Sin

sin = sign = the rainbow seen by Noah (who sent out two birds -- continuing the bird imagery in the paragraph)

"the fairest sin the sunsaw (that's cearc!)" = "the fairest sign the sun saw (that's the arc!)" -- suggests a reversal of perspective; the sun seeing the "sign" of the ark, instead of Noah seeing the sign of the rainbow.

The woman gathering goods in her napsack also collects either a rainbow or an ark.