Which object is most suitable for determining nighttime visibility?

Determining nighttime visibility relies heavily on the ability to perceive objects under low-light conditions. The correct answer, unfocused lights of moderate intensity at known distances, serves as a practical tool for assessing visibility because they can provide a consistent reference point for measuring how far one can see in the dark.

Unfocused lights—such as lights from vehicles, buildings, or other sources that do not create distinct outlines—can be perceived from various angles and distances, allowing observers to gauge visibility effectively. When these lights are of moderate intensity, they can be seen without overwhelming the observer's eyesight, making them a reliable metric for assessing how well one can see at night.

While bright stars, street lights, and moonlight do contribute to nighttime visibility, they each have limitations. Bright stars are often too distant and variable to serve as reliable benchmarks, while street lights can create varying degrees of illumination and often result in stark contrasts that might mislead an observer about true visibility. Moonlight reflecting off surfaces can enhance visibility but is dependent on other atmospheric conditions and the phase of the moon, making it less reliable as a standalone measure.

Thus, unfocused lights at known distances stand out as the most suitable option for accurately determining nighttime visibility.