We have been discussing the recent phenomenon of the rise of people who believe the earth is flat, and discussed Everything You Ever Wanted to Know about the Horizon in the last article, but were not entirely honest with you and we didn't discuss EVERYTHING about the horizon, saving one specific aspect of the horizon for a separate article: horizon drop, or horizon dip.

Image Credit: Geograph
Theodolite Sculpture County Durham, Great Britain

That is because observing horizon drop is possibly one of the easiest and most definitive ways to know for sure yourself that the earth is, indeed, not flat. In fact, if you used a precision instrument called a theodolite, not only could you observe horizon drop, but measure it. And if you measured it in several places around the world, you could not only confirm that the earth is curved, but that it is, indeed, a sphere since it's curved equally at any point on earth.

What is Horizon Drop

Horizon drop is a phenomenon whereby the horizon will actually get lower as one goes higher in altitude. As we discussed in the article How to Prove the Curvature of the Earth With Nothing But a Ladder, the curvature of the earth begins to drop away from you the higher in altitude you go, giving us an increased distance to the horizon. This means that as you go up in altitude, the horizon will get lower and lower on the sphere relative to the observer. This decrease in elevation of the horizon will show up to us as the measurable, repeatable, falsifiable, predictable observation known as horizon drop.

Like most people, this information might be new to you because the horizon always looks like it's pretty much unchanged and remains at eye level. In fact, this is one of the 'mantras' of the flat earth movement, that the horizon always rises to eye level. They say this because it looks like it does!

How Do We Know the Horizon Drops?

While the horizon may look like it 'rises to eye level', as the flat earth mantra goes, or at least it looks like it stays at the same level relative to our vision, the horizon actually drops. However, the horizon drop is small, only a few degrees even when viewing it from the height of an airplane, that our eyes simply cannot register this small amount of drop and it needs to be visualized with some sort of tool that allows us to actually see it.

We know this because humans have been not only observing, but measuring the horizon drop with accuracy using an instrument called a theodolite. For the math geeks, or those interested in learning more about horizon drop in detail, you can read the paper Investigations into the Dip of the Horizon.

Image Credit: Flickr
Surveyor from 1897 with an early theodolite

While you may never have heard of a theodolite before, you've almost certainly seen one at some point in your life when you've seen a surveyor doing his job looking through one of these instruments to survey a plot of land. When you look through a theodolite, it will have crosshairs in the center, much like a sight for a rifle would have.

Surveyors can use their theodolite, ensure it's level, look through the site, then visualize and measure how much the horizon has dropped from eye level, like in the image above. Horizon drop has been known about for a very long time, but has been able to be measured since as long ago as the 16th Century when the first theodolite was invented! Even more evidence that the horizon drop is that manufacturers of virtual reality displays had to make the horizon drop in order to allow users to better estimate distance. We see the horizon drop, but it's too subtle of a drop for us to perceive consciously.

Image Credit: Jesse Kowlowski- Flat Earth Can't Geodesy

How to See Horizon Drop

I did say this was an easy way to prove the curvature of the earth, and while you can find used theodolites on Ebay or at flea markets, you don't need an expensive instrument to see the horizon drop yourself. There are a variety of different ways to see horizon drop that are cheap and you probably already have access to.

To use any of these methods to visualize horizon drop, you'll need to have access to a high place, preferably at least a thousand feet or higher with an unobstructed view of the horizon. The higher you are the more easily you will see the drop without a precision instrument.

1. A level: You can take a regular hardware store level or even a level app on your smartphone,and simply put it on a level surface, then sight down the level to the horizon. Very simple.

2. A Rifle Sight: Take the rifle sight and lay it on or tape it to a level and put the level on a level surface, then look through the rifle sight at the horizon

3. Build a leveling rig or homemade water level: for the DIY kind of people, or for those conspiracy theorists who think that NASA has infiltrated all of the above technology to fake the horizon drop (yes, I've actually had people say this!), you can build a leveling rig for next to nothing. It might look something like this, although there are many ways to make a leveling rig, even using materials you probably already have around the house.

Image Credit: Horizon Drop at Varying Altitudes. Flat Earth Debunked

1. An Auto level: An auto-level is an instrument that looks something like a theodolite, but is a more basic version. You can find them online for around $60

2. A Sight Level: a sight level is a small, hand held instrument that has crosshairs and a level. You can Get One On Amazon for around $15

3. Find a theodolite app to put on your smartphone. Many are free or extremely reasonable like the Hunter Theodolite App

In conclusion, horizon drop is a measurable, repeatable, falsifiable, predictable observation that has been an established fact with repeated observations and measurements for hundreds of years now. The science of geodesy uses, and has used, this measurement for measuring virtually every place on earth that is on a map totaling what is probably billions of observations for the correct map measurements we use every day.

Horizon drop conclusively and easily proves the curvature of the earth with observable science anyone can do... if they choose to. Flat earthers, though, generally choose not to make observations that will disprove the earth is flat. If they did, they would not be flat earthers!

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