# With the naked eye, can one identify a comet that might impact earth?

**URL:** <https://boards.straightdope.com/t/with-the-naked-eye-can-one-identify-a-comet-that-might-impact-earth/508776>\
**Category:** Factual Questions\
**Created:** [September 3, 2009, 6:37pm UTC](https://boards.straightdope.com/t/with-the-naked-eye-can-one-identify-a-comet-that-might-impact-earth/508776 "2009-09-03T18:37:21Z")\
**Posts on this page:** 4\
**Page:** 2

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**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [September 6, 2009, 5:05am UTC](https://boards.straightdope.com/t/with-the-naked-eye-can-one-identify-a-comet-that-might-impact-earth/508776/21 "2009-09-06T05:05:50Z")

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> [@](#):
>
> I selected that picture because it was the only one that showed a landscape which gives you an idea of perspective of the comet’s actual size.

You can’t use terrestrial objects to give a scale to astronomical objects in a picture, because it depends on how close the camera is to the foreground object and what the zoom is. You have to either compare it to other astronomical objects, or to a phenomenon like a rainbow that always has the same angular size.

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**Author:** ![j\_sum1](https://avatars.discourse-cdn.com/v4/letter/j/8baadc/32.png) [@j\_sum1](https://boards.straightdope.com/u/j_sum1)\
**Post date:** [September 8, 2009, 3:55am UTC](https://boards.straightdope.com/t/with-the-naked-eye-can-one-identify-a-comet-that-might-impact-earth/508776/22 "2009-09-08T03:55:15Z")

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Back to the maths. Don’t you also need to record the exact time of the observations? I’m not sure that would have been real easy on Gilligan’s island.

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**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [September 8, 2009, 6:13am UTC](https://boards.straightdope.com/t/with-the-naked-eye-can-one-identify-a-comet-that-might-impact-earth/508776/23 "2009-09-08T06:13:16Z")

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You do need times, but you can make your observation at any time that you can know. Observations made three nights in a row, at the moment when Sirius rises each night, for instance, would work fine. Or more realistically, every night for a month at that time.

You would still need ephemeris tables from a book, but this being the Professor, I think we can assume that he’s memorized all of them.

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**Author:** ![toadspittle](https://avatars.discourse-cdn.com/v4/letter/t/c0e974/32.png) [@toadspittle](https://boards.straightdope.com/u/toadspittle)\
**Post date:** [September 10, 2009, 5:33am UTC](https://boards.straightdope.com/t/with-the-naked-eye-can-one-identify-a-comet-that-might-impact-earth/508776/24 "2009-09-10T05:33:27Z")

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> [@Chronos](#):
>
> You would still need ephemeris tables from a book, but this being the Professor, I think we can assume that he’s memorized all of them.

😛

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