Hours of deep sky darkness for 2021

[StuartT]

I decided to plot out the hours of deep sky darkness (using the APT app) for the rest of the year. Yes, I know.. I need to get out more.

Just in case it's of interest, here it is. It's for my location (Oxford), but should be reasonably good for the south east (ish)

 

Edited August 9, 2021 by StuartT

[Ouroboros]

@StuartT I’m probably being really stupid but I can’t work out what I’m looking at there. Would you mind explaining it. Thanks. 

[StuartT]

15 minutes ago, Ouroboros said:

@StuartT I’m probably being really stupid but I can’t work out what I’m looking at there. Would you mind explaining it. Thanks. 

Deep sky darkness is the period of actual astronomical night (so after astro twilight has ended) and when the moon is below the horizon.

So basically the hours of proper darkness when you can do astrophotography without the moon messing things up.

This diagram shows the actual periods of the various levels of darkness, but without the moon factored in

Edited August 9, 2021 by StuartT

[Ouroboros]

@StuartT Ah … I get why it goes up and down now. Tis the moon.  Sorry. I was being a tad slow there.  

[wulfrun]

5 minutes ago, Ouroboros said:

@StuartT Ah … I get why it goes up and down now. Tis the moon.  Sorry. I was being a tad slow there.  

You weren't alone...same here, I was also puzzled but now it's clear (unlike the skies!)

[Knighty2112]

Ditto. I was mucho confused too! 😉

[AstroMuni]

19 hours ago, StuartT said:

So basically the hours of proper darkness when you can do astrophotography without the moon messing things up.

what is on the Y axis - Is it number of hours of astro darkness?

[Ouroboros]

13 minutes ago, AstroMuni said:

what is on the Y axis - Is it number of hours of astro darkness?

Yeah. That’s how I understand it anyway. 

I wonder why the curve isn’t sinusoidal? I understand why it’s cut off at the bottom. But why also at the top? Why are they asymmetric. 

PS Ah … Is it something to do with the relative positions of the sun and the moon around the time of the new moon? 

PPS And presumably they’re asymmetric because we’re heading towards winter. i.e the hours of true darkness are getting longer day by day. 

Edited August 10, 2021 by Ouroboros

[StuartT]

20 minutes ago, AstroMuni said:

what is on the Y axis - Is it number of hours of astro darkness?

correct

 

8 minutes ago, Ouroboros said:

Yeah. That’s how I understand it anyway. 

I wonder why the curve isn’t sinusoidal? I understand why it’s cut off at the bottom. But why also at the top? Why are they asymmetric. 

PS Ah … Is it something to do with the relative positions of the sun and the moon around the time of the new moon? 

Hmmm... good question! I think it's because at the start of the flat top peaks the moon is new, so the only factor affecting increase in darkness length is the steady increase in night duration (which is virtually linear).

The data is from the Astrophotography Tool app https://www.astrophotography.app/mobile_apps.php

Edited August 10, 2021 by StuartT

[Ouroboros]

@StuartT Yes. OK. So presumably at mid-winter the peak is symmetrical.  If new moon falls near mid winter. 

Edited August 10, 2021 by Ouroboros