What does sunrise/sunset time mean for practical light conditions?
September 26, 2018 11:38 AM   Subscribe

I'm never sure exactly when the world will actually be visible relation to the predicted sunrise or sunset on a certain date. Can you explain this?

Let's say someone wanted to drive in visible conditions, and they learned that "sunrise" would be at 8 a.m. When would it be fairly light outside? How soon after sunset would it be really dark outside?
Assume I understand that clouds etc. impact this. I just mean when does the sunlight actually pervade or vanish from the world in relation to "sunrise" and "sunset?"
posted by nantucket to Science & Nature (8 answers total) 7 users marked this as a favorite
 
You want to read about Civil Twilight and other related terms.

I'm sure one could easily find an app that could give you the times for these things based on the date and lat/long.
posted by bondcliff at 11:40 AM on September 26, 2018 [4 favorites]


The Photographer's Ephemeris is such an app. There's probably one that's simpler/cheaper, since it also has all kinds of stuff about moonrise/set and what direction the things are rising and setting. But it's a pretty cool app.
posted by aubilenon at 12:24 PM on September 26, 2018 [2 favorites]


Civil twilight is definitely the thing you want, and the page bondcliff linked to is good. Depending on light pollution you may be able to see things around you even in nautical twilight (I speak from experience waiting for meteor showers to become visible), but that's not adequate light to drive by.
posted by fedward at 12:44 PM on September 26, 2018 [1 favorite]


My rule of thumb, having lived mostly a little north of center of the US is about 20 minutes of pretty decent light. That fits with the Civil Twight page which says 20-30 minutes. Note that this is noticeably shorter if there are mountains or trees blocking the rising/setting light.
posted by metahawk at 1:23 PM on September 26, 2018


A big part of this depends on the time of year. The angle of the sun's path with the horizon varies 47 degrees between the summer & winter solstices. In winter it intersects the horizon at a much shallower angle, so the sun may be hidden by whatever obstacles are along the horizon well before the official time of sunset. In summer the descends toward the horizon at a much steeper angle so that it disappears from view only a few minutes before the official sunset. This also affects the duration of the afterglow following sunset. The overall effect is that twilight is spread over a longer time both before and after sunset in winter, while in summer it all happens significantly faster.
posted by wps98 at 5:41 PM on September 26, 2018 [1 favorite]


In summer the descends toward the horizon at a much steeper angle ... The overall effect is that twilight is spread over a longer time both before and after sunset in winter, while in summer it all happens significantly faster.

I used to believe this, but it's a misconception (I was going to say "a common misconception", but probably most people never think about this topic at all...). Twilight is longest around the summer solstice, second-longest around the winter solstice, and shortest around the equinoxes. Here are some charts and a Q&A thread with some nice explanations.
posted by aws17576 at 11:19 PM on September 26, 2018 [3 favorites]


The driving laws in your state probably specify that headlights must be on before/after some critical time, such as 20 minutes before sunrise and after sunset.
posted by SemiSalt at 8:05 AM on September 27, 2018


The sunrise module I use takes into account your longitude, latitude, and altitude. One minute is 0.25 degrees, one degree is 4 minutes. So Civil Twilight is (6-.833)*4 = ~20.668 minutes before the absolute first bit of the sun you can see on the horizon.
  Sun Height
    There are a number of sun heights to choose from. The default is -0.833
    because this is what most countries use. Feel free to specify it if you
    need to. Here is the list of values to specify the sun height with:

    *   0 degrees

        Center of Sun's disk touches a mathematical horizon

    *   -0.25 degrees

        Sun's upper limb touches a mathematical horizon

    *   -0.583 degrees

        Center of Sun's disk touches the horizon; atmospheric refraction
        accounted for

    *   -0.833 degrees

        Sun's supper limb touches the horizon; atmospheric refraction
        accounted for

    *   -6 degrees

        Civil twilight (one can no longer read outside without artificial
        illumination)

    *   -12 degrees

        Nautical twilight (navigation using a sea horizon no longer
        possible)

    *   -15 degrees

        Amateur astronomical twilight (the sky is dark enough for most
        astronomical observations)

    *   -18 degrees

        Astronomical twilight (the sky is completely dark)


posted by zengargoyle at 11:53 AM on September 27, 2018 [1 favorite]


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