42
votes
How is this picture of the moon possible?
What is the obstruction creating the crescent considering the moon is between the earth & sun?
The moon. The part of the moon that's dark is in the shadow of the rest of the moon. Because the ...
- 758
24
votes
Is it possible to do moon sighting in advance for 5 years with 100% accuracy?
Note: this answer was posted under duress; though I mentioned in a comment under the question that I was composing an answer, several users have decided to close the question out from under me. ...
- 31.4k
22
votes
How often does a full moon happen on the weekend?
Exactly 2/7 of time is "weekend", There is no exact alignment of the lunar orbit to the length of a week, so a full moon is equally "likely" to be at the weekend
So 2/7 of full ...
- 115k
20
votes
Is there a precise definition of "Supermoon"?
No, there is no special name for the closest supermoon.
Besides, the very name “supermoon” was coined by an astrologer, NOT an astronomer, and it is basically all just media hype. During a so-called “...
- 6,701
17
votes
Accepted
How can the new moon set before the sun, as will happen in some locales on November 13, 2023?
This is because the New Moon is defined as the moment that the moon is at the same ecliptic longitude as the sun. It may be several degrees above or below.
The situation for moonset on Nov 23 is shown ...
- 115k
16
votes
Accepted
The shape of the moon limb/crescent (terminator line)
The line is a semi-ellipse. It is formed by projecting the great circle on the moon formed by the terminator to the plane perpendicular to a line going from the observer to the moon. Projecting a ...
- 115k
15
votes
Accepted
Why moon sighted only in Kerala but not in rest of India?
Think of it like this:
Each hour after New Moon, the Moon is moving away from the Sun. Depending on the position of the Sun, Moon, and observer, it is unlikely to be visible until at least 15 hours ...
- 7,787
15
votes
How is this picture of the moon possible?
The Moon doesn't really rise and set exactly "with the sun", it is usually somewhat north or south of it, because its orbit is somewhat tilted with respect to Earth's orbit.
In this picture ...
- 31.4k
13
votes
Accepted
What is the Terra phase?
The Earth-Moon System is a system where the moon is tidally locked to the planet, but the planet is not tidally locked to the moon. Earth has a 24 hour day, whereas the Moon has a 29.5 Earth day long ...
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13
votes
How is this picture of the moon possible?
It's not a shadow. The only time the moon is shadowed is when it's full and there's a lunar eclipse. What you saw was the moon when most of the side we see was facing away from the sun. At the time of ...
- 1,608
13
votes
Accepted
what percent of moon must be illuminated to be visible with naked eye and clear sky?
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 ...
- 17.1k
11
votes
How does the Moon look like from different latitudes of the Earth?
Here are some answers to your various comments and questions:
you can estimate a direction of south
True, but it would be a very rough approximation at best. In order for it to be accurate, the ...
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11
votes
The shape of the moon limb/crescent (terminator line)
Edit: created a new Codepen that shows the terminator as a gradient, which eliminates the hard line.
Rigorous:
For those not wanting to settle for an approximation, rendering a photorealistic, ...
- 5,542
10
votes
Accepted
How close is a typical New Moon to the Sun in the sky?
Mean angular separation of about 3.2°, based on skyfield Python library simulation of the 2nd millennium.
This is pretty dependent on your viewing location. The ...
- 4,696
9
votes
Accepted
Why can I see the dark side of the moon?
You can generally see the unlit side of the moon when a considerable amount of sunlight is reflected off the earth. This reflected sunlight illuminates the unlit side of the moon. This is referred to ...
- 1,371
9
votes
Accepted
Get a table of Moon phase angle per day
JPL HORIZONS can do that.
With settings like these:
Ephemeris Type : OBSERVER
Target Body : Moon [Luna] [301]
Observer Location : Geocentric [500]
Time Span : Start=2019-09-12, Stop=2019-09-...
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9
votes
Accepted
Is it possible to explain the difference between synodic month and sidereal month by degrees?
A better diagram is something like this:
Day 0. The Moon is aligned with the Sun (New Moon) and some star X behind the Sun (of course :-), as shown by the arrow at 1. (Neither the Sun nor the star ...
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9
votes
Accepted
Is there a precise definition of "Supermoon"?
To directly answer your question: There is no "precise" definition of the term "supermoon."
Term Origin: The origin of the term is generally attributed to the astrologer Richard ...
- 1,903
9
votes
How is this picture of the moon possible?
As you said in your question, the phases of the moon are determined by the positions of the Sun, Moon and Earth in space (it is easy to test out the principle at home using a light bulb and a ball and ...
- 2,867
9
votes
The shape of the moon limb/crescent (terminator line)
You must draw a semicircle of radius R and a semi-ellipse of semi-major axis "R" and semi-minor axis "b".
To calculate "b" knowing the illuminated ratio "n":
$$...
- 1,720
8
votes
What should I consider in order to accurately calculate the moon phase?
The Moon's phase isn't easy to calculate accurately. Fortunately, the 4 main phases (new moon, first quarter, full moon, last quarter) are a bit more stable than the intermediate phases. Good main ...
- 13.1k
8
votes
Accepted
Is there a word or short phrase that denotes the apparently moving part of the lunar limb?
It's called "the terminator line" or simply "the terminator"; where the illumination from the sun terminates.
There's a fuller discussion in EarthSky.org's What is the moon’s ...
- 3,415
7
votes
Why moon sighted only in Kerala but not in rest of India?
The visibility of a narrow crescent moon depends not only on the width of the crescent, but also on the difference between the moon's and sun's altitudes relative to the local horizon.
The greater ...
- 18.2k
7
votes
Accepted
Did NASA/JPL get "waning" and "waxing" backwards in this video?
I think they did make a mistake. The moon waxes after a new moon and wanes before a new moon. To wax means to grow bigger or stronger (and is cognate with "waist"!). To wane means to become smaller ...
- 115k
7
votes
Accepted
How often does a full moon happen on the weekend?
Some further info on the actual distribution pattern.
The synodic month and the week length is descended by 1.53059 days, so one month later the full moon comes 1.53059 days later in the week.
That ...
7
votes
Accepted
Calculate moon illumination given moon age
The algorithm below produces the fraction of moon illumination based on the Julian Day (rather than the moon age). It is from Meeus' Astronomical Algorithms Second Edition chapter 48.
$$
\begin{align}...
- 5,542
6
votes
Is the full moon full everywhere on the planet?
As others have said, the full moon (date and time) is set by when the center of the Moon passes over or under a line that passes from the center of the sun through the center of the Earth. That ...
- 23.9k
6
votes
How to map moon phase number + "lunation" to moon phase name?
The WeatherBit 16-day forecast documentation gives these output field descriptions:
moon_phase: Moon phase illumination fraction (0-1)
...
- 18.2k
5
votes
Accepted
How Soon Could a Waxing Crescent Moon Be Seen?
From the US Naval Observatory: Crescent Moon Visibility:
Naked-eye sightings as early as 15.5 hours after New Moon have been reliably reported while observers with telescopes have made reliable ...
- 4,470
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