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I have read in several places that the orbital period of comet Neowise c2020 F3 is about 6800 years. In several popular media sources it is said that we will be able to see the same comet after ~6800 years.

How is it known that the Earth will be at exactly the right place in order for it to be visible?

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    $\begingroup$ I would interpret this as a figure of speech or a literary device; as a way of saying "it will be close". After all, it is not like as if you could sue those media sources if 6800 years from now it happened that you could not see the comet. Likewise, sometimes it is said "this comet will visit us" while the comet has no intention at all to come close to Earth and say "hi", it is just following its pre-stablished orbit. $\endgroup$ – SJuan76 Jul 19 '20 at 22:34
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    $\begingroup$ I think it's just referring to when the comet will return to the inner Solar System, not specifically near Earth. $\endgroup$ – Barmar Jul 19 '20 at 23:12
  • $\begingroup$ It's in a solar orbit. As long as it remains in a solar orbit and we remain in a solar orbit then the sun is what makes sure that we are in the right place - because both us and neowise are stuck to it by gravity. $\endgroup$ – J... Jul 20 '20 at 19:17
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It is important to appreciate the scale of the comet's orbit compared to the Earth's orbit. The Earth goes round the Sun in a nearly circular path. The average distance is one Astronomical Unit (AU).

C/2020 F3 (Neowise) has an incredibly skinny elliptical orbit. It's closest approach to the Sun (perihelion) is at 0.3 AU, which is inside the orbit of Mercury. However, its furthest distance from the Sun (aphelion) is 500-700 AU.

By way of analogy, if Earth is like a boat that is sailing around a harbour, Neowise is like a ship that comes in from 600km out at sea, turns in the harbour and sails back out again.

As to whether we can see it or not, as @JamesK states, comets are usually visible for several months. It might be possible for a comet to approach, orbit the Sun and recede, entirely in the daytime sky. In that case, we would never see it. However, it is more likely that at least part of its orbit would be in our nighttime sky and so visible at some point. [n.b. by visible, I mean visible to astronomers with telescopes and image processing equipment - not necessarily to naked-eye amateurs].

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  • $\begingroup$ "However, it is more likely that at least part of its orbit would be in our nighttime sky and so visible at some point." Simply by the amount of time it will be close to Earth? How long should that roughly be anyway? $\endgroup$ – Mast Jul 21 '20 at 7:25
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    $\begingroup$ @Mast On the way in, comets are usually spotted a few AU out (roughly between Mars and Jupiter). Neowise was spotted 27/Mar and hit perihelion on 13/Jul so about 3-4 months inbound. Then you have to consider its angular separation from the Sun. If it is coming in from "behind" the Sun, it will be too close to the glare to be seen (think how hard it is to spot Mercury). As soon as it gets a reasonable angle from the Sun, we can see it before dawn or after dusk. It's a trade-off; close to the Sun it's brighter, but too close and it gets lost in the Solar glare. $\endgroup$ – Oscar Bravo Jul 21 '20 at 14:02
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The time of the orbit is not well known but the inclination can't change as much (small changes in velocity due to outgassing can significantly change the orbital period, but can't change the inclination much. So, while we have a good idea of it's orbital track close to perihelion, we just don't know when it will be there.

We don't know where the Earth will be in its orbit when it returns, but a comet is typically visible for several months. It is possible that the Earth will be particularly badly placed, but even then it will be visible

The Earth doesn't need to be in an exact position for the comet to be visible. It may not be a naked eye object next time, but it should still be visible, no matter where Earth is on its orbit.

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Simply because we can see the sun.

The comet will be swinging around the sun, getting closer to it than the Earth does by a large factor. This path cannot always be in-line with the sun or other body orbiting the Earth or the Sun, so there isn't a way for our view to be blocked.

It is plausible that its approach could be mostly co-linear with the sun, or its departure, but not both; its change in angle compared to the sun will be at a different rate, almost certainly different inclination, than Earth's.

Imagine you are walking around a tall skinny (3' radius) pole in the middle of an empty field. You know in 6800 years someone will run up to the pole from some unknown direction, run around it 90 degrees, then run out another direction. You aren't walking fast enough for the pole to block both its approach and its exit from the field.

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  • $\begingroup$ I wonder if there exists a plausible trajectory that would keep a comet entirely in the day-time sky? It would have to be nearly co-planar with Earth (so no inclination), very eccentric (so inward and outward paths are near parallel), have a very low perihelion (to keep in the Sun's glare) and go really fast (to avoid our viewpoint changes too much). I imagine if it was hidden on the way in, it would be very prominent on the way out... $\endgroup$ – Oscar Bravo Jul 21 '20 at 7:13

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