Arc TangentLight is the straightest line there is (for sufficiently local values of straight). We've known this for a while. The next time you happen to be in a Gothic Cathedral, look around the floor for a simple plate of burnished brass, small and square. If one is handy, ask the docent the day and time when the plate be comes useful; ask them the day and time when the sun sends light through a tiny window up there, somewhere, to perfectly illuminate that plate. If you are lucky, it might be that day. The reason the builders did that, those years ago, was to give them a tool to see if the building was settling, sliding, shifting into instability. Should that great pile of stone and glass no longer be true, the plate and the sun would betray it.
The seasons are changing, and I have my own evidence: the sun is once again haunting regions of the sky such that it sends shine though the glass in my front door. In turn, the glass fills the foyer with refracted light; a hundred tiny rainbows, climbing the woodwork on the stairs.
The plumber comes tomorrow.
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