Rocket watchers knew the Falcon 9 launch carrying Iridium 4 was going to be spectacular because the launch window was just after sunset. However it caught much of the general public by surprise and caused a bit of a sensation on social media. Let's talk about the science explaining the shape and behaviour of the launch using Doug Ellison's fantastic footage:

Here's Doug's original video:
   • SpaceX Iridium 4 Launch from Alhambra...  

Also includes a closeup showing Fairing RCS from this video:
   • SpaceX Iridium-4 launch - 12/22/2017