# eli5: How do meteorologists determine when sunrise and sunset occur (down to the minute) if it’s still light or dark outside visually?

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I was looking at the weather app and, from where I live, sunset occurs exactly at 7:16 pm. Outside, the sky is a dark blue but still light enough to see and navigate the world. On the weather app it displays “Civil Twilight”, “Nautical Twilight”, and “Astronomical Twilight”. If these sunset periods occur after 7:16, then how do we know that sunset occurs exactly at 7:16 and not at these other time periods?

In: Planetary Science

well, its really simple. you look at the sun, and the horizon, and when you can no longer see the actual sun its self behind the horizon, that exact moment is sunset. Watch the Sun for long (16 years I think it was? that could be the moon though) enough and you realize just how predictable the sun is. This gives you a math equation to tell you where the sun is.

now you have an equation for it, you dont have to see the sun to know where it is, and you can define all of the twilights based on where the sun is even though you cant see it

Sunset happens when the sun drops below the horizon, not when the sky gets dark.

And for those sunset times, they just calculate it in an idealized scenario where theres no hills or big buildings, just flat land or water all the way out to the horizon.

So you basically just treat the earth like a ball and pick a spot on the ball for your city, and the calculate when they wont be able to see the sun anymore.