Now and then people bemoaning the state of popular understanding of science wonder where the next Carl Sagan is going to be found. By this I trust they mean the person who can communicate what's neat about the universe (pretty near everything) to an audience that thinks science is hard and is terrified by every equation other than E = mc2. Adding in a couple of catchphrases and ease of impersonation would be a bonus, although chances are it'd help making the person popular.
Something in the opening paragraph of Comet stuck me as containing one of those things that made Sagan science writing so Sagan-y. Comet was one of the enormous number of books written to tie in with Halley's Comet's arrival in 1985-86, and it wasn't as successful as Cosmos though my copy has essentially the same physical form and production style, but it's got the same authors at least and as I say one thing struck me. The book starts with the images inspired by ``what would it look like if you were on a comet'', starting in deep space. Out here trillions of orbiting snowbanks and icebergs are stored, gently suspended about the Sun. They cruise no faster than a small propeller-driven aircraft would, buzzing through the blue skies of far-off Earth.
I think it's the propeller-driven spacecraft that makes it. Anyone could write that the comets in the outer solar system are moving at so-and-so many miles per hour and compare that to the speed of light aircraft. Sagan and Druyan dispense with the number and keep the metaphor. It doesn't give something people can calculate with, and maybe the average person wouldn't be able to get the speed of a propeller-driven aircraft right to more than the correct number of digits in the speed, but it's easy for the reader to believe she or he knows that speed, and to understand some of how tiny it is compared to the distances to travel.
I've never seen a guide on skills to develop in writing popular science. Even Isaac Asimov, who described everything else including how he wrote science fiction (``I dunno, I get near the typewriter is all'') doesn't seem to have dropped any explicit hints. But it does seem one of the fundamental tools is the well-chosen metaphor; apparently, one of the others is knowing when to rely on the metaphor and trust you don't need the detail of numbers. Sagan and Druyan don't ignore technical points, but they're able to tuck them in after really good images that let the reader feel confident in the understanding of what's under discussion. That's got to be one of the important tools.
Trivia: The first science essay which Isaac Asimov wrote and received payment for was ``Hemoglobin and the Universe'', which appeared in the February 1955 Astounding Science Fiction. Source: I. Asimov, Isaac Asimov.
Currently Reading: Comet, Carl Sagan, Ann Druyan.