Deflection of the vertical

Okay kiddo, imagine that you live in a big, round ball and you have a special tool that tells you which way is up. This special tool is called a plumb bob. When you hold the plumb bob, the weight at the end of the string will point to the center of the Earth and show you which direction is straight down, or vertical.

But did you know that sometimes the plumb bob doesn't point straight down? It might look like it's tilted a little bit to the side. This happens because the Earth is not a perfect round ball. It's actually a little squished at the top and bottom, kind of like a donut.

This squishing means that gravity, which pulls us toward the center of the Earth, is a little bit stronger in some places than in others. And when gravity is stronger in one spot, it pulls the plumb bob a little bit to the side. This is called deflection of the vertical.

So even though your plumb bob is supposed to point straight down, it might end up tilted to the side because of the Earth's shape and gravity. Scientists have mapped out these deflections so that builders and surveyors can account for them when they're making things like buildings or roads. It's important to make sure everything is level and straight, even when the plumb bob doesn't seem to be pointing straight down!