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I know that similar questions have been answered, but this is very specific.

I know that I need as much darkness as possible, and hence the recommendation for new moon days. But, would you get the same effect during any other day (even on full moon) if moonset is during the afternoon, and therefore there is no visible moon on the sky during the night? Or does the moon still waste the pictures somehow even when there is no line-of-sight?

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    I'm guessing that you might get better quality answers if you re-phrased your question: "How far below the horizon (i.e., how many hours after moonset/before moonrise) must the moon be in order for its light to not interfere with my night-sky photographs?" (Note: I have never seriously attempted to photograph the night sky, so I can't answer that question myself.) – Solomon Slow Jun 3 '20 at 12:30
  • Serious astronomers like new moon nights because they can observe the entire night without interference from the moon. If your only goal is to shoot the milky way, then you only need the moon to be well below the horizon. – Mattman944 Jun 3 '20 at 15:27
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    Does this answer your question? Can you photograph the milky way with a full moon out? – Michael C Jun 4 '20 at 9:56
  • The full moon will never set during the afternoon. It will rise around sunset and set around sunrise. For the moon to set in the afternoon, it must be a waning crescent past last quarter (which will set approximately around noon, depending on the tilt of the Moon's orbit in relation to the plane of the Earth's orbit as well as the tilt of the Earth's axis in relation to the Sun). – Michael C Jun 4 '20 at 10:17
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The short answer is 'no' ... but it's really a qualified no. Here's why:

Position of the Milky Way band

The Milky Way is a band that appears to go all the way around the planet. Roughly described, is the plane of the Milky Way galaxy. In January the night-side of the planet sees the side of the Milky Way facing away from the galactic center -- so the Milky Way band is thinner. In July the night-side of the planet sees the side of the Milky Way facing toward the galactic center.

But since the band of the Milky Way appears to rise (really the planet is rotating) ... the time during the night when the Milky Way appears to be most prominent will vary based on which month or day you plan to do your imaging.

Currently (this being early June), the Milky Way is barely rising after dark in the evening hours. If you want the Milky Way positioned vertically ... you'd be waiting until ... maybe 2-4am. So this brings us back to the Moon.

The takeaway here: The best time to shoot the Milky Way during the night will change depending on the desired placement (e.g. Do you want a vertical Milky Way vs. do you want a Milky Way with a diagonal path or one that arcs across the sky? etc.) The best time to shoot during the night to get your desired placement will vary based on the date.

Moon Phases

While a New Moon is rising and setting with the Sun. The Moon isn't visible at any time during the night. This gives you maximum flexibility.

A First Quarter Moon rises roughly in the middle of the daytime hours and sets roughly in the middle of the nighttime hours. This means if your best time to shoot the Milky Way is in the pre-dawn hours ... the Moon would already have set.

A Last Quarter Moon rises roughly in the middle of the night and sets roughly in the middle of the day. This means if your desired time to image the Milky Way is after sunset (the early part of the night) then the Moon will not have risen in the sky yet.

And of course the Full Moon is up all night long ... there is no good time during the night to shoot during a full moon.

Check the times for moon rise & set. Each night. The Moon will rise, transit, and set about 55 minutes earlier each day.

Atmosphere

The primary reason to avoid shooting when the Moon is up is because of the large amount of light pollution created by the Moon. It's the same reason to travel away from cities -- to get away from the street lights.

Light from the Moon (or any light source) is illuminating particles in the air... dust, smog, humidity, etc. and this means the "black" background of space isn't so black anymore... it's more of a dark gray. That gray starts to become brighter than the fainter stars in the sky (or any other deep sky objects) and makes them more difficult to see... so the image is less dramatic.

Can you have a tiny bit of light pollution? Sure... it's just less optimal and this means you'll need to "stretch" your image data a bit more to try to tease out the Milky Way in your final images. When shooting the Milky Way, you're always going to be heavily post-processing the data to "stretch" it to improve the visibility.

BTW, Astronomical Twilight ends when the Sun is 18° (or more) below the horizon. After that point, the atmosphere is unable to refract any of the Sun's light farther around the planet -- any light you still see once the Sun is beyond 18° below the horizon is coming from something other than the Sun.

Helpful Aids

There are a number of applications that can make it easy to work out the position of the Milky Way as well as the Moon or Sun.

Stellarium is a free desktop planetarium application that runs on Windows, Mac, & Linux machines ... you can't beat the price. There are also a number of non-free apps.

There are also some mobile apps that are nice. I use Sky Safari on my phone (extremely helpful for general astronomy as well). But there is a photography-specific app that I've found called "PhotoPills". The most interesting (to me anyway) feature of PhotoPills is that you can scout locations to shoot during the daytime and use it's augmented-reality feature -- the phone will display the image of your scene. You set the date/time when you'd like to shoot and it will overlay the night sky and Milky Way band so you can evaluate how the Milky Way will appear relative to other elements of your scene ... to work out the best location to set up for the shot. Return back at the time and location to capture the Milky Way exactly as you want it. PhotoPills used to be iOS only, but is now also available on Android.

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  • Thanks for the info that PhotoPills is now available for Android! – Michael C Jun 4 '20 at 10:07
  • It should probably be noted that at high Northern latitudes (or low Southern latitudes) the Sun may never sink more than 18° below the horizon for several weeks or even months, depending on exactly how far north or south one is located, during Summer. After all, above the Arctic circle/below the Antarctic circle the Sun doesn't even sink below the horizon at all for six months! – Michael C Jun 4 '20 at 10:11
  • Thanks Tim, this is a GREAT explanation. My confussion came from the wrong idea of the moon setting/rising on a certain basis, not always the same in each phase. – Roman Rdgz Jun 4 '20 at 13:32
  • @MichaelC very true. This also relates to satellites crossing the image. Since they reflect sunlight, you wouldn't see them if they are completely inside Earth's shadow. But at certain latitudes (high north or low south) that may never happen (they can be visible at any time during the night). – Tim Campbell Jun 4 '20 at 22:23
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I think this is based on a false premise.

Without needing a degree in astrophysics, this simple explanation will do…

Pretty much by definition a new moon is at its zenith at noon & behind the earth at night.
To create a new moon, the sun has to be behind the moon from our perspective, so its visible face is in shadow to us. This means they have to be 'up' at the same time.

A full moon, by the same token is at zenith at night… right where you don't want it if you need darkness.
A full moon is when the sun is shining right on its visible face, which means they have to be on opposite sides of the earth.

See TimeAndDate.com - London, England, United Kingdom — Moonrise, Moonset, and Moon Phases, June 2020 for an example.

So, if you wait for a new moon, it's never going to be in the sky at night.
Problem solved.

By this token you could probably get away with anything before the first quarter, or after the last quarter, though you will probably get some internal reflection in the atmosphere. To know how great an effect that will have on any given phase, you probably would have to ask an astrophysicist ;)

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  • Considering that the Moon is new only for a single moment each month as it is exactly aligned between the Earth and the Sun as viewed from a position perpendicular to the plane of the Earth's orbit, and considering that the Earth is a slightly misshapen sphere, it will be any time of day or night, local time, somewhere on the surface of the Earth each time the Moon is new. – Michael C Jun 4 '20 at 10:01

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