For the #Spacetober prompt Carl Sagan: the famous Arecibo message, an interstellar radio message carrying basic information about humanity and Earth that was sent to the globular cluster Messier 13 in 1974 alongside a view of space with stars & nebulae.
Designed by astronomers who were thinking about the possibility of & communicating with alien life, including Drake, creator of the Drake equation & others, the message was a demo of 1/
human tech more than an actual attempt to communicate; the signal was sent towards globular cluster M13, 25,000 light years away. The message consisted of 1,679 binary digits in < 3 min. The idea was that any intelligent species would recognize 1,679 was chosen because it is a semiprime (the product of 2 prime numbers: 73 & 23), & thus deduce that if they arranged the digits as 73 rows by 23 columns. If you plot the 1s as a black square & 0s as white squares you produce the image in my print 2/
The alternative arrangement, 23 x produces an unintelligible set of characters.
The message encodes the following things visually:
numbers 1-10 (left to right)
atomic numbers of elements H, C, N, O, & P, which make up DNA (1, 6, 7, 8, & 15)
formulas for the chemical compounds that make up the nucleotides of DNA
estimated # of DNA nucleotides in the human genome, & a graphic of the double helix
dimension & graphic of a human & the human population ~4 billion at the time
graphic of the Solar System (including Pluto), indicating which of the planets the message is coming from.
A graphic of the Arecibo radio telescope & the dimension (the physical diameter) of the transmitting antenna dish, also intended to communicate the state of advancement of human tech.
After Drake composed the message in consultation with others, he asked Sagan to
act as "proxy extraterrestrial" - that is, viewing the image he was able to figure out the intent. 4/5
He suggested some tweaks but Drake was confident that the message could work as intended. It was transmitted at a dedication ceremony to mark the remodelling of the Arecibo Telescope in Puerto Rico on 16 November 1974.
5/5