Fig. 5

Three-dimensional plot of day of day length as a function of time of year and latitudes from 56°S to 56°N. Daylight change is expressed in irradiance half-life periods (E _t1/2) per Δ° of latitude. An irradiance half-life period is the time it takes for irradiance levels to double during dawn and halve during dusk. While day length around the solstices changes dramatically in the high latitudes per degree latitude travelled north or south, the dawn and dusk light gradient is also less pronounced at these latitudes. Nonetheless, day length measurement-based latitude estimates can be expected to be about 1.7 times more accurate at 56°N as compared to the equator at solstice. In conclusion, the gradients in day length are greatest during the summer and winter solstices, when the days are longest and shortest in the northern and southern hemispheres; the gradients are weakest during the fall and spring equinoxes