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 ...
hobbs's user avatar
  • 758
25 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. ...
uhoh's user avatar
  • 31.1k
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 ...
James K's user avatar
  • 121k
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 “...
Pierre Paquette's user avatar
19 votes

Why can I see sometimes a horizontal half moon instead of a vertical one?

It occurs because the lit half of the moon points towards the sun, along a great circle in the sky. The half-moon will be 90 degrees away from the sun in the sky. So by (say) 8pm (at equinox for ...
James K's user avatar
  • 121k
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 ...
James K's user avatar
  • 121k
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 ...
James K's user avatar
  • 121k
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 ...
JohnHoltz's user avatar
  • 7,982
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 ...
uhoh's user avatar
  • 31.1k
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 ...
User24373's user avatar
  • 903
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 ...
stretch's user avatar
  • 1,766
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 ...
planetmaker's user avatar
  • 19.4k
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 ...
JohnHoltz's user avatar
  • 7,982
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, ...
Greg Miller's user avatar
  • 5,867
10 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 ...
Stuart Robbins's user avatar
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 ...
notovny's user avatar
  • 4,788
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 ...
BillDOe's user avatar
  • 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-...
Mike G's user avatar
  • 18.7k
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 ...
JohnHoltz's user avatar
  • 7,982
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 ...
Thomas's user avatar
  • 3,416
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": $$...
Albert's user avatar
  • 1,962
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 ...
PM 2Ring's user avatar
  • 14.4k
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 ...
antlersoft's user avatar
  • 3,455
8 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}...
Greg Miller's user avatar
  • 5,867
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 ...
Mike G's user avatar
  • 18.7k
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 ...
James K's user avatar
  • 121k
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 ...
SE - stop firing the good guys's user avatar
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 ...
userLTK's user avatar
  • 24k
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) ...
Mike G's user avatar
  • 18.7k

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