A human with normal vision (20/20) can typically see about 3 miles (5 kilometers) to the horizon when looking horizontally from a height of about 5 feet above the ground. This limit is primarily due to the curvature of the Earth, which causes objects beyond this distance at ground level to be obscured. However, under ideal conditions, such as a clear day and viewing large objects like mountains or tall buildings, humans may see objects up to 50 kilometers (about 30 miles) away, depending on the size and visibility of the object. At night, the human eye's sensitivity to light allows seeing light sources such as a candle flame from about 1.5 miles (2.4 kilometers) away. Additionally, when looking at the sky, humans can see stars, planets, and galaxies that are millions to billions of miles away, although these are seen as faint lights rather than clear images. In summary:
- On flat ground, limited by Earth's curvature: about 3 miles (5 km)
- Large distant objects (mountains, tall buildings) under ideal conditions: up to 50 km (30 miles)
- Candle flame visibility at night: about 1.5 miles (2.4 km)
- Celestial bodies in space: millions to billions of miles away
These distances can vary based on factors such as eye health, elevation, brightness, atmospheric conditions, and size of the object being viewed.
