What are the metaphors used by the poet to refer to the ever-fixed nature of true love in Sonnet 116?

Shakespeare Sonnet
116

Let me not to the marriage of true
minds
Admit impediments. Love is not love
Which alters when it
alteration finds,
Or bends with the remover to
remove:
O no! it is an ever-fixed mark

That looks on tempests and is never
shaken;
It is the star to every wandering
bark,
Whose worth's unknown, although his height
be taken.
Love's not Time's fool, though rosy lips and cheeks

Within his bending sickle's compass come:
Love alters not with his
brief hours and weeks,
But bears it out even to the edge of
doom.
If this be error and upon me proved,
I never writ, nor no man
ever loved.

Note the bolded section of the sonnet.  Here
is where you can find the metaphor referring to the fixed nature of
love.

Love is compared to a lighthouse
(ever-fixed mark) which remains visible during a storm
(tempest).  Love is also compared to the north star
(start), which remains fixed in the sky to guide ships
(wandering
bark).

You may find the attached
website helpful.  It takes Sonnet 116 and paraphrases each
line.