Just passed a very pleasant hour or so doing what passes for gardening at this point in time and this place in space: transplanting cacti. I bought six of the little things at Ikea about a year ago, and imperceptibly they’ve grown. Time for bigger pots. But here’s the weird part.

As soon as I had put the plants into their new, bigger pots, they looked bigger.

This is most perplexing. I just don’t get it. I expected that they would look smaller in a bigger pot. I thought it might bear some relation to the moon illusion, the fact that the moon looks bigger when it is low on the horizon. There are a bunch of explanations for this, some more convincing than others, which seem to throw their hands up and say “we don’t know”.

The illusion seems best explained by the knowledge that we tend to judge size by comparison to things we know, or think we do. High up in the sky, with nothing around to compare it to, the moon looks about moon-sized. Down on the horizon, with buildings and trees and things, it looks much bigger. But why? And why does bending from the waist and looking at the moon upside down, from between your legs, destroy the illusion, as one site says it does? (I have not had the opportunity to check this.)

My cacti, in larger pots, really ought to look smaller, because they are smaller, relative to the size of the pot they are in. If my brain has retained a memory of how big “a cactus pot” is, these pots are quite a lot bigger, maybe double the diameter, so the cactus should look smaller.

I’ve no idea what this really means, whether there is an explanation, or whether that explanation is even worth pursuing. Enlighten me!

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