Some 60 years ago, I was reading a science-fiction book in which an adult was with two children, ca. 12 years old, on a trip of some sort. While they are out walking, he sang “Men of Harlech” to them; the words for a verse or two appear in the book:
“Men of Harlech in the hollow, do ye hear like rushing billow …”, &f.
I recalled nothing else of the book until a few days ago when I suddenly recalled that a planet named “Lepton” figured in the book somehow; I thought this additional detail might be enough to find the book.
The reason for such vagueness, beyond the 60-year time lapse, is that I never finished the book. I was perhaps 9 or 10 years old, but this wasn’t a children’s’ book; the librarian jerked the book from my hands, telling me to “stop pretending to read that book” and go pick a book from the children’s’ section instead. I could read the newspaper before I started 1st grade, so I was by no means pretending, and I wasn’t interested in children’s’ books.
My mother had a somewhat heated talk with the librarian sometime later, and the librarian never bothered me again on subsequent visits. But I was unable to locate the book again.
Sounds like it may be Mr. Bass’s Planetoid by Eleanor Cameron, published 1958. There is not much to the Wikipedia stub, but it mentions both planet Lepton and “Men of Harlech” (in the references). One of the characters has a Welsh name, so that makes sense.
https://en.wikipedia.org/wiki/Mr._Bass%27s_Planetoid
It is absolutely “Mr. Bass’s Planetoid” by Eleanor Cameron. I remember it fondly from my childhood.