# What does 'a binary mobile' mean in the year of 1983?

I read from the book SICP

Exercise 2.29. A binary mobile consists of two branches, a left branch and a right branch. Each branch is a rod of a certain length, from which hangs either a weight or another binary mobile. We can represent a binary mobile using compound data by constructing it from two branches (for example, using list):

What's a 'binary mobile', I search exhaustively by words and images yet find just a mobile phone? Nevertheless, it make no sense to say "a binary mobile phone"

The book was written in 1983, what did mobile mean then?

• I don't think your context has anything to do with mobile phones. It's the kind of "balancing toy" suspended over a baby's cot, for example. Where each horizontal rod hangs from a string tied round its central balance point, and may have two or more strings going down to support additional horizontal rods. Your writer calls it a binary mobile because each string exactly divides the two sides of a rod by weight. I hadn't seen this specific sense before, but it's essentially the same idea as a "balanced binary tree" data structure in programming (perhaps that's the "origin" here). – FumbleFingers Jan 4 '20 at 13:47
• mobile (noun) A decorative structure that is suspended so as to turn freely in the air. You'll often see the basic structure used for wind chimes, for example. – FumbleFingers Jan 4 '20 at 13:50
• @FumbleFingersReinstateMonica I feel like that would have worked well as an answer. – snailplane Jan 4 '20 at 14:27
• The reference to "comound data" made me think about a "binary tree" which is a purely virtual concept. – Mike Brockington Jan 8 '20 at 13:33

Here is a picture of a mobile like the one in the question

• OMG, amazing answer. – AbstProcDo Jan 4 '20 at 15:47
• It would be amazing if it included the answer to the puzzle (which I don't know). – Michael Harvey Jan 4 '20 at 15:50
• Wolfram – James K Jan 4 '20 at 16:24
• Though strictly the puzzle is not a binary mobile, as one branch has both a weight and a mobile. – James K Jan 4 '20 at 16:27
• Not sure I'd want to put a a mobile weighing a couple of hundred kilos above a child's cot... – Daniel Roseman Jan 5 '20 at 14:59

In 1983, mobile as a noun meant a type of sculpture (Wikipedia)

The OED gives mobile n 4 as coming from French usage of 1931.

1. a. A sculpture consisting of hanging or pivoting pieces of metal, plastic, etc., in abstract or (more recently) representational shapes, connected by wires and threads so as to be able to move and rotate in response to air currents or when propelled by an internal mechanism. Now chiefly historical.

The quotations in the OED are interesting:

1932 Art News "Mr. Calder..calls his newest phase, ‘Mobiles’. This brand new art form, signifying abstract sculptures which move..were [sic] first shown in Paris in February."
1949 Archit. Rev. "Alexander Calder's work on the ‘stabile’ is not as well known in England as is his work on the now well established ‘mobile’. In fact Calder has always done ‘still’ sculpture, and the term stabile, given to it by Hans Arp, appears to be some months older than the name mobile, which was invented by Marcel Duchamp."

Here's one of the Calder's sculptures:
Alexander Calder, Red Mobile, 1956, Painted sheet metal and metal rods, a signature work by Calder, from Wikipedia.