dead reckoning

v. intr. finding yourself bothered by somebody's death more than you would have expected, even if they were only an abstract presence in your life, like a lighthouse in the distance that suddenly goes dark, leaving you with one less landmark to navigate by.

In navigation, dead reckoning is the practice of using your prior course to extrapolate your subsequent position. It can be useful on starless nights but often leads to cumulative errors; if you don't often check your position against new data, you might end up completely lost.