APOD: The Dance of Venus and Earth (2020 Jun 03)

[APOD Robot]

The Dance of Venus and Earth

Explanation: Every time Venus passes the Earth, it shows the same face. This remarkable fact has been known for only about 50 years, ever since radio telescopes have been able to peer beneath Venus' thick clouds and track its slowly rotating surface. This inferior conjunction -- when Venus and Earth are the closest -- occurs today. The featured animation shows the positions of the Sun, Venus and Earth between 2010-2023 based on NASA-downloaded data, while a mock yellow 'arm' has been fixed to the ground on Venus to indicate rotation. The reason for this unusual 1.6-year resonance is the gravitational influence that Earth has on Venus, which surprisingly dominates the Sun's tidal effect. If Venus could be seen through the Sun's glare today, it would show just a very slight sliver of a crescent. Although previously visible in the evening sky, starting tomorrow, Venus will appear in the morning sky -- on the other side of the Sun as viewed from Earth.

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[Ann]

Speaking of Venus, there is a lovely portrait of Venus by Thierry Legault in the Recent Submissions thread.

Ann

[Jerome68]

I did not know about that fact, super interesting. Sad is not possible to appreciate it because of the clouds in Venus.

By the way, do you know any source where I can read, why Venus rotation is so slow compared with the rest of the planets? That is another fact that has always got my attention about this planet

[GeoXXXXX]

I did not know about that fact, super interesting. Sad is not possible to appreciate it because of the clouds in Venus.

By the way, do you know any source where I can read, why Venus rotation is so slow compared with the rest of the planets? That is another fact that has always got my attention about this planet

Try...
https://space.stackexchange.com/questio ... -slow#2290
Stackexchange is a favorite of mine.

Eric

[JohnD]

The APOD blurb, that says that the Earth "surprisingly dominates the Sun's tidal effect" leads to a 1969 article, whose abstract starts, "The observed spin-orbit resonance of Venus, whereby the same side of Venus faces the Earth at each inferior conjunction, cannot be explained adequately by gravitational interaction with the Earth alone." It then goes on to speculate (I think, I cannot read the entire article, thanks to Elsevier's ridiculous publishing policy. I mean, charging you to read a fifty year old paper!) that a "solar atmospheric tide" may contribute to the orbital locking.

I don't understand that - solar? In the Sun's atmosphere? Or in that of Venus, caused by the Sun? And after fifty years, do we have no more certain an explanation?

[orin stepanek]

Liked the video animation!

[Iksarfighter]

Ce verrouillage gravitationnel est fabuleux, merci de nous l'avoir fait découvrir !

[Jerome68]

wow.... thanks a lot GeoXXXXX!!, actually my mind just blow up at this moment by reading another thing I had no idea.... the rotation seems going slower over the time :O... love the link!

By the way, I always thought that is the reason for Venus does not have a strong magnetic field, and in consequence, no water and no live... you think that makes sense?

[Chris Peterson]

The APOD blurb, that says that the Earth "surprisingly dominates the Sun's tidal effect" leads to a 1969 article, whose abstract starts, "The observed spin-orbit resonance of Venus, whereby the same side of Venus faces the Earth at each inferior conjunction, cannot be explained adequately by gravitational interaction with the Earth alone." It then goes on to speculate (I think, I cannot read the entire article, thanks to Elsevier's ridiculous publishing policy. I mean, charging you to read a fifty year old paper!) that a "solar atmospheric tide" may contribute to the orbital locking.

I don't understand that - solar? In the Sun's atmosphere? Or in that of Venus, caused by the Sun? And after fifty years, do we have no more certain an explanation?

https://en.wikipedia.org/wiki/Atmospheric_tide

[GeoXXXXX]

Your very welcome Jerome!
I just love that site, it’s like a more disciplined, smarter Quora (which I love too but it has gone downhill a bit, too many stupid questions).
As fot the Venusian magnetic field or lack thereof (although it does have a very weak one from it’s atmosphere’s interaction with the solar wind) is a question that has not been settled. One theory has that a large impact that created the moon also stopped the Earth’s interior from stratifying and stopping the interior rotation that keeps the field charged...
https://phys.org/news/2017-12-doesnt-ve ... phere.html

Chris can give a better answer than mine I am sure...

Eric

[Flyboy1331]

The same face of Venus also faces the Earth during opposition? That what the animation appears to depict.

[Iksarfighter]

Finally Venus is like a moon of the Earth.

As tidal forces varie like 1/D^3 maybe when Earth is very close it is stronger than the Sun.

[JohnD]

The APOD blurb, that says that the Earth "surprisingly dominates the Sun's tidal effect" leads to a 1969 article, whose abstract starts, "The observed spin-orbit resonance of Venus, whereby the same side of Venus faces the Earth at each inferior conjunction, cannot be explained adequately by gravitational interaction with the Earth alone." It then goes on to speculate (I think, I cannot read the entire article, thanks to Elsevier's ridiculous publishing policy. I mean, charging you to read a fifty year old paper!) that a "solar atmospheric tide" may contribute to the orbital locking.

I don't understand that - solar? In the Sun's atmosphere? Or in that of Venus, caused by the Sun? And after fifty years, do we have no more certain an explanation?

https://en.wikipedia.org/wiki/Atmospheric_tide

Thank you, Chris! So, caused by Venus' atmosphere. I'm amazed, dumbfounded. The Moon has no atmosphere but has been locked into synchronous rotation with the Earth, and I understand that the Earth's rotation is slowing down so that one day it will be locked to the Moon. But not for millions of years (?). So intuitively it's unimaginable that Venus, 25 million miles away its closest, should be influenced as much as the Moon, which is a hundred times nearer all the time! And so affected by Earth's gravity 10,000 times less! Quantum chromodynamics? Trivial compared to orbital mechanics!

[Chris Peterson]

The APOD blurb, that says that the Earth "surprisingly dominates the Sun's tidal effect" leads to a 1969 article, whose abstract starts, "The observed spin-orbit resonance of Venus, whereby the same side of Venus faces the Earth at each inferior conjunction, cannot be explained adequately by gravitational interaction with the Earth alone." It then goes on to speculate (I think, I cannot read the entire article, thanks to Elsevier's ridiculous publishing policy. I mean, charging you to read a fifty year old paper!) that a "solar atmospheric tide" may contribute to the orbital locking.

I don't understand that - solar? In the Sun's atmosphere? Or in that of Venus, caused by the Sun? And after fifty years, do we have no more certain an explanation?

https://en.wikipedia.org/wiki/Atmospheric_tide

Thank you, Chris! So, caused by Venus' atmosphere. I'm amazed, dumbfounded. The Moon has no atmosphere but has been locked into synchronous rotation with the Earth, and I understand that the Earth's rotation is slowing down so that one day it will be locked to the Moon. But not for millions of years (?). So intuitively it's unimaginable that Venus, 25 million miles away its closest, should be influenced as much as the Moon, which is a hundred times nearer all the time! And so affected by Earth's gravity 10,000 times less! Quantum chromodynamics? Trivial compared to orbital mechanics!

The Earth will never be tidally locked to the Moon. The Sun will have melted both long before that can happen.

Presumably the atmospheric tides have significant impact on Venus because its atmosphere is so dense.

[Ann]

I didn't pay all that much attention to this APOD. So, is it correct that the ultra-slow rotation of Venus is actually caused by the fact that our sister planet is tidally locked to the Earth?

Or have I misunderstood everything?

Ann

[edgardine]

Ann, it is not yet understood.

When I was interested in the lower conjunction, I searched which region of Venus faces us, and the answer lies in the toponymy: Alpha Regio and Beta Regio (first regions perceived by radar images from Earth). There is however a difference of 4 hours and 5 minutes between each synodic revolution. For Earth, a difference of 4 hours every 583 days would quickly lead to a complete rotation (23h 56mn), but Venus performs hers in 243 (terrestrial) days. Over time, the face it presents to us in the lower conjunction slightly changes.

[Jerome68]

I read here and there that one of the potential reasons for that, is some big impact to Venus back in the days, which slow day severely the rotation speed. Something like happened to the Earth which created the Moon, but to Venus with worse results, this would make sense right?

[Chris Peterson]

I didn't pay all that much attention to this APOD. So, is it correct that the ultra-slow rotation of Venus is actually caused by the fact that our sister planet is tidally locked to the Earth?

Or have I misunderstood everything?

Ann

The slow rotation of Venus is probably unrelated to the Earth. Something happened early in the formation of the Solar System that flipped its rotation axis and resulted in a slow retrograde rotation. Nor is Venus in a 1.6 year orbital resonance with Earth, as the caption suggests. Venus is close to that, with a near 13:8 ratio, but it isn't a true resonance, just a coincidental match. It is, however, close enough to resonant that a gravitational interaction between Earth and Venus could have created an orbit-rotation resonance that results in Venus presenting the same fact towards us at inferior conjunction. That isn't exactly tidal locking (because Venus does not present the same face towards us all the time), although it's a related phenomenon. That said, I don't think it is conclusively established that this relationship is locked. It's possible that it's just a coincidence of rotational and orbital times (like the near orbital resonance) and the face we see will drift over time, if observed for centuries or millennia.

[neufer]

The slow rotation of Venus is probably unrelated to the Earth. Something happened early in the formation of the Solar System that flipped its rotation axis and resulted in a slow retrograde rotation.

<<Alex Alemi's and David Stevenson's 2006 study of models of the early Solar System at the California Institute of Technology shows Venus likely had at least one moon created by a huge impact event billions of years ago. About 10 million years later, according to the study, another impact reversed the planet's spin direction and caused the Venusian moon gradually to spiral inward until it collided with Venus.>>

[JohnD]

Oh! So it's coincidence, nothing to do with tidal locking, atmospheric tides or the Great Venusian Panjandrum?
John

[Chris Peterson]

Oh! So it's coincidence, nothing to do with tidal locking, atmospheric tides or the Great Venusian Panjandrum?
John

The near orbital resonance is just a coincidence. But that same near resonance might play a role in the orbital-rotational relationship that seems to keep the same place on the surface rotating into the same position at each inferior conjunction... meaning the latter might not be a coincidence at all, but something that follows physically as a consequence of a coincidence.

[Jerome68]

The near orbital resonance is just a coincidence. But that same near resonance might play a role in the orbital-rotational relationship that seems to keep the same place on the surface rotating into the same position at each inferior conjunction... meaning the latter might not be a coincidence at all, but something that follows physically as a consequence of a coincidence.

I normally will agree with you, but it also shows the history of science that a coincidence is hiding some big discover, wouldn't be this the case? (I don't pretend to say I know it, but, why not? also, the moon is showing us the same face, why not a way to interact we don't know would explain it?). Coincidences in science are very rarely (in my opinion)

[Chris Peterson]

The near orbital resonance is just a coincidence. But that same near resonance might play a role in the orbital-rotational relationship that seems to keep the same place on the surface rotating into the same position at each inferior conjunction... meaning the latter might not be a coincidence at all, but something that follows physically as a consequence of a coincidence.

I normally will agree with you, but it also shows the history of science that a coincidence is hiding some big discover, wouldn't be this the case? (I don't pretend to say I know it, but, why not? also, the moon is showing us the same face, why not a way to interact we don't know would explain it?). Coincidences in science are very rarely (in my opinion) :)

Because we have a very, very good understanding of the underlying science involved in orbital dynamics. There are many examples of coincidental near orbital resonances throughout the Solar System. They are expected to occur based on simple statistics, and the observed probabilities are consistent with that. There is no reason at all to believe that some unknown factor is at play.

The uncertainty with respect to whether there is a physical mechanism creating the apparent orbital-rotational synchronization between Earth and Venus exists because of a lack of complete information (this involves complex elements such as Venusian atmospheric dynamics). That lack of information doesn't exist when it comes to the actual orbits of the two planets.

[Jerome68]

That is right, the small details right? sorry for my comment I think it doesn't make any sense, I feel I shouldn't put it hehe. But thanks a lot for the nice explanation

[rwe]

The myth that Venus is somehow locked to Earth was started in the 1960s by Radar Astronomers when they found Venus showed the same face to Earth at inferior conjunction. This was debunked by I. I. Shapiro, et al. in a paper "Nonresonance Rotation of Venus" ApJ letters, Jun 1, 1971. The side of Venus that faces the Earth at inferior conjunction actually changes, in a single Venus pentagram (8 years, 5 inferior conjunctions) where Earth sub-West Longitude varies by 10 degrees within the petals and the pentagram rotates about 30 degrees per 100 years. In the year 2450, it will have rotated 180 degrees so if a Radar Astronomer the would "see" the other side from the 1960s side facing earth. Sharpiro concluded that it is an interesting coincidence similar many others in the solar system such as Mars having a near 24 hour day.