APOD: Young Moon Meets Evening Star (2011 Dec 01)

[APOD Robot]

Young Moon Meets Evening Star

Explanation: Now appearing as planet Earth's evening star, brilliant Venus shines in western skies at twilight. Standing above a rugged horizon and warm sunset colors, the twilight's celestial beacon was joined last Saturday by a Moon 35 hours young in this gorgeous skyscape. The close pairing of Venus and Moon is known as a conjunction. Not visible in the frame, fleeting planet Mercury has fallen from evening skies, sinking deeper into the sunset glow below the young crescent Moon. The scene was captured while trekking in northern Portugal's Peneda-Geres National Park.

<< Previous APOD

Discuss Any APOD

Next APOD >>

[/b]

[owlice]

Oh, I'm glad to see this! When I first saw the submitted image, I thought it lovely. Still do!

[starstruck]

Gorgeous!

With just a hint of Earthshine too . . . . what a beautiful view!

[Indigo_Sunrise]

Images like this one are some of my favorites! I love the colors of the sky in twilight or early dawn, especially when planets and/or the moon are featured.

Absolutely fabulous image!

[neufer]

[orin stepanek]

the twilight's celestial beacon was joined last Saturday by a Moon 35 hours young

I liked the photo; and I saved it for my background collection; but a moon 35 hours young is a phrase that I don't quite get! So pardon my ignorance and somebody might be kind enough to explain it?

[Chris Peterson]

I liked the photo; and I saved it for my background collection; but a moon 35 hours young is a phrase that I don't quite get! So pardon my ignorance and somebody might be kind enough to explain it? :?

It means that the image was made when the Moon was just 35 hours past "new", which was the point in the lunar cycle where the Moon was between the Earth and Sun (in conjunction with the Sun). It is the point where you can have a total solar eclipse, in the fairly rare case where the three bodies are on the same plane. It is also the point where you have the minimum amount of Moon illuminated as seen from Earth.

The Moon usually needs to be at least a few hours past new in order for any crescent to be detectable (usually with some sort of optical aid). It is possible, however, to image the Moon at the instant it is new, depending on its distance from the Earth-Sun plane. 35 hours past new is a very small crescent, but one that is easily seen.

[neufer]

I liked the photo; and I saved it for my background collection; but a moon 35 hours young is a phrase that I don't quite get! So pardon my ignorance and somebody might be kind enough to explain it?

It means that the image was made when the Moon was just 35 hours past "new", which was the point in the lunar cycle where the Moon was between the Earth and Sun (in conjunction with the Sun). It is the point where you can have a total solar eclipse, in the fairly rare case where the three bodies are on the same plane. It is also the point where you have the minimum amount of Moon illuminated as seen from Earth. The Moon usually needs to be at least a few hours past new in order for any crescent to be detectable (usually with some sort of optical aid). It is possible, however, to image the Moon at the instant it is new, depending on its distance from the Earth-Sun plane. 35 hours past new is a very small crescent, but one that is easily seen.

[color=#0000FF]A crescent moon can be seen over palm trees at sunset in Manama, Bahrain, marking the beginning of the Muslim month of Ramadan[/color]

---

<<Ramadan (Arabic: رمضان) is the ninth month of the Islamic calendar, which lasts 29 or 30 days. It is the Islamic month of fasting, in which participating Muslims refrain from eating, drinking, smoking and sex during daylight hours and is intended to teach Muslims about patience, spirituality, humility and submissiveness to God. Muslims fast for the sake of God and to offer more prayer than usual. Compared to the solar calendar, the dates of Ramadan vary, moving backwards by about eleven days each year depending on the moon. For the year of 1432 Hijri, the first day of Ramadan was determined to be August 1, 2011. Muslims believe Ramadan to be an auspicious month for the revelations of God to humankind, being the month in which the first verses of the Qur'an were revealed to the Islamic prophet, Muhammad. The word Ramadan is derived from an Arabic root R-M-Ḍ, as in words like "ramiḍa" or "ar-ramaḍ" denoting intense heat, scorched ground and shortness of rations.

Hilāl (the crescent) is typically a day (or more) after the astronomical new moon. Since the new moon indicates the beginning of the new month, Muslims can usually safely estimate the beginning of Ramadan. There are many disagreements each year, however, on when Ramadan starts. This stems from the tradition of sighting the crescent moon with the naked eye; as such, there are differences for countries in different parts of the globe. More recently, however, some Muslims are leaning towards using astronomical calculations to avoid this confusion.>>

[APODFORIST]

With just a hint of Earthshine too . . . . what a beautiful view!

I just thought about that:

Is on new moon an Earthshine possible too? If so I guess it would be hard to detect ...

[Chris Peterson]

Is on new moon an Earthshine possible too? If so I guess it would be hard to detect ...

Earthshine is greatest on a new Moon. Of course, if the Moon is very close to being astronomically new, it is so close to the Sun that the sky around it is bright, and the resulting poor contrast can make the earthshine hard to see. So visually, we typically see the brightest earthshine when the Moon is a couple of days on either side of new- still near conjunction, but far enough that the sky around it is pretty dark.

[Ann]

Today's APOD is very beautiful. The skyscape is lovely, and the lake looks mysterious and almost otherworldly, as if it had been, perhaps, a methane lake on Titan. Except there would have been nothing to see in the skies of Titan, except orange rain.

Ann

[APODFORIST]

@Chris Peterson:
Sounds plausible

[APOD Grok]

How lovely! Bravo Benjamim Ribeiro! The sky over my house looked very much like this last evening. So striking! Too bad the view here doesn't include as beautiful a background as Portugal's Peneda-Geres National Park.

[Sam]

From the first link, "Venus shines," I learned that the dichotomy of Venus (disc is exactly 50% illuminated) always occurs 4-6 days before or after greatest elongation.

How is this so? I thought that at greatest elongation the Sun-Venus-Earth angle is 90°. If this is true, shouldn't dichotomy and greatest elongation be simultaneous?

Sam

[saturn2]

Venus and Moon, a beautiful image.

[TNT]

What a beautiful landscape! I really like how Venus and the Moon are together. The sunset adds to it as well.

[alter-ego]

From the first link, "Venus shines," I learned that the dichotomy of Venus (disc is exactly 50% illuminated) always occurs 4-6 days before or after greatest elongation.

How is this so? I thought that at greatest elongation the Sun-Venus-Earth angle is 90°. If this is true, shouldn't dichotomy and greatest elongation be simultaneous?

Sam

That is only true if the orbits are perfect circles. The picture shows exaggerated elliptical orbits to more clearly answer your question.

Position 1 = 50% Illumination
Position 2 = Greatest Elongation

[Sam]

The picture shows exaggerated elliptical orbits to more clearly answer your question.

Wow, that picture really helps, thanks!
However, I also found this page:

The general geometry of how the dichotomy occurs is easy to understand. It is where the inferior planet’s position is at right angles to the Earth and Sun near the time of maximum elongation, and somewhere between 45° and 47°. One common misconception is that dichotomy exactly corresponds with the greatest elongation East (or West) of the Sun. This is not true, and the reasons for this disparity are the different eccentricities of the two independent orbits of Venus and Earth. When Venus has its greatest elongation east or west of the Sun, the predicted time of dichotomy can be either slightly early or later. Often these differences never exceed more than one day, but this is independent of the four-day difference between the observed and predicted dichotomies.

For reasons, which are still uncertain, the predicted time of the Dichotomy of Venus or Schröter Effect is never the same as the observed event. This difference averages about four to six days earlier or later than expected, depending on the Sun’s side that Venus (or Mercury) is placed. For example, the eastern elongation was predicted for the afternoon of 10th June 1999, this suggests the observed 50% phase will be more like the 4th or 6th June.

Where are those extra 2-5 days coming from?

Sam

[alter-ego]

The picture shows exaggerated elliptical orbits to more clearly answer your question.

Wow, that picture really helps, thanks!
However, I also found this page:

The general geometry of how the dichotomy occurs is easy to understand. It is where the inferior planet’s position is at right angles to the Earth and Sun near the time of maximum elongation, and somewhere between 45° and 47°. One common misconception is that dichotomy exactly corresponds with the greatest elongation East (or West) of the Sun. This is not true, and the reasons for this disparity are the different eccentricities of the two independent orbits of Venus and Earth. When Venus has its greatest elongation east or west of the Sun, the predicted time of dichotomy can be either slightly early or later. Often these differences never exceed more than one day, but this is independent of the four-day difference between the observed and predicted dichotomies.

For reasons, which are still uncertain, the predicted time of the Dichotomy of Venus or Schröter Effect is never the same as the observed event. This difference averages about four to six days earlier or later than expected, depending on the Sun’s side that Venus (or Mercury) is placed. For example, the eastern elongation was predicted for the afternoon of 10th June 1999, this suggests the observed 50% phase will be more like the 4th or 6th June.

Where are those extra 2-5 days coming from?

Thanks for the follow-up. I must admit I was not aware of this, but I'm certain there is no magic in the explanation, it should come down to understanding the details more completely, and there may not be only one answer. I'll share my thoughts:
- Observational clarity of the dichotomy (straightness of the terminator) has some uncertainty, and Venus' atmosphere is not a hard-surfaced, perfectly spherical billiard ball. Therefore there may be some bias due to the atmosphere and how it attenuates sunlight, which brings me to my next thought,
- The simple geometry model assumes a spherical planet (I believe). We do know one interesting behavior; there are almost exactly 5 (5.0001) Venusian solar days between each closest approach to Earth (i.e. Earth sees the same side of Venus at each closest approach). In my mind, this apparent "tidal locking" with Earth raises a big question wrt the dichotomy: Is there enough of a transient (but resonant) gravitational effect that can change the shape (surface and or atmosphere) enough to cause an observable curved terminator near greatest elongation? I do know that Mariner V revealed a small apparent oblateness which counters this suggestion, but I mention it anyway because, like the dichotomy problem, "tidal locking" is not well understood either. I am open for a connection between the two phenomena.

Those are my best on-the-fly speculations.