APOD: The Shifting Tails of Comet NEOWISE (2020 Aug 12)

[APOD Robot]

The Shifting Tails of Comet NEOWISE

Explanation: Keep your eye on the ion tail of Comet NEOWISE. A tale of this tail is the trail of the Earth. As with all comets, the blue ion tail always points away from the Sun. But as Comet C/2020 F3 (NEOWISE) rounded our Sun, its ion tail pointed in slightly different directions. This is because between 2020 July 17 and July 25 when the featured images were taken, the Earth moved noticeably in its orbit around the Sun. But the Earth's motion made the Sun appear to shift in the sky. So even though you can't see the Sun directly in the featured image(s), the directions of the ion tails reveal this apparent solar shift. The Sun's apparent motion is in the ecliptic, the common plane where all planets orbit. The featured five image composite was meticulously composed to accurately place each comet image -- and the five extrapolated solar positions -- on a single foreground image of Turó de l'Home Mountain, north of Barcelona, Spain Comet NEOWISE is no longer the impressive naked-eye object it was last month, but it can still be found with a small telescope as it heads back to the outer Solar System.

<< Previous APOD

This Day in APOD

Next APOD >>

[jks]

Hi,

Please be advised that currently the date given above the image(s) is 2020 August 13.

jks

[XgeoX]

16 of the last 37 APODS have featured NEOWISE, that has to be an APOD record! Not complaining BTW, I’ve grown fond of the little fella!

[Boomer12k]

Bye-Bye comet...thanks so much for coming...

:---[===] *

[orin stepanek]

Nice!

[Tszabeau]

Very interesting and informative. I wonder what the point at which those lines, if extended, would cross might be called or signify?

[RJN]

Hi,

Please be advised that currently the date given above the image(s) is 2020 August 13.

jks

Thank you for pointing that out. The date on the main NASA APOD has now been corrected to be 2020 August 12. We apologize for the oversight.
- RJN

[johnnydeep]

Very interesting and informative. I wonder what the point at which those lines, if extended, would cross might be called or signify?

Hmm. My first thought is that all the lines would intersect at the sun itself. But I'm not very sure of that.

[Chris Peterson]

Very interesting and informative. I wonder what the point at which those lines, if extended, would cross might be called or signify?

Hmm. My first thought is that all the lines would intersect at the sun itself. But I'm not very sure of that.

The lines don't intersect. The Earth is moving in one orbit, the comet in another. The lines look close to intersecting here simply because the images were made over just a few days, so nothing had the opportunity to move very much. If we followed it for longer, it would become apparent that there is no common point of intersection.

[Sa Ji Tario]

The alidades that were taken intersect at a point that is one of the foci of the comet's orbit, as a dependence of the Sun it is the Sun who is in that focus. Because of the little angles it looks like it's on Earth, but like Chris said, it's somewhere else. -

[MarkBour]

I think it would be important for this exercise to take the images of the comet at the exact same time, local time each night. Then using the approximation of drawing the lines (rays, actually) as a reverse extension of the ion tail. I don't know how well the ion tail approximates the anti-solar direction, perhaps it is near enough perfect for this purpose. It would be my first inclination (sorry for that pun) to extend the rays until they intersected and say "that's where the Sun is." But as @Chris Peterson and the APOD caption point out, the Sun's apparent position at a given time of day shifts along the ecliptic, so there is no single point of intersection (though it might look like there is for a short period of days).

But what has this APOD done? It has taken a region on the surface of the earth (somewhat consistent in altitude) and inscribed the point of intersection of these rays with the ground. Will this draw out a version of the analemma? To the extent that the ion tail is giving a precise direction pointer to the Sun and that the comet can act as a proxy for an observer on the Earth, this should work. It's a cleverly-noticed indicator of the direction to the Sun at a time of day when the Sun is below the observer's feet. However, the comet is a point way off the Earth and it also moved across the sky during this time. Therefore, I believe the figure these points would eventually draw out would be some perverted version of the standard analemma.

Also, thanks, @Sa Ji Tario, I had to look up "alidades" to learn what those were.

[Qfwfq78]

Hello Mark, as quick note, the way in which i've located the Sun's position was the following: i've captured screenshots of Stellarium of the days in question, enabling view below horizon. I then added these screenshots on PTGui Pro using stars above horizon as control points against my own panorama. This worked much better than I thought it would, I had to do only minor adjustment for positions to match with tails after this. Regards, Ignacio

[MarkBour]

Hello Mark, as quick note, the way in which i've located the Sun's position was the following: i've captured screenshots of Stellarium of the days in question, enabling view below horizon. I then added these screenshots on PTGui Pro using stars above horizon as control points against my own panorama. This worked much better than I thought it would, I had to do only minor adjustment for positions to match with tails after this. Regards, Ignacio

Thanks, Ignacio. It made a lovely and intriguing picture for today!