The Moon you see is one second old,
and then a third; the sunlight sent
back off its dust has only told
you where it was, not where it went.
For light is quick, but not so quick
it gets here without any wait:
seven times round Earth with every tick,
and still, from stars, it’s always late.
The Sun is eight whole minutes gone
before its morning reaches you;
if it went out, you’d carry on
in sunshine for those minutes too.
Sirius, bright above the hill,
sent out the light you’re seeing now
nine years ago, when you were still
too small to ask the sky for how.
And that pale smear, Andromeda,
a whole galaxy, spun and bright,
sent what you see before there were
people like us to see its light.
It flew two million years and more
through nothing, through the cold and black,
to end its journey on the floor
of your own eye, and not come back.
So when they tell you, “Look ahead”,
lie on the grass and look up too:
the sky’s a letter, posted, read
long after it was sent to you.
A small experimentMake a light-age map of tonight's sky
- On a clear evening, use the star map to identify the Moon, one planet if any are up, and three bright stars.
- Look up how far away each one is (the app usually tells you).
- Work out how old the light is: for stars, the distance in light-years is the age in years.
- Draw a quick sketch of the sky and write each light's age next to it. Mark any star whose light left before you were born.
What's going on?
Every object in the sky is shown to you as it was when its light set off. The Moon is about 1.3 seconds out of date, planets a few minutes to hours, and stars years to centuries. Looking up is looking back.