One item, everything we know about it, and everything it is connected to.
Words we use: Mystery A question nobody has answered yet. Clue from people Something people wrote down or told long ago: a chronicle, a diary, a story, a carving. Clue from nature Something nature recorded by itself: tree rings, ice, rock layers, or the leftovers of an exploded star. Hunch Our guess that a clue from people and a clue from nature are about the same event. Book or paper Where we read it, so anyone can check our work. Step Something we did, in the order we did it.
Goryeosa volume 47: '四年八月癸亥，日中有黑子，大如雞卵', about 31 August 1278. A naked-eye sunspot near the solar maximum that Miyahara et al. reconstruct for about 1278.