Arecibo Message

From Wikipedia:

The Arecibo message is a radio message that was beamed into space at a ceremony to mark the remodeling of the Arecibo radio telescope on 16 November 1974. It was aimed at the globular star cluster M13 some 25,000 light years away because it was a large and close collection of stars that was available in the sky at the time and place of the ceremony.[1] The message consisted of 1679 binary digits (equivalent to nearly 205 bytes) transmitted at a frequency of 2380 MHz and modulated by shifting the frequency by 10 Hz, with a power of 1000 kWatt. The beam was extremely narrow (giving a power equivalent to 20 trillion Watts if it were omnidirectional) making it the strongest man-made signal ever sent. The entire transmission lasted 169 seconds and was not repeated.[2] The number 1679 was chosen because it is a semiprime (the product of two prime numbers) and therefore can only be broken down into 23 rows and 73 columns, or 73 rows and 23 columns. This assumes that those who read it will choose to arrange it as a rectangle. The information arranged the first way (23 rows, 73 columns) produces jumbled nonsense, but if arranged the second way (73 rows, 23 columns) it forms the image shown on the right (assuming the bits are arranged from left to right and the rows are arranged from top to bottom; otherwise a flipped image is formed), which is assumed to be recognizable as data.[3]