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Morning and Evening Comet

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#1 Alex McConahay

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Posted 14 July 2020 - 02:39 PM

If this has been discussed in this forum before, please redirect me to where........I could not find it. 

 

Yesterday, at 4:30 am from Southern California, I saw the comet above the north eastern horizon. 

Last night at about 9:15 or something, I saw it on the northwest horizon. 

 

My brain cannot figure out how I was able to do this. What is the motion of the comet/trick of perspective/ or whatever that makes it possible to see the comet in the morning on one side of the sun and then later that same day in the evening, apparently on the other side of the sun? It is preceding the sun on rising, but following the sun on setting. 

 

Is there a three-dimensional(ish) illustration of this somewhere?

 

Thanks.

 

Alex

 



#2 randcpoll

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Posted 14 July 2020 - 02:44 PM

That freaked me out at first too. Then I realized it is so far north it is almost 'circumpolar'. That was my "Ohh!" moment.



#3 Alex McConahay

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Posted 14 July 2020 - 02:52 PM

I get the circumpolar part----kinda like the big dipper or Cassiopeia never set. They just go round and round......

 

But the comet is shining with reflected light. Where is it in relation to the earth/sun that it can look like it is both leading (in the morning, up before the sun) and following (still not set after the sun has) the sun.

 

Is there an illustration somewhere than can help me get my head around that?

 

Alex



#4 ButterFly

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Posted 14 July 2020 - 02:54 PM

The ecliptic is angled with respect to the horizon.  That's what makes it possible.  That angle changes with latitude.  Watch the comet in Stellarium (in EQ mode without skyglow) from different latitudes.  From the South pole, where it is winter, the sun never rises today.  From the North pole, where it is summer, neither the Sun nor the comet set.



#5 Sam M

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Posted 14 July 2020 - 03:22 PM

There's a 3d model that you can play with here...

https://theskylive.com/c2020f3-info



#6 beggarly

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Posted 14 July 2020 - 04:03 PM

Morning view:

Morning view.jpg

Evening view:

Evening view.jpg

Trajectory:

Trajectory.jpg

 

 



#7 ButterFly

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Posted 14 July 2020 - 04:08 PM

There's a 3d model that you can play with here...

https://theskylive.com/c2020f3-info

It's too slow for its orbit to matter.  Today, if one can see the comet before sunrise and after sunset from their location, any object with that RA/Dec will be visible before and after sunset today, and every year thereafter on the same day from that location.  The angle of the ecliptic is what sets sunrise and sunset for a particular location on a particular day, and thus the band of RA/Dec for which this is possible from a particular location on a particular day.



#8 ButterFly

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Posted 14 July 2020 - 04:22 PM

HIP 37023 is a nearby star at present.  Follow it over the course of a year.



#9 Alex McConahay

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Posted 14 July 2020 - 04:32 PM

Thanks, everybody. As long as I have been at it, I still don't understand it all. 

Alex



#10 chrysalis

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Posted 14 July 2020 - 04:59 PM

I **think** Venus can also do this.



#11 chrysalis

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Posted 14 July 2020 - 05:08 PM

Thanks, everybody. As long as I have been at it, I still don't understand it all. 

Alex

Don't get hung up on the three-dimensional geometry. As others have said, it's really a function of of what the comet's astronomical coordinates are versus which astronomical coordinates are above the horizon. When these synch up in the morning AND in the evening - think the circumpolar analogy where a star is visible in the morning and in the evening sky - the comet will be visible in both parts of the day.

 

What stretched MY brain is figuring out the orientation of the tail as the comet circles the sky...but of course, it always points away from the sun!


Edited by chrysalis, 15 July 2020 - 04:00 AM.


#12 ButterFly

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Posted 14 July 2020 - 05:10 PM

Watch it first, the understanding will follow.

 

For a given latitude and time, one can find the RA/Dec of the horizon as a function of azimuth.  Find that for sunrise, ask which RA/Dec is above the horizon.  Find that also for sunset, and again ask which RA/Dec is above the horizon.  Find the intersection. That is the band of RA/Dec that can do this for a given location on a given day.  The subset of circumpolar stars never set, so they are included in the above.




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