As I sit here in the quiet Summer night,
Suddenly, from the distant road, there comes
The grind and rush of an electric car.
And, from still farther off,
An engine puffs sharply,
Followed by the drawn-out shunting scrape of a freight train.
These are the sounds that men make
In the long business of living.
They will always make such sounds,
Years after I am dead and cannot hear them.

Sitting here in the Summer night,
I think of my death.
What will it be like for you then?
You will see my chair
With its bright chintz covering
Standing in the afternoon sunshine,
As now.
You will see my narrow table
At which I have written so many hours.
My dogs will push their noses into your hand,
And ask -- ask --
Clinging to you with puzzled eyes.

The old house will still be here,
The old house which has known me since the beginning.
The walls which have watched me while I played:
Soldiers, marbles, paper-dolls,
Which have protected me and my books.
The front-door will gaze down among the old trees
Where, as a child, I hunted ghosts and Indians;
It will look out on the wide gravel sweep
Where I rolled my hoop,
And at the rhododendron bushes
Where I caught black-spotted butterflies.

The old house will guard you,
As I have done.
Its walls and rooms will hold you,
And I shall whisper my thoughts and fancies
As always,
From the pages of my books.

You will sit here, some quiet Summer night,
Listening to the puffing trains,
But you will not be lonely,
For these things are a part of me.
And my love will go on speaking to you
Through the chairs, and the tables, and the pictures,
As it does now through my voice,
And the quick, necessary touch of my hand.

A "penumbra" is a shadow, a sort of half-shadow.  The "umbra" is the main shadow.  If you see a total eclipse of the moon, you are in the umbra area.  If you see a partial eclipse, you are in the penumbra.

This poem has a momento mori theme. This means "remember that you will die." She is talking about her own death in the poem. The shadow mentioned in the title, penumbra, symbolizes the shadow of death.  She is just in the edge of the shadow.  It is not imminent, but approaching from the distance.  When she is gone, the dog will ask Ada with his eyes what has become of her.  Otherwise, the house and the world will continue in her absence.