How does true love correspond to the polar star in Sonnet 116?

In Sonnet 116, Shakespeare never uses the word "polar" star, but it is what he means when he refers to a star that is an "ever-fixed mark" and "the star to every wand'ring bark [ship at sea]." This star, very bright in the sky, appears to hang right over the North Pole. It is steady and reliable—never changing—so sailors can use it for navigation.
The speaker of this sonnet is comparing true love to the polar star. Like the pole star, true love never changes. It can always be relied on. Even when the beloved changes by aging and becoming less attractive, such true love never alters or fades. It is based on an attachment between two souls that goes far deeper than looks.
Other stars may move across the sky and change position, just as false lovers change from one beloved to another, but true love, like the unchanging polar star, will always stay the same.