And watched a swiftly-passing train.
He heard the whistle's changing pitch
And proceeded to explain:
What he heard with his own ear
Was mathematically clear
(His brain was logical and swift),
'Twas owing to the Doppler shift!
Now this simple, clever deed
Is used for measuring the speed
Of galaxies. A man named Hubble
From his telescope's clear bubble,
Amazed us all, after he said,
“Their signals shift toward the red!”
The conclusion: rather terse -
We're in an expanding universe...