"Mother, any distance greater than a single span" is a beautiful poem about a son growing up and away from his mother.
In the first stanza, the narrator addresses his mother.
Mother, any distance greater than a single span
requires a second pair of hands.
You come to help me measure windows, pelmets, doors,
the acres of the walls, the prairies of the floors.
He is acknowledging the fact that she is a help to him, and he sets up this idea of measuring.
In the second stanza, we are given the image of the mother holding on to a spool of tape, letting the son go farther and farther away.
You at the zero-end, me with the spool of tape, recording
length, reporting metres, centimetres back to base, then leaving
up the stairs, the line still feeding out, unreeling
years between us. Anchor. Kite.
He compares the mother to an anchor while he compares himself to a kite. The reader gets the sense that as the years pass, "unreeling years between us," the tape measure gets longer and longer as the son goes farther and farther away.
In the third stanza, we are presented with the image of an astronaut:
I space-walk through the empty bedrooms, climb
the ladder to the loft, to breaking point, where something
has to give;
two floors below your fingertips still pinch
the last one-hundredth of an inch...I reach
towards a hatch that opens on an endless sky
to fall or fly.
The astronaut is always tied to the space ship by a cord. Like the astronaut, the son is continuing up and up, farther and farther away. The mother holds on to "the last one-hundredth of an inch" and the poem leaves us at the moment right before we find out what happens to the son. Does he fall? Does he fly?
The main message of the poem is that children have a base from which they explore the world: they have their mother. Over the years, they explore the world, traveling farther and farther away. Yet the mother will hang on till the last moment, until it's time for that child to fly on their own. And even then, she will hang on.