We can identify the 2nd quarter 'person' with the 'worm' Rigi:

... By repeating a charm, she opened the two shells and slipped inside. She could see nothing, because the sun and the moon did not then exist; and then, she could not stand up because there was not enough room in the shellfish. Constantly hunting about she at last found a snail. To endow it with power she placed it under her arm, lay down and slept for three days. Then she let it free, and still hunting about she found another snail bigger than the first one, and treated it in the same way. Then she said to the first snail: 'Can you open this room a little, so that we can sit down?' The snail said it could, and opened the shell a little. 

Old-Spider then took the snail, placed it in the west of the shell, and made it into the moon. Then there was a little light, which allowed Old-Spider to see a big worm. At her request he opened the shell a little wider, and from the body of the worm flowed a salted sweat which collected in the lower half-shell and became the sea. Then he raised the upper half-shell very high, and it became the sky. Rigi, the worm, exhausted by this great effort, then died. Old-Spider then made the sun from the second snail, and placed it beside the lower half-shell, which became the earth ...

Careful reading leads to conclusions:

There were two snails, a little one and a bigger one. The first, the little one, became the moon and her location was in the west. The bigger snail became the sun and his location was in the east.

Both snails first needed power, and that was accomplished by 3 days in the armpit while Old Spider lay down. At winter solstice movement stops, a time for lying down. The 3 nights (rather than days) of complete darkness are those between 365 and 362 (= 180 + 182) when 'gods are born'. The armpit is a center for re-creation.

Rigi is neither of the two snails but another being, a very powerful 'worm'. Exhausted he dies at summer solstice. In his stead the salty sea emerged and sun is not placed in the east until the worm has gone.

Rigi is obviously a wordplay on ragi (sky).