If you’ve ever stood on a dock and watched a boat sail away you know that at some point the boat seems like it just disappears. Even when you can see the water extending on and on to what seems like forever, it seems like the boat has disappeared over the horizon.
That’s when you likely realize that you’re not actually looking at a never-ending expanse of water after all. But how does that happen? And why?
What is the Horizon on the Ocean?
The horizon is considered the point where the Earth and the sky meet. When you’re looking out over the ocean this is the line between the water and the apparent edge of the sky, which generally appears higher than the water leading up to it.
The horizon actually exists at all times, but you may not notice it as much when you’re walking around a city or an area where there are a lot of obstacles in the way.
If you find yourself in a field or looking down a very long street you may be able to see the horizon line out ahead of you. It may actually be further than three miles simply because there are typically obstacles around the horizon, such as trees or buildings.
These obstacles can help you keep a reference better than the flat, empty space you would get on the ocean.
On the ocean or anywhere else, the horizon is simply where the curve of the Earth is enough that it wraps around itself. After all, the Earth is a large ball and that means you’ll never be able to see all the way around by looking at the top.
Try picking up a beach ball and holding it up to your eye level. You can see across the top of the ball but eventually the curve of the ball is so much that you can’t see it anymore. If you’re holding the ball close enough to your face it will look as though there’s an edge to the ball that is disappearing into the sky, it’s own horizon.
How Far is the Horizon on the Ocean?
When we look at the horizon on the ocean it will depend slightly on how high you are above sea level. In general, the horizon is approximately 3.1 miles out from where you are. This is based on an eye level of approximately 5 feet 7 inches above sea level.
If you’re standing above sea level, you may be able to see slightly further out for the same reason that you can see further when standing on a tall building or a ladder. You have a further range of sight from there and you’ll find yourself able to watch things happening at a greater distance, which includes boats going over the horizon. Remember, however, that you likely won’t be able to judge how far away the horizon is by looking at the ocean because the flat expanse tends to play tricks on your mind.
You should also keep in mind that when the weather is colder it can actually increase the distance that you can see.
For example, Antarctica has a larger sight line in many instances because of the increased atmospheric refraction that occurs as a result of the cold weather.
This can allow you to see a distance that is much greater than the three miles we’ve discussed, though this is generally more common over solid surfaces. Still, you can look out to a large distance over the ocean in this area.
Why Does the Ocean Horizon Look Higher?
When you look out at the horizon it tends to look higher because of a trick of your eyes. The horizon is actually right at your eye level but when you’re looking out it appears to rise up above the rest of the water that is around you. This is because of the way your mind evaluates what is right in front of you versus what is at a distance.
The truth is that you’ll always see the horizon this way. It doesn’t matter how much above sea level you get, the horizon always looks like it’s at the same distance.
The reason for this is that you are able to see a further distance away when you’re higher up, but your eyes continue to look straight out from where you are to find the horizon.
Even when you can look down and see that the ocean is far below you, you’ll look out and still feel like the horizon is at eye level.
That’s because the horizon, to your eyes, absolutely is at eye level. But because the water below you is not and the water before you is you feel like the horizon is rising.
Why Do Boats Disappear Over the Horizon?
The horizon (known as skyline as well) is the line that separates the sky from the Earth. In other words, this is the furthest distance you can see before the curvature of the Earth makes objects disappear. When a boat reaches that far distance you are unable to see it anymore because the curve of the Earth makes it appear to sink out of sight.
If you’re standing higher above sea level, you may be able to see out a little further. Also, keep in mind that the exact distance where you stop being able to see something will not necessarily be five kilometers or three miles.
That’s where the horizon line is but depending on the height of the boat you’re watching you may be able to see at least part of it for a little longer. That’s because it takes a little bit of time for it to fully vanish over the horizon.
Smaller boats might actually seem to disappear even faster because they are closer to ‘ground level’ and have less to extend over the horizon.
Because the Earth is round, you’re not able to look out across it forever. Your eyes have a maximum sight line and the Earth starts to curve around.
The slope is very minimal (which is why people can live on all parts of the Earth) but as you’re staring out at it your eye notices that there’s an ‘edge’ to what you can see.
It’s partially because of the lack of anything else in your way or anything else that you can compare that ‘edge’ to. Having obstacles like trees or buildings can influence how far you can see.
That means if there are several boats out on the water you may feel like you can see further, or if there is a lighthouse or other obstacle that you can use as a reference point.
How Far Can You See a Boat Before it Disappears Over the Horizon?
You will typically be able to see a boat that is out about three miles or about 5 kilometers away from you. Beyond this distance is where you will see the horizon and the boat will appear to pass over that horizon and start to sink.
If you’re watching a large boat you may be able to see it for slightly further than this distance because it takes time for the entire boat to disappear over the horizon.
You will likely see that the lower portion of the boat disappears first and then you can only see the mast. Then, gradually, the mast will start to disappear over the horizon as well.
The boat itself might be further than three miles out, but your eyeline is still about the same. If you’re watching a small boat it may seem to disappear even faster because of it’s distance above the water to start.
This is also going to depend on conditions such as the weather at the time (whether there’s fog or rain or snow) and the temperature (hot or cold). When the sky is clear you will be able to see a much further distance than you would if there is anything to limit your visibility.
You may not even realize there’s fog in the air but the slightest amount could cloud your vision. Likewise, colder weather actually clears atmospheric refraction, which means you can see an even further distance.
If you’ve ever noticed that things seem much crisper and clearer in the winter you’ve already experienced this.
Conclusion
When it comes to the horizon a lot of it is actually about the way your mind and your eyes communicate. And it’s about the way that your mind and your eye can play tricks on you.
Once you understand how the horizon works, however, you might just have a better understanding of what’s happening when you’re watching those boats head out to sea.
It doesn’t have to be as mysterious as it might have seemed. In fact, there’s a perfectly logical and rational explanation for what you’re seeing.
And everyone else is seeing it too. It’s all about the curvature of the Earth.
References:
https://www.godownsize.com/do-boats-disappear-over-horizon/
https://www.livescience.com/32111-how-far-away-is-the-horizon.html