Figure 1.

Illustration of the principle of a solar heading indicator, showing the trajectories that would be followed by (a, b, c) a bird that always flew at a constant heading angle with respect to the sun, and (d, e) a bird that flew at a heading angle corrected for drift at a constant rate of π/12 radians/h. The trajectories are drawn using ephemeris data for the Royal Observatory Greenwich, at (a, d) the winter solstice, (b, e) the spring equinox and (c, f) the summer solstice. Each trajectory covers an entire 24 h period for the insight that this yields; that period during which the sun would not be visible is shown by a dashed line; the filled circles mark hourly intervals. To read the graphs, simply pick a release time (in GMT) and follow the trajectory forward in time from this point. Note that there is little curvature apparent in any of the graphs on a timescale of much less than an hour, and that an almost straight trajectory can result on a timescale of much longer than an hour if drift is corrected at a constant rate of π/12 radians/h (i.e. 15°/h). The inset to (b) shows an enlarged view of the trajectory between 1200 and 1300 hours, to provide a better indication of the extent of the deviation from straight flight in this simple case in which the bird follows an elliptical trajectory. Ephemeris data were obtained using the NASA Jet Propulsion Laboratory's HORIZONS software (Giorgini et al., 1996).