Songfacts®:

• When Björk's longtime collaborator and friend, Icelandic poet Sjón, was asked by a newspaper to write a Christmas poem, he composed this celebration of light and seasons. She explained: "Sjón wrote this poem which I really, really love and it's called 'solstice,' which is basically a Christmas carol, and it's sort of about the fact that the four seasons are because of the tilt of the earth."
• Björk plays the song with a specially commissioned pendulum-harp, which embodies the idea of gravity that was central to the song's inspiration. The instrument was developed by scientists at the Massachusetts Institute of Technology and comprises four 10-foot pendulums with cylindrical harps attached to the end. As each pendulums swings they gain momentum causing the harp strings to ring out. Andy Cavatora of the MIT Media Lab, told Spinner that Bjork's desire to control elemental forces inspired his design. "Bjork's vision was to incorporate forces of nature into the show, like lightning and magnetism. She likes pendulums as a manifestation of gravity, and I like them a lot as natural oscillators, too. Just like a guitar string or organ pipe or the LC circuits of an analog synth, pendulums transform one kind of energy into another and back again. In this case, it's gravitational potential energy and kinetic energy."
• The pendulum-harp's design changed a great deal over time. Bjork recalled to Stereogum: "I gave him the song I'd already written and then he said 'to play that you need more than one pendulum, you actually need 38 pendulums.' And I was like 'WHAT?!' and I was like, 'Oh that's a shame and then he's like because its 38 arms it needs to be like 30 feet tall otherwise they'll hit each other.' And I was like, 'Damn ok, oh because they're 38 and they're 30 feet tall they need to be made out of aluminum.' And I was like 'WHAT?' and 'because it's aluminum it has to be motorized' and I was just like 'Ok…this is not going the right way.' And then he'd worked on it for a whole year so it was really really difficult. I just had to stop it I said, 'Listen it has become the opposite of what its original intention was that it was effortless.' It was Spinal Tap reverse. We [now have] four pendulums that are each a few notes. You can hang them either in the ceiling or on a branch or something. They're about two meters tall and made of wood."
• Each track on Biophilia is accompanied by an app and this song's app was inspired by the poem's comparison to the solar system with a Christmas tree. At the centre is a sun from which rays of light can be pulled to form a circular harp of strings plucked by orbiting planets.
• Biophilia finds Björk attempting to break the typical 4/4 time signature and this song features 7/4 and 6/4 times bars.