Penny is Desmond's "constant" partly in a physics sense and partly in an emotional sense. She's always been there for him. She waits for him forever (just like Penelope did for Odysseus, another sailor lost at sea for many years), except in "The Constant" she's wearing a wedding ring! She married the guy that she and Desmond talked about just before he started running stairs. (Remember, Penny's father tells Desmond in the car outside the military prison that Penny was engaged?) How "constant" is she if she gave up on him and married some other guy?

Of course, it's kinda hard to blame her. He was gone a long, long time! But still, she his constant, she's supposed to wait!

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"Helen, do you remember that authentic Australian walkabout I told you about?"