* Astronomy

Title: Variation of the Diameter of the Sun as Measured by the Solar Disk Sextant (SDS)
Authors: Mark Reynolds, Shyeh Loi, Tara Murphy, Jon Miller, Dipankar Maitra, Kayhan Gultekin, Neil Gehrels, Jamie Kennea, Michael Siegel, Jonathan Gelbord, Paul Kuin, Vanessa Moss, Sarah Reeves, William Robbins, Bryan Gaensler, Rubens Reis, Robert Petre

The balloon-borne Solar Disk Sextant (SDS) experiment has measured the angular size of the Sun on seven occasions spanning the years 1992 to 2011. The solar half-diameter -- observed in a 100-nm wide passband centered at 615 nm -- is found to vary over that period by up to 200 mas, while the typical estimated uncertainty of each measure is 20 mas. The diameter variation appears as if it might be cyclic, although it is not in phase with the solar activity cycle; thus, the measured diameter variation is not simply an observational artifact of surface activity. The SDS is described here in detail, as is the complete analysis procedure necessary to calibrate the instrument and allow comparison of diameter measures across decades.

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