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hazy and partially clouded skies, because the
position of the zeros is unchanged by cloud. No
other method shows this invariance.
However, much remains uncertain about insect
navigation. For the algorithm to be accurate the top
of the robot or drone must be pointing towards the
zenith. Insects may do this using the 3 ocelli on the
top of their heads (Goodman, 1970). But how they
would do this is not clear. Many insects also have
ommatidia in sets of three with the polarization axes
of the 3 sets differing by about π/3 (Labhart, 1988;
Wehner, 2001). There are several possibilities. If 3
ommatidia from different sets point at the same
patch of sky they could be used to calculate the
polarization angles χ, even in a cloudy sky.
Alternatively they might help identify in one of them
the signs of the corrections ±γ and ±δ if less than 4
zeros are visible. Yet another possibility is that they
are looking also for the 4 zeros where χ = ±π/4 + π/3
and the 4 zeros where χ = ±π/4 - π/3. The algorithm
above might then be repeated for each of the 3 sets
of zeros, giving 3 different approximations for the
solar azimuth. All these possibilities are being
investigated.
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