Tree Trunk Illusion: Does it Disappear Over the Horizon?

Ryan Bruch · Jan 6, 2015

Supposed there is a tree on a perfectly flat plane. If an observer in front of that tree moves further and further away from the tree, the tree would look smaller and smaller. When the observer moves further and further away, would the lower part of the tree's trunk seem to disappear as if sinking below the horizon?

Doug Huffman · Jan 6, 2015

No. Why would it?

Ryan Bruch · Jan 6, 2015

A person claimed that it does and that he had observed it before, on Earth though. These are his illustrations:

Three hatched circles labelled A, B and C of decreasing size, each resting on a horizontal hatched bar labelled D below them, arranged left to right.

Perspective diagram of a row of trees of decreasing height receding from left to right toward a vanishing point labelled W, with converging dashed lines from A and E, horizontal lines labelled A, B, C, D and E.

Line diagram of a figure walking away along a flat line CD, with points A, B and E labelled and dashed sight lines converging toward the far horizon.

Line drawing of three steam locomotives on a flat track, the nearest large and the farthest small, with smoke plumes rising from each and a dashed line connecting their tops to show the apparent sinking.

Diagram of a row of flags receding into the distance above a flat water surface, with dashed horizontal lines labelled A, B, C and D showing the flag tops and bases converging toward the horizon.

Doug Huffman · Jan 6, 2015

Earth is curved, an oblate spheroid.

gsal · Jan 6, 2015

In more words, what Doug is telling you is that YOU were the one that specified a condition of a perfectly flat surface, thus, there is no such thing as a horizon. So, what is it? a flat surface or an sphere...please state your problem correctly and fully form the beginning.

Doug Huffman · Jan 6, 2015

In Nathaniel Bowditch's American Practical Navigator is listed a most useful thumb-rule for estimating the distance to the horizon for practical heights of eye.

Tree Trunk Illusion: Does it Disappear Over the Horizon?

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