What does he think may have been his mistake?

We watched it apart, and perhaps that was my mistake,
letting the half-darkness fall over you in the city,

while I traced its spreading hand across the fields,
following the rooks, flying in threes to roost.

But as the sun became quarter, then half moon,
it unlocked in me, and I saw us connected again,

by the day’s slowing to monochrome, by the mid-day midnight breeze
and by the moon’s shadow passing over and between us.

It was, however, just a trick of the light,
as I learnt, on returning and calling you that night;

listening to your voice down the line,
cooled by his presence, eclipsed and clipped.

And then, on going to sleep, the dream –
his shadow falling across your up-looking face –

his shadow, falling across your memory of me.

Where is she and where is he?
Why might watching an eclipse together have been significant for them?