Is Triton ever hidden behind Neptune?

[Albertgauss]

TL;DR Summary

Triton ever hidden behind Neptune?

As you know, the Neptune moon of Triton has an inclination of

129.812° (to the ecliptic)
156.885° (to Neptune's equator)
129.608° (to Neptune's orbit)

according to Wikipedia. The rotation axis of Neptune itself 28.3 degrees with respect to the solar system ecliptic.

If I were to look at Neptune from Earth, would I ever see Triton go behind Neptune? It is hard to tell from animations and pictures on the web if this happens or not.

If Triton does spend time behind Neptune, occulted as seen from Earth, is it possible to know what percentage of its orbit it is hidden behind Neptune?

 

[jedishrfu]

While not answering your question, I did find this recent event where Triton occulted a star and astronomers used it to study Tritons atmosphere indirectly.

https://www.space.com/39895-neptune-moon-triton-flashes-gaia- spacecraft .html

With respect to your questions, I'm sure they know Triton's orbital position in great detail as evidenced by the star occultation.

As to whether it orbits behind Neptune, I couldn't find any specific.

 

[Drakkith]

Summary:: Triton ever hidden behind Neptune?

If I were to look at Neptune from Earth, would I ever see Triton go behind Neptune? It is hard to tell from animations and pictures on the web if this happens or not.

Yes. It must behind the planet at some point from our point of view. As Neptune moves in its orbit, Triton's orbit appears to rotate. Eventually the orbit will be such that it will have 'rotated' to put the moon behind Neptune. Note that it's not really rotating, it's just a result of the changing perspective. This may not occur for decades, but it will happen eventually.

 

[Albertgauss]

I see, so its a pretty rare event then.

For example, the moons of Uranus below will never be behind the planet, unless, like you say, every so many decades when the planet's orbit is just right in some extraordinary way.

The other extreme, is that of Jupiter's moons, which, because Jupiter rotates so coplanar to the elliptic, its moons go behind Jupiter half the time (very generally).

My guess is that the consensus is that

Triton orbits Neptune more similarly to how the moons of Uranus orbit Uranus than how the moons of Jupiter orbit Jupiter.

Is this correct?

 

[snorkack]

Yes, and it is fairly easy to know when and how much it happens. No, it is not "rare", even though it does not happen for 76 years. Nor is it "rare" for Uranus´ satellites to be behind Uranus.
My rough logic of computation. Note that this is a rule of thumb, I may omit minor corrections.
Triton goes about 355 000 km from Neptune (omitting to account for eccentricity of Triton)
Triton has about 40 degree orbital inclination (direction does not matter)
Sin 40 is about 0,64 (passes check - sin 30 is 0,5 and sin 45, 0,707).
Therefore Triton gets about 227 000 km from Neptune´ s orbit.
Neptune is about 49 500 km diametre (I am omitting to account for flattening). Triton about 2700. What do you count as "Triton behind Neptune"? Whole Triton behind, or also a part of Triton? If you mean whole Triton, it leaves about 46 800 km space behind Neptune. So Triton must be at most 23 400 km from Neptune at conjunction.
I should think that Neptune must be at most arcsin(23 400/227 000) from Titan´ s node. Which I get to be about 5,9 degrees.
So rounding off one more number, Triton goes behind Neptune for 6 degrees each side of each node. Making total 24 degrees of 360 degree orbit - 1/15 of the 164 year orbit of Neptune. Meaning 11 years in 2 5 and a half year periods, separated by 76 years of no occultations. But in each of the 5 and a half year periods, since Triton has orbital period of under 6 days, there are over 300 occultations in a row. Something that happens over 300 times in 82 years is not really rare even if never happens for 76 years.

 

[Albertgauss]

I attached a picture of what I was thinking. I drew it out, crudely and it seems like Triton would never be blocked by Neptune as seen from Earth. I followed the post up until

"I should think that Neptune must be at most arcsin(23 400/227 000) from Titan´ s node. Which I get to be about 5,9 degrees."

I do see that the sine of 23 400/227 000 is 6 degrees, but I can't visualize what triangle this in this context. I may not have the nodes in the attached picture in the correct place from the nodes you mentioned in the reply.

Yes, you are correct about ignoring flattening, etc.

"whole triton or a part of triton goes behind Neptune" --> whichever is easiest is fine.

I didn't understand everything after: "So rounding off one more number..." This must be due to some precession I did not consider.

Does the picture I have correspond to the "76 years of no occultations"? That seems to make the most sense to me.

"Meaning 11 years in 2 5 and a half year periods", my guess is that I would have to redraw my picture so that Triton's orbit has a lower orbital inclination during these times where it does cross into the shadow on the backside of Neptune and can't be seen from Earth.

 

[Ibix]

The point is that the plane of Triton's orbit is fixed with respect to the fixed stars. At some point during Neptune's orbit that plane is edge-on to us, and it is possible for Triton then to be behind Neptune.

 

[DaveC426913]

This about right? Did I miss anything?

Boy it's really hard to represent multiple overlapping shapes in 3D...

 

[Albertgauss]

Oh, I see. I'm glad I posted a picture. The way you have it, DaveC426913, Triton certainly can pass behind Neptune. I didn't even think Triton could orbit that way, as evidenced by my drawing that was wrong. I did try to make Neptune and Triton orbit to scale.

The only adjustment I would make to DaveC426913 picture is that orbit radius of Triton around Neptune is 6 times the diameter of Neptune. In the pic above, the orbit should be much bigger and Neptune much smaller. I don't know if that would help to visualize or not, as the math would be the same.

Yes, I agree, the 3D pictures are hard.

I think for me, I got the information I wanted. To actually see the numbers of smorkak would certainly need a picture. I'm good for here. If anyone else has comments, that's fine, but I feel my question was answered and I appreciate everyone's help.

 

[DaveC426913]

I did try to make Neptune and Triton orbit to scale.

The only adjustment I would make to DaveC426913 picture is that orbit radius of Triton around Neptune is 6 times the diameter of Neptune. In the pic above, the orbit should be much bigger and Neptune much smaller. I don't know if that would help to visualize or not, as the math would be the same.

Trying to make anything to scale - whether planets or orbits - will only serve to obfuscate the relationships.
In fact, even my first pic is trying to take in too much. Here is a zoomie of the right side of the first pic.

The key to the whole thing is to keep in mind that nether Neptune's rotational axis nor Triton's plane of orbit change as they revolve about the Sun. Just like Earth, Neptune has its own Pole Star (theoretically) in the fixed background of stars, as does Triton's orbit.

At two points in Neptune's orbit about the sun, Triton's orbit about Neptune is seen edge-on. That's when Triton passes behind Neptune from Earth's PoV.

*nb" I have used the word eclipse sloppily. Should really be occultation.

 

[Albertgauss]

Sounds good. I understand. Both pictures actually work well.

 

[Vanadium 50]

Just like Earth, Neptune has its own Pole Star (theoretically) in the fixed background of stars

Delta Cygni. A great bar quiz question.

 

[snorkack]

I attached a picture of what I was thinking. I drew it out, crudely and it seems like Triton would never be blocked by Neptune as seen from Earth.

Thanks! It is useful to explain my points, though I do not want to alter your drawing myself.
The point is that the direction Neptune-Earth relative to the orbit of Triton changes with orbit of Neptune (and of Earth).

I followed the post up until
"I should think that Neptune must be at most arcsin(23 400/227 000) from Titan´ s node. Which I get to be about 5,9 degrees."

I do see that the sine of 23 400/227 000 is 6 degrees, but I can't visualize what triangle this in this context. I may not have the nodes in the attached picture in the correct place from the nodes you mentioned in the reply.

Sometimes they are, sometimes they are not.
Or rather, Earth is sometimes at one place relative to nodes, sometimes at another.
You depict the situation at antinode, where Triton passes at the maximum distance (227 000 km) from Neptune.
Now imagine situation one quarter of orbit - 41 years - later, where the direction to one node is precisely the direction to Earth.
Then we see Triton´ s orbit as a circle exactly edge on - a straight line which is tilted 40 degrees to Neptune´ s orbit but goes precisely behind the dead centre of Neptune. Where Triton is duly hid behind Neptune. Note that the ends of the straight line are still 227 000 km from Neptune´ s orbit, but the centre of the line goes through centre of Neptune.
And now imagine the situation where the direction to Earth is near but not precisely the direction to node.
A circle seen nearly but not exactly edge on is a narrow ellipse. Narrow enough that the narrow middle of the ellipse fits behind Neptune´ s disc.
Consider the limit - where Triton grazes just under the edge of Neptune´ s disc.
5,9 degrees is the angle between the direction Neptune-node (where Triton passes behind the dead centre of Neptune), along the plane of Neptune´ s orbit, to the direction where Triton is seen in grazing occultation.

Yes, you are correct about ignoring flattening, etc.

"whole triton or a part of triton goes behind Neptune" --> whichever is easiest is fine.

I didn't understand everything after: "So rounding off one more number..." This must be due to some precession I did not consider.

Does the picture I have correspond to the "76 years of no occultations"? That seems to make the most sense to me.

Yes. The whole orbit is 164 years. So the half orbit from one node to another is 82 years (omitting complications like eccentricity of Neptune orbit). Of this, 6 years is the time when Earth is so close to either node that eclipses happen, and the remaining 76 years have no occultations.

"Meaning 11 years in 2 5 and a half year periods", my guess is that I would have to redraw my picture so that Triton's orbit has a lower orbital inclination during these times where it does cross into the shadow on the backside of Neptune and can't be seen from Earth. View attachment 266172

Inclination is the same. If you put Earth on the direction to node, you can still watch the direction to ecliptic and Neptune´ s orbit, and see that the straight line along which Triton orbits has a certain inclination to Neptune´ s orbit.

 

[Albertgauss]

Hi all,

Thanks! It is useful to explain my points, though I do not want to alter your drawing

It is perfectly fine if you want to alter the drawings. I made it with a PowerPoint and I have a copy are ready on my hard drive. It takes a few minutes, but all that one has to do is download the JPEG, insert it into PowerPoint, insert the text and drawing objects as needed, save the PowerPoint as a JPEG, then read upload it to the website. It’s no big deal but it really helps a lot. For instance, if I didn’t make this picture then I don’t think people would have known where I was confused. The sloppy orbit you see for Triton was drawn with the powerpoint, insert shape, curly string tool.

I made two more pictures, and I think I am finally seeing better what Dave’s pictures represent. Slide 2 shows the position I think that Snorkack is getting out where occultation is possible; Slide 3 is my own attempt to make a drawing that coincides with Dave’s pictures.

Slide 2 below

Slide 3 below

I realize that one of the mistakes that I made was to assume that the anti-node orbit where occultation is not possible was always facing Earth. I see what Dave’s pictures mean now – trying to draw it out on my own – that Triton’s orbit is fixed with respect to Neptune. As Neptune goes around the center of the solar system, I see that how Earth views Tritons orbit is what changes and provides Trition’s occasional occultation from earth.

Slide two also shows my attempt to draw out the triangle that gives the 5.9°. Still does leave one question, though: if the radius of Triton’s orbit around Neptune is 335,000 km, should not that distance be used instead of the 227,000 km to make the triangle that gives the angle?

 

[DaveC426913]

Slide two also shows my attempt to draw out the triangle that gives the 5.9°. Still does leave one question, though: if the radius of Triton’s orbit around Neptune is 335,000 km, should not that distance be used instead of the 227,000 km to make the triangle that gives the angle?

When Triton's orbital plane is edge-on to our line of sight, the tilt of its plane is irrelevant - so: yes, use the 335,000km distance.

 

[snorkack]

When Triton's orbital plane is edge-on to our line of sight, the tilt of its plane is irrelevant - so: yes, use the 335,000km distance.

But the question was about making the triangle.
355 000 km would be the appropriate number if sine of the tilt were 1, so tilt is 90 degrees. Like Uranus´ satellites are close.
Compare satellites of Jupiter. When are they not occulted by Jupiter?
Callisto orbits at distance 1 883 000 km from Jupiter.
But the inclination of Callisto´ s orbit is just 2 degrees to ecliptic.
Meaning Callisto only ever gets something like 66 000 km from Jupiter´ s orbit. Even at antinode, Callisto is not far enough to pass around rather than behind Jupiter.

We have node (where the satellite´ s orbit is edge on and the satellite always passes behind planet), and antinode (where satellite passes at maximum distance, which can still be quite close). So what we are looking for is the point where the satellite just grazes the planet,

 

[DaveC426913]

But the question was about making the triangle.

Eh, OK. I guess I didn't follow what the OP was doing.

 

[stefan r]

Rings of Saturn might be easier to visualize. Sometimes the rings are big fat and glaringly obvious in a telescope (Galileo said Saturn "had ears"). Sometimes you look edge on and they rings effectively disappear in a normal backyard telescope. Triton is a planet instead of a disc but it is orbiting in a plane. We should see the orbital plane edge on twice per Neptune's orbit around the Sun.