New Scientist explains that the recipients, if there are any, could answer in about 25 years – enough time for a radio message to travel the 12 light years to GJ 273b, a habitable planet in Canis Major, and back:
This missive to the stars was sent on the anniversary of the “Arecibo message,” a radio transmission beamed toward a distant star cluster in 1974 from the Arecibo radio telescope in Puerto Rico. The Arecibo message contained information on the planets of our solar system, the structure of DNA, a cartoon-like picture of what a human being looks like, and other basic information about the earth and its inhabitants.
This new message – beamed from an antenna in Norway over roughly eight hours over a three-day period in October – is simpler and may be more readily understood, Vakoch says.
It begins with information about counting, arithmetic, geometry, and trigonometry, and includes a description of the radio waves that carry the message, as well as a tutorial on clocks and timekeeping, to see if any potential inhabitants of GJ 273b have an understanding of time similar to our own.
Clock instructions. Be really funny if hostile aliens destroy our planet and everything we’ve ever known over, like, “The big hand is on the 12.”