<p>The typical stated time after maximum phase angle is ~15 hours before it's visible to the naked eye, but this would depend a bit on the declination of Luna relative to that of Sol. In any case, a good estimate would probably be around at least 0.3% illumination.</p>

CC BY-SA 4.0
2023-04-23T20:55:01.140Z
+1 While answers should try to draw from or cite supporting sources or show some work to support assertions, it may not be so easy in this particular case.
uhoh
CC BY-SA 4.0
2023-04-21T11:57:10.783Z
2023-04-22T12:35:30.413Z
2023-04-22T12:35:30.413Z
Outis Nemo
<p>Under perfect sky conditions the Moon doesn't need to be illuminated at all, and you might experience a solar eclipse. In other months it is approximately nearly a day after new moon.</p>
<p>Even during new moon the Moon's dark side will be illuminated by the Earth shine, thus it is not totally dark. So even without an eclipse one can in principle also spot the new Moon in the vicinity of the Sun. Practically it's a matter of contrast, and that makes the problem surprisingly difficult.</p>
<p><a href="https://astro.ukho.gov.uk/moonwatch/nextnewmoon.html" rel="noreferrer">https://astro.ukho.gov.uk/moonwatch/nextnewmoon.html</a> gives a good overview over where and when you can spot the new moon with which optical aids (and if any are needed). According to that the earliest you can see it with the naked eye is around 15..23h after new moon and 12..19h with a telescope. The exact duration between new moon and first visibility <a href="https://astro.ukho.gov.uk/moonwatch/background.html#yallop_criterion" rel="noreferrer">varies</a>, depending on where the Earth and Moon are in their respective orbits, also due to the the inclination, thus the height of the Moon in the sky - which in turn affects visibility and contrast:</p>
<blockquote>
<p>Predicting the first sighting of the new crescent moon has been attempted since the time of the Babylonians and perhaps earlier still. These early methods relied on the age of the Moon and the time difference between moonset and sunset. More modern methods use the angular separation of the Sun and Moon, the altitude difference between the Sun and Moon, their relative azimuths and the width of the crescent. Some methods even try to model atmospheric effects to improve the prediction and to address the effect of relevant atmospheric phenomena.</p>
</blockquote>
<p>They also give an <a href="https://astro.ukho.gov.uk/download/NAOTN69.pdf" rel="noreferrer">extensive technical treatise</a> on the problem describing their approach(es) in detail.</p>
<p>You can also visit <a href="https://astro.ukho.gov.uk/surfbin/first_beta.cgi" rel="noreferrer">this site</a> to have it tell you for your location the first visibility.</p>

CC BY-SA 4.0
2022-03-02T19:32:20.033Z
2022-07-08T19:07:11.087Z
2022-07-08T19:07:11.087Z
planetmaker
<p>Originally, the new Jewish month was determined by the <a href="https://www.chabad.org/library/article_cdo/aid/526874/jewish/The-Jewish-Month.htm" rel="noreferrer">sighting of the new crescent moon.</a></p>
<p>There are <a href="https://www.paulcarlisle.net/mooncalendar/" rel="noreferrer">online calendars</a> that give percentage of moon that is illuminated.</p>
<p>Using the computations from these calendars, what is the minimum illumination percentage for someone to be able to unambiguously see it with naked eye assuming perfect sky conditions?</p>

CC BY-SA 4.0
2022-03-02T21:41:59.137Z
A friend of mine, and author of "The Six Inch Lunar Atlas", was pretty big on this, and encouraged a competition among our astronomy club to be the first to see a new moon when conditions were right.  More than just the illuminated fraction comes in to play, the angle the sun-moon line makes with the horizon plays a big role.  E.g.  If the sun is not very far below the horizon, the sky will still be quite bright and can wash out the moon, even if it's more illuminated than other times when it was easily visible.  My friend would track this, and announce when a good viewing was coming.
Greg Miller
<moon-phases>
what percent of moon must be illuminated to be visible with naked eye and clear sky?
Bob
# what percent of moon must be illuminated to be visible with naked eye and clear sky?

(Asked by: username_0 on 2022-03-02)

Originally, the new Jewish month was determined by the [sighting of the new crescent moon.](https://www.chabad.org/library/article_cdo/aid/526874/jewish/The-Jewish-Month.htm)

There are [online calendars](https://www.paulcarlisle.net/mooncalendar/) that give percentage of moon that is illuminated.

Using the computations from these calendars, what is the minimum illumination percentage for someone to be able to unambiguously see it with naked eye assuming perfect sky conditions?

**Tags:** moon-phases
> **Comments:**
> **username_1**: A friend of mine, and author of "The Six Inch Lunar Atlas", was pretty big on this, and encouraged a competition among our astronomy club to be the first to see a new moon when conditions were right.  More than just the illuminated fraction comes in to play, the angle the sun-moon line makes with the horizon plays a big role.  E.g.  If the sun is not very far below the horizon, the sky will still be quite bright and can wash out the moon, even if it's more illuminated than other times when it was easily visible.  My friend would track this, and announce when a good viewing was coming.  

---

## 2 answers

### Answer by username_4 on 2022-03-02. Score: 13 | Accepted answer

Under perfect sky conditions the Moon doesn't need to be illuminated at all, and you might experience a solar eclipse. In other months it is approximately nearly a day after new moon.

Even during new moon the Moon's dark side will be illuminated by the Earth shine, thus it is not totally dark. So even without an eclipse one can in principle also spot the new Moon in the vicinity of the Sun. Practically it's a matter of contrast, and that makes the problem surprisingly difficult.

<https://astro.ukho.gov.uk/moonwatch/nextnewmoon.html> gives a good overview over where and when you can spot the new moon with which optical aids (and if any are needed). According to that the earliest you can see it with the naked eye is around 15..23h after new moon and 12..19h with a telescope. The exact duration between new moon and first visibility [varies](https://astro.ukho.gov.uk/moonwatch/background.html#yallop_criterion), depending on where the Earth and Moon are in their respective orbits, also due to the the inclination, thus the height of the Moon in the sky - which in turn affects visibility and contrast:

> Predicting the first sighting of the new crescent moon has been attempted since the time of the Babylonians and perhaps earlier still. These early methods relied on the age of the Moon and the time difference between moonset and sunset. More modern methods use the angular separation of the Sun and Moon, the altitude difference between the Sun and Moon, their relative azimuths and the width of the crescent. Some methods even try to model atmospheric effects to improve the prediction and to address the effect of relevant atmospheric phenomena.

They also give an [extensive technical treatise](https://astro.ukho.gov.uk/download/NAOTN69.pdf) on the problem describing their approach(es) in detail.

You can also visit [this site](https://astro.ukho.gov.uk/surfbin/first_beta.cgi) to have it tell you for your location the first visibility.


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### Answer by username_2 on 2023-04-21. Score: 1

The typical stated time after maximum phase angle is ~15 hours before it's visible to the naked eye, but this would depend a bit on the declination of Luna relative to that of Sol. In any case, a good estimate would probably be around at least 0.3% illumination.

> **Comments:**
> **username_3**: +1 While answers should try to draw from or cite supporting sources or show some work to support assertions, it may not be so easy in this particular case.  

---

