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tokens: galaxy. Some arcs are longer than others, some may wind in the opposite direction to the majority and some ‘arcs’ are actually just noise mistaken for an arc. We found that the most reliable measure of the winding direction of the galaxy was a length-weighted vote of the winding direction of all the detected spiral arcs. Figure 3. Steps sparcfire (Davis Hayes 2014 ) takes in describing a spiral galaxy image. (a) The centred and de-projected image. (b) Contrast-enhanced image. (c) Orientation field (at reduced resolution for display purposes). (d) Initial arm segments found via hierarchical agglomerative clustering (HAC) of nearby pixels with similar orientations and consistent logarithmic spiral shape, overlaid with the associated logarithmic spiral arcs fitted to these clusters. (e) Final pixel clusters (and associated arcs) found by merging compatible arcs. (f) Final arcs superimposed on image (a). Red arcs wind S-wise, cyan arcs wind Z-wise. To demonstrate that this measure is unbiased with regard to spin direction, we refer to Fig. 4 , which shows a scatter plot of the left-to-right mirrored versus unmirrored value of the galaxy's pitch angle as measured by sparcfire ; in a perfect reversal, the two should be negatives of each other. We find that in 29 094 out of 29 250 cases (99.47% of cases), the two are negatives of each other to within 10 −4 deg. Even more relevant to this paper, we find that in all but five cases (99.983% of cases), the chirality of the mirrored image is correctly flipped compared to the unmirrored case. Thus, the chirality determination of sparcfire is unbiased, with respect to flipped images, to better than two parts in 10 4 . (It is instructive to discuss why a deterministic algorithm like sparcfire is not 100% unbiased to image flips; we discuss some of the issues in the appendix.) Figure 4. Galaxy-level pitch angles reported by sparcfire using unmirrored and left-to-right mirrored input images across 29 250 ‘clear’ spiral galaxies (see text for definition). These galaxy-level pitch angles are calculated as the arc-length-weighted average of all arcs agreeing with the dominant winding direction (as determined by an arc-length-weighted vote). We see that for almost all galaxies the measured pitch angle almost exactly negative, as it should be. The diagonal line gives y = − x ; cases on this line are visually underrepresented due to overlap. More importantly, only five cases out of 29 250 disagree on chirality, showing that sparcfire is chirality unbiased to a level of almost one part in 10 000. We then ran sparcfire on the entire Galaxy Zoo sample of galaxies, in order to determine the chirality of galaxies in an unbiased manner. However, we still used the human GZ1 determination of P S + P Z to select which galaxies actually display spiral structure. The second quarter of Table 2 details the statistical significance of the winding direction bias, as a function of the human confidence in observed spiral structure,