Every morning before I sit down to write I go look at the sky above Nairobi, where I live part-time. I’ve got a big picture window facing north, wide enough to frame great masses of clouds and a half-dozen falcons playing in thermals as if life is nothing but joy. What I love about this view is it’s always different. Nairobi almost never has blue skies nor gray skies, but always a mix of the two: heavy thunderclouds laboring with bellyfuls of rain and fringed at the top with ice-blue from the sun behind, or soft eraser-marks of cirrus clouds over a mat of turquoise, or cumulus clouds as perfectly-shaped as a flock of sheep marching north towards some heavenly pasture.

And yet I simultaneously associate the sight of this particular sky with constancy. It’s always different, but it’s always there. Gazing at it makes me feel calm, anchored, safe, ready to start the day with the knowledge that something beyond my plate glass is steadfast for me.

Oddly, I don’t think I would have this feeling if the Nairobi sky were pure blue. Its changeability helps me see its steadfastness. The psychologist William Warren, who studies how human beings perceive and navigate by things in their environments, told me that “there’s a lot of evidence for this [kind of perception] on a short time-scale: our attention is grabbed by ‘transients,’ or quick changes.” We don’t notice nor feel thankful for our dining-room table’s changelessness; we never say, “Thank God for my solid floor today.” For me, the Nairobi sky’s unbroken promise to daily share its infinite variety felt like a more wondrous gift than, say, the truly permanent pavement below it.

We sometimes presume constancy and change are opposed to each other. “He’s my rock.” “She’s changed.” With that presumption, we can deny our lives, and our loved ones, cycles of change. In fact, change and constancy are related. We recognize this when we speak in metaphors. Often our metaphors of constancy are built around things that actually wax and wane. The Psalmist wished his reign “will be established forever like the moon, the faithful witness in the sky.” Keats and Joni Mitchell alike wished to be as constant as a star — which can’t be seen half the time. Faith includes a cycle of absence, of doubt; otherwise it’s dogma, default.

Might it be, too, that modern life has oddly diminished our ability to perceive constancy and stability in the world by seeking too much pure permanence? More and more stores are open 24 hours a day. Heat and air conditioning keeps us from experiencing the full wheel of the seasons. Not very many of us tend plants. Nothing endures “as April’s green endures,” the poet Wallace Stevens declared, a hundred years ago.

Now the supermarket lettuce aisle is green as April all year — and we are less anchored for it.