From the window of my studio here in the west of Ireland I look out into the wide Atlantic. This is the westernmost point of Europe; next comes North America, thousands of miles away and drifting farther away each day at a rate of about an inch a year.
There are lots of theoretical and empirical reasons for believing that the Atlantic widens, but the clincher is that the drift can be directly measured. One method, borrowed from astronomers, is known as Very Long Baseline Interferometry. Two widely separated dish antennae -- in Europe and the United States, say -- record radio signals from a distance source, typically a galaxy or quasar billions of light years away. The slight difference in the arrival times of the signals are recorded with atomic clocks. If these differences are measured for at least three different sources, it is possible to calculate the distance between the two receivers with centimeter accuracy. This has now been done over a long enough period to show that, yes, the continents are drifting apart.
I love the idea that radio waves that have been traveling across the universe for billions of years can be harnessed to confirm that the Atlantic widens -- has in fact been widening since that time 200 million years ago when I could have walked dry-shod from Dingle to Boston. What a glorious stretch of our imaginations.