Ah… can’t use the sun phase item then (unless I say it isn’t equal to all the pre-sunset states). I’ve gone with the DSL route - thank you @rlkoshak and @rpwong .
Rich is the expert here…I just like his design patterns. 
I should have mentioned that I have multiple rules that reference my time of day item, which is why I prefer that approach. Also, I try to keep things as simple as possible so that my non-programmer brain can grasp them.
Pointing Stuart to the ToD Design Pattern was exactly correct. But while OH 3 is still new, I also like to actually answer the original question, even if it may or may not be the best overall approach. The more answers to stuff like “how do I use But only if…?” there are on the forum, hopefully the fewer questions we will need to answer about them later on. 
@stuart475898, absolutely, if you have two or more rules that care about sunrise/sunset you should look at using the time of Day DP.
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I just use the but only if part of the rule section and say:
LocalSun_SunPhaseName != DAYLIGHT
That pretty much covers sunset to sunrise.
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Actually… that probably would do just as well. Not sure why I didn’t think of this??? Thanks @ubeaut
I use the sun phase, it is a alternative.
I guess this will depend on location.
Here (mid Norway) it will be after sunset, but there is no need for outdoor lightning mid summer.
I have light sensors as well so the lights only come on if the lux level is low.
I also use lux levels but I get them from the astro binding “diffuse radiation” value as I don’t have a light sensor yet. I use it in many rules including motion sensor lighting, christmas lighting (with 3 lighting levels defined for all on, only inside on and all off), exterior lighting auto-off triggers.
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I use the Phillips hue motion sensors as they have zigbee and have lux, motion and temperature sensors. I use zigbee2mqtt to talk to OH3.
Do you find it works well - that is, the lights generally come on when they need to? Could you give an example of where/how you use it? E.g. when radiation is below some value, toggle a virtual switch that OH uses to turn a light on when motion detected?
Sounds like an interesting way to solve the problem or light levels
Things
astro:sun:home @ "World" [ geolocation="##.####, ##.####, ###" ]
Items
Number:Intensity Diffuse_Radiation "Diffuse radiation [%.2f %unit%]" (Home, Sensors) { channel="astro:sun:home:radiation#diffuse" }
Rules
//------------------------------------------------------------------------------------------------
//-------------------------- Night light controll --------------------------
//------------------------------------------------------------------------------------------------
rule "Night light controll"
when
Item Diffuse_Radiation changed or
Item Someone_Home changed
then
var DiffRad = Diffuse_Radiation.state as Number
Diffuse_Radiation.persist
if (DiffRad <= 40 && Someone_Home.state == ON) {
if (Cor_night_light.state != ON) Cor_night_light.sendCommand(ON)
} else {
if (Cor_night_light.state != OFF) Cor_night_light.sendCommand(OFF)
}
end
//------------------------------------------------------------------------------------------------
//-------------------------- Christmass tree controll --------------------------
//------------------------------------------------------------------------------------------------
rule "Christmass tree controll"
when
Item Diffuse_Radiation changed or
Item Someone_Home changed or
Time cron "0 0 8 1/1 * ? *" or
Time cron "0 0 21 1/1 * ? *"
then
var Number hours = now.getHourOfDay
var DiffRad = Diffuse_Radiation.state as Number
if (DiffRad <= 80 && Someone_Home.state == ON && hours >= 8 && hours < 21) {
if (Sonoff_Powerstrip.state != ON) Sonoff_Powerstrip.sendCommand(ON)
} else {
if (Sonoff_Powerstrip.state != OFF) Sonoff_Powerstrip.sendCommand(OFF)
}
end
Night light in the corridor turns on and stays on when it gets dark enough for that light to be of any help
Christmas Tree turns on if diffused light radiation is low enough for the tree lights to be visible but no earlier than 8 in the morning and turns off no later than 9 in the evening. In my case the number is rather high because the Christmas lights are bright and are still visible.
I also use a Someone_Home Item for presence detection and there is no reason for any of those lights to be ON while there’s nobody home.
Because the diffused radiation value is calculated based on geoposition, it increases and decreases without fluctuations so there is no need for any hysteresis. If I was using a real lux sensor, I would use if ((DiffRad + 5) <= 40) {...} else if ((DiffRad - 5) > 40) {...}.
I’m also interested in this. How accurate do you find the diffuse radiation over the course of the day and in varying weather conditions? For example, if there was a thunderstorm that suddenly darkened the sky and then cleared up, would the diffuse radiation keep up? It would be amazing if openHAB knew to turn the lights on when the sky suddenly darkened, then turn them off when it brightens up.
I’ve set up an item to monitor the changes over the course of a day, but I’m very curious about your experience with it.
@rpwong -
“if there was a thunderstorm that suddenly darkened the sky and then cleared up, would the diffuse radiation keep up? It would be amazing if openHAB knew to turn the lights on when the sky suddenly darkened, then turn them off when it brightens up.”
I believe that is only truly possible with an active sensor. Diffused Radiation is calculated based on the earths rotation by the Astro binding and not a measurement of current weather conditions and local light. I am also looking into options for doing exactly this - measure light levels and trigger rules based on the immediate conditions 
+1! 
For now though, using the Astro bindings Sun Phase channel and the Civil Dawn/Dusk state has been working just fine here!
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My active light sensors are great. It is dark and raining here in Australia today and the motion sensors are setting on the lights even though it is only 4pm.
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As d0t said, the astro binding calculates the light level only based on sun’s position and earth’s rotation. Cloud cover is not taken into account. I trialed my values based on how far from the windows is each light and how much scattered light gets there on an average day. Sometimes the lights go on when it’s too bright or go off when it’s too dark but in general it works OK. I do not have an active light sensor, so this is the only way I could figure out how to get as close to knowing the real lighting conditions as possible.
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Gotcha. It’s definitely a good way to turn things on (which I’m already doing with my light sensors), but I was hoping it could also provide some intelligence to turn things off. In retrospect, that would be too much to hope for. Still, a really interesting approach.
You could use a weather service to modify Astro’s (maximum) prediction, to improve the guesswork. I’ve no idea how much you take off for “overcast” or “rain”, but a few days experimentation might give a start.
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I was thinking that, too. The ultimate solution would be to monitor a radar map, but that is waaaaaayyyy beyond my skill set.
What I do works fairly well. It’s not perfect but serviceable. I simply use OWM’s cloudiness channel which is a percent. If it’s over 60% I turn the lights on. Even here where weather is so localized that one could be in full sun in a downpour it always turnes the lights on when they are needed and infrequently turns them on when they are not needed.
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That’s a good idea! I’ll try that combined with diffused radiation value when I come back and see if that provides any better results. Thanks for the tip
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