Event in April 1947

[puzzleman]

What on one day in April 1947 covered an area of 18000000000 square kilometres

[moose!]

could it be....

"The largest sunspot ever recorded was visible in March and April 1947 and covered an area of over 7,000 million square miles; about a hundred Earths could be fitted into this area!"

[smudge]

1947 was a special year, as it was the year I was born :0). It was also one of the harshest winters on record. I know this has nothing whatsoever to do with the question, but just thought I'd share that with the young ones.