to the interpolation points that are present in VI/49’s “bound_20.dat” file).

I am also curious to know whether the gaps/overlaps are present in the original 1930 publication (“Délimitation scientifique des constellations”, Cambridge University Press), the electronic data transcribed by Barry Rappaport, or possibly even in the earlier work on boundaries of southern constellations published by Benjamin Gould (“Uranometria Argentina”, 1877 and 1879). It seems strange that the IAU data exhibit these issues but I suppose that many astronomers may simply use data from the miscellaneous VI/42 and VI/49 catalogues in VizieR, rather than the online IAU data.

Although Google Earth already contains an option for “IAU Constellation Boundaries” (see “Sky Database | Backyard Astronomy” under Layers), enabling this feature only shows inactive color regions that fade out when the user zooms in. The download below contains an enhanced version that is interactive, which helps with the identification of a specific location within a region and it provides quick navigation options to online resources. If you know of other good online resources about the 88 constellation regions, feel free to leave a comment. Perhaps I can add these also to the balloons that popup when you click on a constellation.

Download the plug-in for Sky in Google Earth:

Screenshot of Sky in Google Earth with Plugin for Constellation Boundaries