SunSketcher: Precise Measurement of the Solar Shape via a Network of Smartphone Eclipse Observations

The shape of the solar limb is determined by sub-surface flows that reflect not only solar differential rotation, but also an assortment of other flow patterns associated with the 22-year solar cycle, the emergence of active regions, and so forth. The goal of the citizen science SunSketcher project is to measure the size and shape of the Sun to an unprecedented level of accuracy (a few parts per million), in order to constrain these flow fields. To accomplish this goal, we developed a smartphone app that calculates the times of second and third contacts at the phone’s GPS location and commands the phone camera to take a series of images spaced by about half a second near these times, when “Baily’s Beads” are visible. All data is uploaded to a central server for later analysis. Although the raw smartphone images are rather unspectacular, their real value is in the timing information, with each image time-tagged to about a millisecond. The sequences of images collected from the myriad of participant phones, spread both along and across the eclipse path, will reveal the precise times when the Baily’s Beads appear and disappear, from different vantage points. Detailed knowledge of the lunar limb profile from each phone’s vantage point can also be obtained using the LOLA database and associated software. Then, using an assumed solar limb profile, a prediction can be made of the Baily’s Bead timings and compared with those from the time-tagged phone images. This comparison drives adjustments to the assumed solar limb profile, eventually arriving at a best-fit overall profile. The overall accuracy of the measurement is set by a combination of factors, and is limited principally by the precision to which the phone’s location is determined by GPS and the accuracy to which the lunar topography is known. Each of these is of the order of 10 m, corresponding to about 4 km on the Sun, an order of magnitude better than current precision, and we expect that SunSketcher’s massive oversampling will improve the precision substantially beyond this level as analyses continue. In this initial report we will mainly describe the operation itself, with statistics on the data obtained on April 8.