Dans le ciel

Dans le ciel is a French astronomy program tied to the magazine Ciel et Espace, and its structure is as regular as the sky it covers. Each month begins with a list of what is worth observing. A conjunction between Venus and Jupiter. Zodiacal light low in the west. A one-day-old crescent Moon passing two degrees above Jupiter. A partial solar eclipse. Mercury at its greatest western elongation, the mirror image of the eastern one noted the month before. David Fossé presents, and the program credits Cyril Birnbaum and Sébastien Fontaine for the observing advice that fills out each entry. The pleasure of the show is in how those three talk to each other. When the phrase age of the Moon comes up, one of them admits to a moment of panic, having briefly assumed the question meant four and a half billion years rather than one day. From there the conversation runs to a trick for working out lunar age, a nervous joke about a formidable blue-violet volume titled Astronomical Algorithms, and a straight answer about the Moon completing its circuit in roughly 27 days and 8 hours. Sébastien explains that the Moon travels about its own diameter every hour, around 1.5 degrees, adding up to some 13 degrees a day, which is why it so often shifts from one zodiac constellation to the next overnight. Practical questions get practical answers. Will a good pair of binoculars show Io, Europa, Ganymède and Callisto. What processing does a planetary image need, and where can the tutorials be found. There is history here too, and a sense of perspective: chasing a transit or an eclipse now means a few hours on a plane, where it once meant months or years of travel, with no guarantee of returning alive or of seeing the event at all. Episodes point listeners toward the Nuits des Étoiles in early August and toward a special issue on a total eclipse that shipped with viewing glasses. A second strand, La science en chemin, answers a question listeners keep asking: how does someone become an astronomer. It takes the microphone onto the paths of the Meudon Observatory. In one episode, astrophysicist Miguel Montargès describes the effort of a team of thirty colleagues to explain why a star faded, the conclusion, published in Nature in 2021, being that its surface cooled locally, dust condensed in gas the star had thrown off years before, and the combination hid it from view. He notes that the investigation ran largely from home during the confinements. Nicolas Franco and Elisa Savette are credited as realisers. The result is a show that respects both the equipment and the wonder.