View Full Version : Astonomy help

02-21-2019, 01:45 AM
At a dark site, it's only going to be dark when the sun is down and the moon is down or small.

It seems like there has to be a tool that will tell me specific dark hours for a site but my google fu has failed me.

Or is there not a tool because REAL astronomers just know or ... ?

02-21-2019, 02:30 AM
There are several sites that allow you to set location and date. Based on that, it will tell you moonrise / moondown & sunrise / sundown. There are smartphone apps for that too.

Location or site alone won't help you w/o date. If the date isn't important and you need it dark at a certain time, I'm sure you can find a new mood date.

02-21-2019, 02:57 AM
so there isn't a "dark hours" tool I need a place and a date and look up and correlate the sunlit hours, moonlit hours and phases?

(and the position of what I want to see to make sure there won't be ground in the way and the weather and .....)

Will it average out to about a quarter of a lunar month?

02-21-2019, 05:04 AM
Hmm. I know not. Maybe there is. Check out a group called the International Dark Sky Society (Association?). Or maybe there are some astronomy-related photography sites that do such a thing. Like you, they're after windows of most darkness.

As for position, that's a little tougher. Even if you're on a hill, there might be trees. Even with all that, there's weather. Can't help you there.

02-21-2019, 09:14 PM
Location, date and time are needed for all the calculators I know of. If the date and time aren't critical to your story, just say "The sun had fully set but the moon had not yet risen, resulting in nearly total black out conditions. Perfect for what he intended."


02-21-2019, 10:49 PM
There are online calculators online for this (at least there used to be, I had to use one for a project about 18 months ago) - but as others have said, you are going to need a lot of details to make it accurate, if you are certain of all of those you should be able to input them and get a good idea. I think the Astronomy Reddit page also had links for this.

Good luck!

02-22-2019, 11:00 AM
Islamic prayer time calculators would tell you that information. Google "prayer time calculator" Type in your location and the date you want the info for. The important info would be "maghreb" = sunset, "isha'" = the beginning of night, i.e. when it's fully dark (astronomical twilight ends), "fajr" = first light, i.e. dawn twilight begins. The time between isha and fajr is when it's fully dark. Assuming no light pollution, that's when you can see the most stars. You can usually print out charts that give you the prayer times for each day for various months.

ETA: the above is correct for Sunni Muslims. I don't know if Shia Muslims do prayer times the same or not.

Please note that at some latitudes it never gets fully dark in the summer. Where I live (south coast of UK) astronomical twilight never ends and it's never fully dark in June, and for part of May and July. There are different kinds of twilight. Civil twilight means it's still light enough to do ordinary stuff without needing artificial lights. Civil twilight ends at this latitude so it seems dark at midnight in summer, but astronomical twilight persists for the whole night. (Further north you get persistent civil twilight in summer, and obviously once you get in the arctic circle you get midnight sun in summer.) The rest of the year you get proper night. That said, there's too much light pollution in the town where I live for anyone to notice the difference.

On a prayer time calculator, the months when you have twilight persisting, Muslims use a formula to work out what time to do the isha and fajr prayers. The prayer time calculator will notify whether it's calculated based on actual night/twilight or using a formula, so you'll know if and when you don't get fully dark nights in the summer. I have no idea what Muslims in the arctic circle do to calculate prayer times.

You don't need to put exact times in your story but you do need to be in the right ball park. You can't have astronomers in the UK seeing stars at 8pm in June that can only be seen when the sky's fully dark. Even people with no interest at all in astronomy or Islamic prayer times will know that the sun won't have even set yet.

02-22-2019, 11:20 AM
Astonomy help



02-22-2019, 11:47 AM
At a dark site, it's only going to be dark when the sun is down and the moon is down or small.

It seems like there has to be a tool that will tell me specific dark hours for a site but my google fu has failed me.

Or is there not a tool because REAL astronomers just know or ... ?

Is this any help?


Kenneth Keltarin
03-07-2019, 09:13 PM
I hope this is helpful...

Depending on the phase of the moon, it will be visible at specific parts of the night. A full moon actually rises at roughly the same time the sun sets, meaning that during a full moon, there will be a big glowing object in the sky for a full 24 hour period. Full moons also look full not just on the day of the moon, but the day before and after. The opposite is true of the new moon. The new moon actually rises at the same time as the sun, which is why you don't see it at night--it's not there.

Half moons, crescent moons and gibbous moons are more complicated. A first-quarter half moon (where the right-half is lit in the Northern Hemisphere and the left-half is lit in the Southern Hemisphere) rises midday/noon and sets at midnight. A last or third-quarter moon (left-half lit in Northern Hemisphere, right-half lit in Southern Hemisphere) is the opposite: it rises in the middle of the night and doesn't set until midday. Crescents fit between new moons and quarter moons, and gibbous fit between quarter moons and full moons.

To answer the original question, at night it will be darkest at anytime during a new moon, the second-half of the night for a first quarter-moon, the whole night for a full moon, and the first-half of the night for a last-quarter moon. Wikipedia also has a chart listing all of this information but I picked all this up during my astronomy class back in college, it's fairly common knowledge for astronomers and has been for centuries! :)

03-11-2019, 09:27 AM
I think I got it.

In the north,
new moon dark,
then dark early
no dark
dark late (or early in the morning)

details wikipedia or calculator.

Thank you all