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We performed an image search for "Comet Holmes," using the Yahoo Web searchengine, on 2010 April 1. Thousands of images were returned. We astrometricallycalibrated---and therefore vetted---the images using the Astrometry.net system.The calibrated image pointings form a set of data points to which we can fit atest-particle orbit in the Solar System, marginalizing over image dates anddetecting outliers. The approach is Bayesian and the model is, in essence, amodel of how comet astrophotographers point their instruments. In this work, wedo not measure the position of the comet within each image, but rather use thecelestial position of the whole image to infer the orbit. We find very strongprobabilistic constraints on the orbit, although slightly off the JPLephemeris, probably due to limitations of our model. Hyperparameters of themodel constrain the reliability of date meta-data and where in the imageastrophotographers place the comet; we find that ~70 percent of the meta-dataare correct and that the comet typically appears in the central third of theimage footprint. This project demonstrates that discoveries and measurementscan be made using data of extreme heterogeneity and unknown provenance. As thesize and diversity of astronomical data sets continues to grow, approaches likeours will become more essential. This project also demonstrates that the Web isan enormous repository of astronomical information; and that if an object hasbeen given a name and photographed thousands of times by observers who posttheir images on the Web, we can (re-)discover it and infer its dynamicalproperties.

Searching for comets on the World Wide Web: The orbit of 17P/Holmes from the behavior of photographers