Before leaving Prague,
He had found an unpublished manuscript of Johannes Kepler,
And the last instrument made by Tycho Brahe.
http://www-history.mcs.st-andrews.ac.uk/history/Mathematicians/Brahe.html

In his walks around the city,
He would think about these two—
the one German and the other Danish,
the younger apprenticed to the older
(for Brahe was the Imperial Mathematician
when Kepler came to study with him)—
And their brief meeting in Prague.

He imagined
Brahe's onion-domed observatory,
Uraniburg (the castle of heaven),
On a small island, three miles long, between Denmark and Sweden,
Where huge instruments
(later stored and eventually lost),
Amidst multiple clocks and timekeepers,
And an alchemical oven,
Were mounted
To measure distant stars,
And to trace the movement of the planets
Around the sun.

He imagined Brahe's excitement, in 1572,
When he saw the new star,
A stella nova
That was in fact a dying one.

And he imagined
The ambitious 30-year-old Kepler,
Fresh from publishing
The Cosmographic Mystery,
Arriving in Prague
To meet, study, and argue with Brahe,
And, upon Brahe's death,
To assume not only his title of Imperial Mathematician
But also to take over his unpublished research.
http://www-history.mcs.st-andrews.ac.uk/history/Mathematicians/Kepler.html

What did they talk about,
The Young Astronomer wondered,
In those two years?

He spent days
Imagining their exchanges.

Surely they must have
— he speculated hopefully,
for the subject of time
was already obsessing him as a young man—
Tackled time as well as space?

But the 1601 fragment of Kepler's writings
And Brahe's last instrument,
Which the Young Astronomer had discovered
And studied feverishly,
Gave him no clues regarding this.

Brahe and Kepler?

The conversations
These two might have had
In the city of Prague in 1600?

Surely
Not standard fare for a young astronomer
Studying in post–World War II Europe.

As an old man,
And poet-scholar,
He often thought about Brahe and Kepler,
As he sat solitary,
Night after night,
In the Old Observatory,
Watching the Falling Star.