You don't need to be very high up to see the horizon. The exact height depends on a few factors:

* Earth's curvature: The Earth is a sphere, so the horizon is a curved line that gets further away as you go higher.

* Atmospheric conditions: Clear air allows you to see further than hazy or foggy air.

* Observer's height: Taller observers can see the horizon from a lower altitude.

General rule of thumb:

* A typical person standing at sea level will see the horizon about 3 miles (5 kilometers) away.

* At 35,000 feet (10,668 meters), a typical cruising altitude for commercial airliners, the horizon is about 230 miles (370 kilometers) away.

Formula:

There's a formula to calculate the distance to the horizon:

* Distance (in miles) = √(altitude (in feet) * 1.5)

Therefore, you can see the horizon at relatively low altitudes, even as low as a few hundred feet. The higher you go, the farther away the horizon will appear.