E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Guards of the Pole.
The two stars and in the Great Bear. Shakespeare, in Othello, ii. 1, refers to them where he says, the surge seems “to quench the guards of the ever-fixéd pole.”
1
“How to knowe the houre of the night by the [Polar’] Gards, by knowing on what point of the compass they shall be at midnight every fifteenth day throughout the whole year.”—Norman: Safegard of Sailors (1587).
and 
in the Great Bear. Shakespeare, 
