Rewritten version of this story (much clearer), done with the help of Valkyrie Vamp (Thank you so much!)
It was a priceless moment and one that John would be unlikely to ever forget.
With a sense of triumph, exhilaration and downright amusement, he catalogued away the look on Sherlock's face when he realized that John had figured out the break in the case before him.
Groups of murder victims had been found spread around London in a vaguely circular fashion but with no apparent rhyme or reason. A group of three friends here, a family there, occasionally one or two victims who would be decorated with pieces of rock or even the singularly bizarre one who was buried in dirt.
Dirt, John had mused hopelessly. He had given up trying to pay attention to Sherlock's progressively more profaned rambling over half an hour ago. The brief respite from his own train of thought was a welcome relief.
How often did people think about ending up back in the earth? Earth. Like the planet. Planet Earth was part of the solar system. Sherlock Holmes apparently didn't have room in his head to know about the solar system. To even know or care that humanity and the planet existed in a circle of planets that orbited around the sun.
He debated for a moment the sense in 'deleting' that particular fact and yet retaining the main points of a children's nursery rhyme.
Looking around at the photographs and the maps of London that had deigned to wallpaper the living room of 221b was frustrating. Cityscapes and road maps were dotted by tiny colored tacks and pins, marking the locations of crime scenes in a seemingly random order. There was a picture here, but what?
Sherlock was pacing the well-trodden floor, sporting the three nicotine patches that John had deemed as the maximum allowance without being unreasonably detrimental to his health. They knew there was one piece to be added to the picture, and it was a picture, as Sherlock had assured John, but who was the target and where? But in order to figure that they had to know what they were looking at.
Sherlock moaned and paced and asked questions out loud but John was staring at the map. It wasn't a perfect circle the killer had created and the varying number of people looked like planets themselves on the map. The startlingly large family that had got Lestrade to invite Sherlock onto the case was easily twice the size of the other victims when plotted on the map.
And for some reason it simply hit John, the solar system. Their killer was forming a solar system in London!
All that was missing was the sun…
"Sherlock-"
"I need my skull back."
"Sherlock!"
"Please John, I can barely think-"
John took one deep breath, "Planets."
Sherlock whirled around to gape at him.
He quickly figured out the rest after that. Figured out that the killer's method was by bomb, figured out that the intended target was the British Museum and together they ran to stop him (it was definitely a 'him' according to Sherlock, he was also an astronomy professor and he was very upset that Pluto was no longer a planet).
As they ran though London John couldn't contain himself, "I only have room for things that are really, really important." He told Sherlock smugly as they dodged several tourists.
Sherlock Holmes spared John one disparaging glance over his shoulder.
"Shut up."