ever-fixed mark (5): i.e., a lighthouse (mark = sea-mark).
Compare Othello (5.2.305-7):
Be not afraid, though you do see me weapon'd;
Here is my journey's end, here is my butt,
And very sea-mark of my utmost sail.
the star to every wandering bark (7): i.e., the star that guides every lost ship (guiding star = Polaris). Shakespeare again mentions Polaris (also known as "the north star") in Much Ado About Nothing (2.1.222) and Julius Caesar (3.1.65).
Whose worth's unknown, although his height be taken (8): The subject here is still the north star. The star's true value can never truly be calculated, although its height can be measured.
Love's not Time's fool (9): i.e., love is not at the mercy of Time.
Within his bending sickle's compass come (10): i.e., physical beauty falls within the range ("compass") of Time's curved blade. Note the comparison of Time to the Grim Reaper, the scythe-wielding personification of death.
edge of doom (12): i.e., Doomsday.
Compare 1 Henry IV (4.1.141):
Come, let us take a muster speedily:
Doomsday is near; die all, die merrily.