Shadow bands

Shadow bands are thin, wavy lines of alternating light and dark that can be seen moving and undulating in parallel on plain-coloured surfaces immediately before and after a total solar eclipse.

[2][3][4] The shadows' detailed structure is due to random patterns of fine air turbulence that refract the collimated sunlight arriving from the narrow eclipse crescent.

The degree of collimation in the light gradually increases as the crescent thins, until the solar disk is completely covered and the eclipse is total.

However, they are so small that even the visible disk of planets (Venus, Jupiter, etc) are larger than the "dancing" scale, and so their brightness appears to remain steady.

Similarly, shadow bands are essentially the "twinkling" of the Sun's thin crescent in the seconds surrounding totality.