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In my question How was Earth's "quasi-satellite" 2016 HO3 "first spotted" and it's orbit determined? I link to two videos of simulations of views of 2016 HO3's orbit seen in two different frames.

In this NASA JPL video (above) the view is rotating around the sun following Earth. You can see the earth move slightly closer and farther from the sun since the Earth's orbit is not quite circular.

This video (above) from http://arksky.org/calendar/alerts/714-what-is-it-the-strange-new-object-2016-ho3 shows a projection of 2016 HO3's motion against the stars as seen from Earth's location, but in a fixed direction. You can see the the sun and planets tend to follow the ecliptic, while 2016 HO3 does a figure-eight every year.

The NASA JPL news brief announcing the discovery of 2016 HO3 states:

The asteroid's orbit also undergoes a slow, back-and-forth twist over multiple decades. "The asteroid's loops around Earth drift a little ahead or behind from year to year, but when they drift too far forward or backward, Earth's gravity is just strong enough to reverse the drift and hold onto the asteroid so that it never wanders farther away than about 100 times the distance of the moon," said Chodas. "The same effect also prevents the asteroid from approaching much closer than about 38 times the distance of the moon. In effect, this small asteroid is caught in a little dance with Earth."

note: Paul Chodas is the manager of NASA's Center for Near-Earth Object (NEO) Studies

I think that the fact that it's orbit slowly oscillates on the order of decades (tens of orbital periods) with respect to Earth's orbit means it is in a 1:1 resonance with Earth. But I'm using the term orbital resonance without knowing it's exact definition - if there is one. Perhaps it's a soft term - some orbits may be clearly in resonance, others only roughly in resonance.

Is there a good working definition of orbital resonance, and is 2016 OH3's orbit in 1:1 resonance with Earth's orbit?

bonus: Are Trojan asteroids (at a planet's $L_4$ or $L_5$ triangular libration points) also considered to be in a 1:1 resonance orbit?

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1:1 resonance means that each year the asteroid will be in approximately the same place as viewed from Earth as it was the previous year. This as compared to Pluto, which is in a 3:2 resonance with Neptune, meaning for every 3 Neptune years, Pluto will make 2 years. These should be a result of gravitational interaction between the two bodies.

And yes, Trojans are considered 1:1 resonance, see Wikipedia.

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    $\begingroup$ You definitely get the "bonus points" for the Trojans :) But for the question of resonance I think it has to be more than that. There can be coincidental periodic coincidences of positions unrelated to mutual gravitational interaction, and there can also be fleeting synchrony too weak to call true resonance. I can't believe I managed to cram so many big words into one sentence! Try again. To be called a resonance, the repeatability needs to be influenced or supported by the two things interacting with each other. If you can't show that mathematically, I think you can't call it a resonance. $\endgroup$ – uhoh Jun 19 '16 at 12:34

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