Verifying Earth’s Curvature Over a 10km Distance

The question of whether the Earth is spherical or flat has been discussed for centuries, but modern science approaches it through measurement, geometry, and observation. One practical way to examine the shape of the Earth is to calculate the expected curvature over a known distance and determine what height difference should exist between a straight line connecting two points and the actual surface of the Earth. This section presents a geometric calculation used to estimate the curvature over a distance of 10 kilometers across a body of water, such as a lake.

Author: Marcin Szolke (Scholke)

About the Author: More information at https://lov111vol.com/about-marcin-scholke

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Geometric Model of Earth

In scientific models, the Earth is approximated as a sphere with an average radius of about 6,371 kilometers. While the Earth is actually an oblate spheroid (slightly flattened at the poles), the spherical approximation is sufficiently accurate for calculations over distances of a few kilometers.

If two points on the Earth’s surface are connected by a straight line through space, that line forms a chord of the Earth’s spherical surface. The actual surface between those points forms a circular arc. Because the arc follows the curvature of the sphere, the midpoint of the arc lies slightly above the chord. The vertical distance between the midpoint of the arc and the chord is called the sagitta.

This sagittarius represents the amount of curvature present over that distance.

MathematicalFormula?

For a circle or sphere, the sagittahhh can be calculated using the following geometric formula: h=R−R2−(L2)2h = R – \sqrt{R^2 – \left(\frac{L}{2}\right)^2}h=R−R2−(2L)2

where:

• RRRistheradiusofthesphere
• LLList the length of the chord
• hhhisthesagitta(the maximum height of the arc above the chord) For Earth:

R≈6,371,000metersR\approx6,371,000\,\text{meters}R≈6,371,000meters

Calculationfora10kmDistance

Let us consider a straight line between two points that are 10 kilometers apart. L=10,000 metersL = 10,000 \, \text{meters}L=10,000meters

First, we divide the distance by two:

L2=5,000meters\frac{L}{2}=5,000\, \text{meters}2L=5,000meters

Substituting the values into the formula:

h=6,371,000−6,371,0002−5,0002h=6,371,000-\sqrt{6,371,000^2- 5,000^2}h=6,371,000−6,371,0002−5,0002

Carrying out the calculation gives:

h≈1.96metersh\approx1.96\,\text{meters}h≈1.96meters

This means that if a perfectly straight line were drawn between the two end points 10km apart, the Earth’s surface at the midpoint would lie approximately 1.96 meters above that straight line.

ApproximateFormulaforSmallDistances

For relatively short distances compared to the Earth’s radius, as impler approximation can be used: h≈L28Rh \approx \frac{L^2}{8R}h≈8RL2

Using the same values:

h≈(10,000)28×6,371,000h\approx\frac{(10,000)^2}{8\times6,371,000}h≈8×6,371,000(10,000)2

This again produces a value close to 1.96 meters, confirming the result obtained from the exact formula.

ComparativeTableofCurvature

Distance(km)	CurvatureHeightatMidpoint(m)	ApproximateVisualEffect
1	0.002	Nearly imperceptible
5	0.49	Small but measurable
10	1.96	Smallbut measurable
20	7.85	Visiblewithprecisetools
50	49	Easily measurable
100	196	Significant

This table illustrates how the curvature grows quadratically with distance, making it difficult to perceive over short spans but increasingly noticeable over longer distances.

InterpretationoftheResult

The calculated value represents the theoretical curvature expected if the Earth behaves like a sphere with the given radius. Over a 10 km span, the curvature is small but measurable in principle. The midpoint of the Earth’s surface would be nearly two meters higher than the straight line connecting the endpoints.

However, observing this directly in a real environment is significantly more complicated than the geometric calculations suggest. Several physical factors influence measurements across long distances over water.

AtmosphericRefraction

Light traveling through the atmosphere does not move in perfectly straight lines because the air density changes with altitude. This effect, known as atmospheric refraction, bends light slightly downward toward the Earth’s surface.

As a result, distant objects can appear slightly higher than their true geometric position relative to the Earth’s curvature. Refraction can partially compensate for the expected drop due to curvature, sometimes reducing it by roughly 10–20 percent depending on atmospheric conditions.

In extreme temperature gradients, refraction can produce mirages, which can significantly distort the apparent position of objects.

SurfaceIrregularities

Bodies of water, such as lakes, are not perfectly smooth reference surfaces. Waves, wind, and small variations in water level introduce additional uncertainty in measurements. Even small wave heights can exceed the magnitude of curvature expected over a few kilometers.

Because of this, precise curvature measurements often require controlled surveying techniques rather than simple visual observation.

SurveyingandtheVerticalReference

Another important factor is how measurement instruments are aligned. Surveying instruments such as levels and theodolites are aligned with the local vertical direction, which is defined by gravity. Since gravity always points toward the Earth’s center, the vertical direction itself follows the curvature of the Earth.

This means that standard leveling procedures do not create a perfectly straight line in space. Instead, they produce a line that remains perpendicular to gravity at each point along the path. Over long distances, this line naturally follows the Earth’s curvature.

Therefore, special methods are required to measure deviations relative to a true straight line in three- dimensional space.

PracticalObservationsOverWater

Many observational tests related to Earth’s curvature involve viewing distant objects across lakes or oceans. When an object moves farther away from the observer, the lower parts of the object gradually disappear below the horizon. This effect is consistent with the geometric curvature predicted by the spherical Earth model.

For example, when a ship sails away across these seas, the hull disappears before the mast. Similarly, distant shorelines often appear partially hidden by the horizon when viewed from low elevations.

These phenomena can be explained by the curved geometry of the Earth’s surface, combined with the finite height of the observer above that surface.

Scale of Curvature

One reason curvature can be difficult to perceive in everyday life is that the Earth’s radius is extremely large relative to human-scale distances. Over one kilometer, the curvature is only about two centimeters. Even over ten kilometers, it is less than two meters.

This gradual curvature is why large bodies of water often appear visually flat over short distances.

The relationship between distance and curvature grows with the square of the distance.As a result, curvature becomes more noticeable over tens or hundreds of kilometers.

Approximate curvature values are:

• 1km: about2cm
• 5km: about 0.49 m
• 10km: about 1.96 m
• 20km: about 7.85 m

• 50km: about 49 m

These values illustrate how slowly the Earth’s surface deviates from a straight line.

ExperimentalConsiderations

To experimentally verify curvature over a 10 km distance, a measurement system would need to establish a straight reference line between two elevated points while measuring the vertical position of the water surface at the midpoint.

Such an experiment would need to control for:

• atmosphericrefraction
• instrumentalignment
• wavemotion
• thermalgradients
• precisedistancemeasurement

Because of these complications, high-precision geodetic surveys and satellite measurements are typically used to determine the Earth’s shape with high accuracy.

Conclusion

The geometric model of a spherical Earth predicts that over a distance of 10 kilometers, the midpoint of the Earth’s surface lies approximately 1.96 meters above the straight line connecting the endpoints. This value follows directly from the mathematics of circles and the known radius of the Earth.

While the curvature is relatively small at this scale, it increases rapidly with distance and becomes clearly measurable using appropriate scientific methods. Careful measurements that account for atmospheric and environmental factors allow researchers to test these predictions and refine models of the Earth’s shape.

This geometric approach demonstrates that simple mathematical reasoning can be used to estimate the expected curvature of the Earth’s surface across measurable distances.

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About the Author:

Marcin Szolke (Scholke) is an independent researcher and science communicator focusing on physics, geodesy, and scientific literacy. More information can be found at

https://lov111vol.com/about-marcin-scholke

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  • *thinbold