We cannot see beyond the horizon because of the curvature of the Earth (unless you think the Earth is flat, in which case, please do not read any further as it may deflate your illusions). Here’s a breakdown of why this happens:

1. Earth’s Curvature

• The Earth is a sphere (roughly), and as you move further away from an object, the surface of the Earth gradually curves downward. This causes objects to eventually fall out of view as the Earth’s surface blocks them. The point where the Earth curves enough to block your line of sight is called the horizon.
• Any object beyond this point is below your line of sight, making it invisible unless you increase your elevation (e.g., by going to a higher vantage point like climbing up the mast).

2. Line of Sight

• Our ability to see depends on having an unobstructed “line of sight” between our eyes and the object. The Earth’s curvature prevents us from having a straight, unobstructed line of sight to objects beyond the horizon. As a result, we can only see things up to the horizon, and the curved surface blocks anything further away.

3. Height and Perspective

• The distance to the horizon increases with height because being higher up allows you to see “over” more of the Earth’s curvature. This is why the horizon seems much further away from a mountain or tall building, and you can see a larger area. From a low height, like standing on the beach, the horizon is much closer.

4. Atmospheric Effects

• Light can bend slightly in the atmosphere due to refraction, which sometimes allows us to see objects just beyond the geometric horizon, but this effect is minor. It does not significantly extend our line of sight.
• In extreme cases, like with mirages, atmospheric conditions can cause light to bend enough that objects just beyond the horizon seem to appear. Still, these are unusual and dependent on specific weather conditions.

5. Size and Distance of Objects

• The larger an object is, the further away it can be while still visible. For instance, a ship’s mast might remain visible while the lower part of the ship is hidden below the horizon because the mast is tall enough to extend above the curved horizon. Conversely, if you are on watch in the “crow’s nest” at the top of the mast, you will be the first on the ship to see another boat approaching from beyond the horizon.
• For celestial objects (like the Sun or stars), the horizon doesn’t block them because they are incredibly far away, and the Earth’s curvature is negligible compared to those vast distances.

You cannot see beyond the horizon because the Earth’s surface curves downward, blocking your view of distant objects. The horizon is the furthest point at which the Earth’s curvature allows you to see, and objects beyond that are hidden below the curve unless you are high enough to extend your view.

Sailors are interested in knowing how far the horizon is for several important reasons:

1. Navigation and Situational Awareness

• The distance to the horizon gives sailors an idea of the limits of their visible surroundings. By knowing how far they can see, they can estimate when objects such as ships, land, or navigational markers like a lighthouse will appear or disappear from view.
• This is particularly important when approaching the coastline, as recognising landmarks or avoiding hazards (such as shallow waters or obstacles) can depend on how far they can see.

2. Detection of Other Vessels

• The horizon marks the furthest distance at which another vessel or object (like a buoy or lighthouse) can be seen. Knowing the horizon’s distance helps sailors detect ships early enough to avoid potential collisions, especially in busy shipping lanes.
• In naval or maritime security scenarios, knowing the horizon’s range helps detect potential threats or targets.

3. Estimating Distances

• Understanding how far away the horizon is helps sailors gauge distances on the open sea. For example, if a ship or landmark first appears on the horizon, the sailor knows it’s approximately that far away.
• This can help when navigating through channels or around headlands where timing or distance estimation matters for safety and efficiency.

4. Communication with Visual Signals

• In the age before radio and GPS, ships relied on visual signals like flags and signal lamps. The distance to the horizon determined the range at which these signals could be seen, affecting communication with other vessels.
• Even today, knowledge of how far visual communication devices can be effective is crucial.

5. Navigation in Celestial Observations

• In traditional celestial navigation, sailors use the horizon as a reference point when taking measurements of celestial bodies (like the sun or stars) with a sextant. Accurate horizon distance is essential for making these measurements precise and determining their position on the sea.

6. Weather Observation

• The distance to the horizon can also warn sailors early about incoming weather. By observing cloud formations or weather patterns near the horizon, sailors can prepare for changing conditions like storms or squalls.
• Early detection of weather changes can be life-saving on the open sea.

7. Lighthouse on a Nautical Chart

On nautical charts, lighthouses are marked to help sailors navigate and identify their position relative to land and hazards. The following details are commonly found for lighthouses on a chart:

1. Lighthouse Symbol: Typically represented by a small black dot with a black or magenta circle around it.
2. Light Characteristics: The light’s flashing pattern, colour, and timing (e.g., “Fl W 10s” means a white light flashing every 10 seconds).
3. Height: The elevation of the light above the water level, usually in meters or feet.
4. Range of Visibility: The maximum distance the light can be seen in clear weather, often indicated in nautical miles (e.g., “18M” means it is visible from up to 18 nautical miles away).
5. Position: The lighthouse’s geographic coordinates (latitude and longitude).

Let’s have a quick look at how this looks on the chart…

• FL3 means that you can see three short flashes.
• 10s means that after each series of three flashes, there are 10 seconds of no light.
• 62m means that the lighthouse is 62 meters above sea level.
• 25m means you can see this lighthouse from 25 nautical miles away in clear weather conditions.

These details allow sailors to recognise and use the lighthouse for navigation.

Summary

In summary, knowing the distance to the horizon is essential for navigation, safety, and effective communication at sea. It helps sailors maintain situational awareness and aids in making critical decisions related to course-plotting, collision avoidance, and weather forecasting.