The Doldrums

The great English poet, Samuel Coleridge, is referring to the doldrums in the lines below, excerpted from his longest poem, penned in 1798, The Rime of the Ancient Mariner.

All in a hot and copper sky,
The bloody Sun, at noon,
Right up above the mast did stand,
No bigger than the Moon.

Day after day, day after day,
We stuck, nor breath nor motion;
As idle as a painted ship
Upon a painted ocean.

Many of the sailors on Coleridge's ship succumbed from lack of water, their parched condition resulting from being on a sailing ship, becalmed in the doldrums.

How do we know he refers to the doldrums, not the horse latitudes?