On overcast days, when they finally dip beneath the cloud cover, they look like spaceships, gliding from the heavens with their lights ablaze. And they’re coming right at you. This is how it feels, anyway, on the stretch of beach and boardwalk just before South Boston’s Castle Island — about 19 benches out from the parking lot, to be precise — which is one of the best places around to plane-watch. When the wind is right, jets big and small approach Logan International Airport from the south-southwest, and their course brings them in over the glassy waters of Pleasure Bay, landing gear out, Blue Hills in the background. Slowly, the deep rumble gathers into a roar, building and building until it sweeps across the sky overhead. (The bigger the jet, the better.) And the planes usually come at a good clip; Logan averages about 550 landings daily. I like to bring my kids here when we’re dropping grandparents off at the airport. We marvel together at these lumbering birds and how effortlessly they seem to float onto the waiting runways.