Chart · shift-scroll to zoom · drag to pan or rotate · ctrl-drag to box-select a region · click to mark a point, again to remove it · drag a point to move it · drag a readout to move its label, drop it back on the point to re-place it
cos H = (sin alt − sin φ sin δ) / (cos φ cos δ) · usable = overlap of [RA − H, RA + H] with the astronomical dark window (Sun below −18°), in local sidereal time