If you missed marking the longest day in the Northern Hemisphere, you can still celebrate the latest sunset..
Sungraph is a cute calculator to find sunrises and sunsets.. (OS X).
We've talked about why sunrises and sunsets don't line up with the expected day. I've put the question to people with reasonable educations and generally find that most physicists and astronomers can offer an explanation (they've thought about it before), but it baffles most people until they puzzle it out ... the critical elements are the tilt of the Earth's axis and its elliptical orbit.
If you want to cheat, check out the Analemma page.
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