Meaning of “mooned in a window”

meaning of “mooned in a window”

The verses below is a poem titled For Sale By Robert Lowell

Ready, afraid

of living alone till eighty,

Mother mooned in a window,

as if she had stayed on a train

one stop past her destination.

Here I cannot visualize the underlined part of the poem.

A dictionary says the meaning of MOON in this context is to gaze absently. (Am I right?)

According to the poem in Korean translation it means Mother looked out the window daydreaming.

But I cannot buy this explanation since if this is the case the line should be Mother mooned out the window not Mother mooned in a window. (Am I right?)

If I am right I still have no picture of this line.

Could you help me out?