When the Astronomer stops seeking — Treason by Nature

A lost Pleiad leads to a missing husband in the Arctic, which hints that a sailor doubts his compass when sailing near the North pole. Treason comes eventually, no matter what they do.

When the Astronomer stops seeking 5
For his Pleiad's Face-
When the lone British Lady
Forsakes the Arctic Race

When to his Covenant Needle 5
The Sailor doubting turns-
It will be amply early
To ask what treason means.
(F.957/J.851)
[1] stops seeking:: the astronomer knows that the lost Pleiad can never be found, but he will not stop seeking (like Lady Franklin) except after his death.
[2] Pleiad:: the lost Pleiad, a lover lost forever. Pleiad's singular form is a hint.
[3, 4] British Lady, Forsakes:: Lady Jane Franklin (1791-1875) did not forsake to seek her husband even that Franklin was reported died in 1847 by McClintock in 1859. This poem was written around 1865. The last search for Franklin was in 1875.
[5, 6] Needle, Sailor doubting:: compass doesn't work in polar regions.
[7] ample early:: it's not happened yet, but definitely will.
[8] treason means:: to know that treason happens, irresistibly.

The Seven Stars were supposed to be the seven daughters of Atlas, . . . They were all united to the immortal gods, with the exception of Merope who married Sisyphus, king of Corinth, and whose star therefore is dim and obscure among the rest of her sisters. The "lost Pleiad," the "sorrowing Merope," has long been a favorite shadowy creation of the poetic dream. ─ Hours at home, Volume I (1865)

In many a nameless being we retrace,
Whose course and home we knew not, nor shall know,
Like the lost Pleiad seen no more below.
The Poetical Works of Lord Byron (1861)

European News. — The screw steamship Fox, Captain McClintock, sent by Lady Franklin to the Arctic regions in search of the traces of Sir John Franklin's expedition, had returned to England, . . . Sir John Franklin had died on June 11, 1847, and the total deaths to date bad been nine officers and fifteen men. ─ Friends' Intelligencer, Volume 16 (1860)