I How does the perihelion passage affect the orbital period of comet Neowise?

[Buzz Bloom]

TL;DR Summary

The diagram of Neowise's orbit in the cited Wikipedia article seems very odd.

The orbit diagram

https://en.wikipedia.org/wiki/C/2020_F3_(NEOWISE)#/media/File:Comet_2020_F3-skyview.png'​

in the Wikipedia article

https://en.wikipedia.org/wiki/C/2020_F3_(NEOWISE)​

shows some very odd spirals.

My first guess is the picture shows what Neowise does in one Earth year (mostly 2020) as the Earth completes it's orbit. The caption says, "7 day motion", apparently referring to the separation between orbital points displayed. Another possible interpretation is that the diagram represents one complete Neowise orbit, and each loop of the sprial represents what happens to Neowise in different Earth years from Earth's perspective. That is, the loop in the sky gets smaller as Neowise moves further away, and vice versa. The point where the two spiral cones touch corresponds to the aphelion.

If my guess is wrong, can someone please explain what is happening regarding these spirals in Neowise's orbit?.

The following is a quote from the article.

This perihelion passage will increase the comet's orbital period from about 4500 years to about 6800 years.​

Can someone explain how the perihelion passage causes this change in orbital period?

 

[Vanadium 50]

The diagram of Neowise's orbit in the cited Wikipedia article seems very odd.

That's because it's not an orbit.

 

[Ibix]

It looks like it's the comet's path across the sky, with its position marked weekly. I expect you'll find 52 positions marked per loop of the spiral, because we are observing from the Earth and you are seeing the parallax from our orbital motion superimposed on the comet's own orbital motion.

 

[phyzguy]

Summary:: The diagram of Neowise's orbit in the cited Wikipedia article seems very odd.

The orbit diagram

https://en.wikipedia.org/wiki/C/2020_F3_(NEOWISE)#/media/File:Comet_2020_F3-skyview.png'​

in the Wikipedia article

https://en.wikipedia.org/wiki/C/2020_F3_(NEOWISE)​

shows some very odd spirals.

My first guess is the picture shows what Neowise does in one Earth year (mostly 2020) as the Earth completes it's orbit. The caption says, "7 day motion", apparently referring to the separation between orbital points displayed. Another possible interpretation is that the diagram represents one complete Neowise orbit, and each loop of the sprial represents what happens to Neowise in different Earth years from Earth's perspective. That is, the loop in the sky gets smaller as Neowise moves further away, and vice versa. The point where the two spiral cones touch corresponds to the aphelion.

If my guess is wrong, can someone please explain what is happening regarding these spirals in Neowise's orbit?.

The following is a quote from the article.

This perihelion passage will increase the comet's orbital period from about 4500 years to about 6800 years.​

Can someone explain how the perihelion passage causes this change in orbital period?

I think you've gotten the answer to your first question, which is that this is a plot of the position of the comet on the sky of the Earth, with the points being 7 days apart. You're right in that each loop of the spiral is one Earth year.

On your second question, comets don't follow simple Keplerian orbits. They are spewing out streams of gas and dust, and these streams act like rocket engines and change the comet's orbit. This effect is especially pronounced as the comet passes perihelion, where it gets the most solar heating and spews out the most evaporating gases. So the orbital elements after perihelion passage can be significantly different than before perihelion passage. In extreme cases, the comet simply boils away during perihelion passage and nothing is left but a cloud of dust.

 

[sophiecentaur]

The confusing thing is that the tight spirals can be wrongly thought of as what happens when the comet is nearest. In fact, its the single large loop that shows the parallax around the time its orbital angular velocity is greatest and it distance from us is least. Then the gassing adds another factor.
I always take my hat off to astronomers who find that sort of thing second nature - after a lot of experience and hard work, no doubt.