Why have ‘sixty degrees of longitude… / Vanished’?

Last night I did not fight for sleep
But lay awake from midnight while the world
Turned its slow features to the moving deep
Of darkness, till I knew that you were furled,

Beloved, in the same dark watch as I.
And sixty degrees of longitude beside
Vanished as though a swan in ecstasy
Had spanned the distance from your sleeping side.

Consider his thoughts and feelings when separated from his partner. Look at the words ‘same dark watch as I’.