Is illumination in the sky due to moonlight measured in any way? I'm trying to understand if I can make a model to understand at what level of illumination a person can clearly identify another person in a 1-meter range if six people with a similar physical characteristics are standing in a line.

Why am I doing this?

A friend of mine is being falsy accused of a crime. A woman is saying that she saw my friend at a place in the moonlight during a crime. In reality, he was with me that day at that time, and there is no way he was at the crime scene 50 miles away. I want to help him and his advocate with science. The date is February 16th, 2020.

  • $\begingroup$ You need an expert witness. In the UK, the Royal Greenwich Observatory supply such a service I believe. $\endgroup$
    – ProfRob
    Commented May 12, 2023 at 7:10
  • $\begingroup$ Usually light pollution produces way more light than the moon. The moon only looks brighter because it comes from one spot in the sky. $\endgroup$ Commented May 12, 2023 at 12:11

1 Answer 1


Illumination is measured in "lux". This is a measure of luminous flux, per unit area on the ground.

It varies from about 50000 lux on a bright sunny day, to about 1 at twilight, to about 0.1 on a typical moon-lit night, to 0.001 or less under starlight. This is why you will see more stars on nights when the moon has set.(source)

Our eyes are pretty marvellous things, they can adapt to different levels of illumination. Inside a house has about 1% of the illumination of outside, and yet you hardly notice as you walk inside a house. The usual scale of the brightness of astronomical bodies is logarithmic, to compensate for how our eyes perceive brightness. How this is can be applied to criminal law is beyond the scope of this answer.

  • $\begingroup$ Thanks. Is it fair to assume a person can see but not identify if he is inside a room with no light but there is a good amount of moonlight outside? how do i design an experiment to test it? $\endgroup$
    – Tony Paul
    Commented May 12, 2023 at 19:18
  • 1
    $\begingroup$ There is no astronomy in your followup question. You are a not an independent expert witness, so no "experiment" you do would be admissible anyway. $\endgroup$
    – James K
    Commented May 12, 2023 at 21:27
  • 1
    $\begingroup$ @TonyPaul Welcome to Astronomy Stack Exchange! For your followup, consider Law Stack Exchange but read through some questions first to get an idea of the style of questions that are well-received there. $\endgroup$
    – uhoh
    Commented May 12, 2023 at 23:17

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