Magnitude of celestial body bright enough to cast shadow

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Magnitude of celestial body bright enough to cast shadow

Postby Alg. » Tue Oct 22, 2013 8:40 am UTC

Hi, guys.

I'm doing a quiz on astronomy and came up with a question "What natural celestial bodies are bright enough to cast shadows on Earth?". The answers are Sun, Moon and Venus.
However another person who's reviewing questions demands proof (Articles at NASA and digitalsky turned out to be not enough).

So, does anyone have any idea how can I calculate the magnitude of dimmest object that is still bright enough to cast shadow? Assuming no Moon or other bright light sources are present.
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Re: Magnitude of celestial body bright enough to cast shadow

Postby Carnildo » Wed Oct 23, 2013 9:07 am UTC

"Cast a shadow" is pretty vague: with sensitive enough instruments, you could measure the shadows cast by every star individually. In order to answer this, you'd need to define what shadows do and don't count. My personal definition would be that a light source can cast a shadow if it is at least as bright as every other light source combined; you might want to use a different threshold (eg. one based on the minimum brightness difference perceptible at your chosen ambient light level).
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Re: Magnitude of celestial body bright enough to cast shadow

Postby davidstarlingm » Wed Oct 23, 2013 1:31 pm UTC

The dimmest stars visible to the naked eye are +8 magnitude. Ostensibly, you could use this to predict the smallest light interval discernable to the naked eye, and use that to find what bodies cast visible shadows.
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Re: Magnitude of celestial body bright enough to cast shadow

Postby mfb » Thu Oct 24, 2013 10:03 pm UTC

Carnildo wrote:"Cast a shadow" is pretty vague: with sensitive enough instruments, you could measure the shadows cast by every star individually. In order to answer this, you'd need to define what shadows do and don't count. My personal definition would be that a light source can cast a shadow if it is at least as bright as every other light source combined;
With this definition, only the sun and and the moon are the only natural* astronomical** objects that can do this on a regular basis (edit), unless you have a very cloudy night with a very bright object shining through a hole in the clouds.

*Iridium flares can easily outshine all stars combined
**A fire can cast shadows, for example
Last edited by mfb on Sun Oct 27, 2013 8:08 pm UTC, edited 1 time in total.
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Re: Magnitude of celestial body bright enough to cast shadow

Postby ElWanderer » Fri Oct 25, 2013 10:31 am UTC

How about comets? I'd have thought they can be much brighter than Venus (citation needed).

Edit: Wikipedia says Venus has a max apparent magnitude of -4.6, much brighter than Comet Hale-Bopp or the great comet of 1811 (magnitude 0). But Wikipedia also suggests Mars and Jupiter can cast shadows...
Last edited by ElWanderer on Fri Oct 25, 2013 10:45 am UTC, edited 1 time in total.
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Re: Magnitude of celestial body bright enough to cast shadow

Postby Tass » Fri Oct 25, 2013 10:43 am UTC

ElWanderer wrote:How about comets? I'd have thought they can be much brighter than Venus (citation needed).


They can, although they are much more spread out, so if you get a shadow it will be much more diffuse. Also really bright ones are rare. Ison was originally predicted to mayby get as bright as the moon, but this estimate has been heavily downgraded.
Last edited by Tass on Fri Oct 25, 2013 10:50 am UTC, edited 1 time in total.
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Re: Magnitude of celestial body bright enough to cast shadow

Postby ElWanderer » Fri Oct 25, 2013 10:48 am UTC

Tass wrote:
ElWanderer wrote:How about comets? I'd have thought they can be much brighter than Venus (citation needed).


They can, an upcoming one might get as bright as the moon. But they are much more spread out, so if you get a shadow it will be much more diffuse. Also really bright ones are rare.

Good point. Just edited my post while you were replying - apologies.
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Re: Magnitude of celestial body bright enough to cast shadow

Postby Tass » Fri Oct 25, 2013 10:51 am UTC

ElWanderer wrote:
Tass wrote:
ElWanderer wrote:How about comets? I'd have thought they can be much brighter than Venus (citation needed).


They can, an upcoming one might get as bright as the moon. But they are much more spread out, so if you get a shadow it will be much more diffuse. Also really bright ones are rare.

Good point. Just edited my post while you were replying - apologies.


Ah. And I just looked Ison up and edited my post while you were replying.
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Re: Magnitude of celestial body bright enough to cast shadow

Postby PM 2Ring » Sat Oct 26, 2013 7:33 am UTC

Some supernovae have been bright enough to see during the day, so they'd surely be bright enough to cast a shadow.

See http://en.wikipedia.org/wiki/List_of_supernovae
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Re: Magnitude of celestial body bright enough to cast shadow

Postby eternauta3k » Tue Nov 05, 2013 7:45 pm UTC

You need to know the level of noise in your image. If all sources of light add a predictable intensity, then you're limited by the brightness resolution of your detector like Carnildo said.
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Re: Magnitude of celestial body bright enough to cast shadow

Postby Quizatzhaderac » Thu Nov 07, 2013 9:13 pm UTC

PM 2Ring wrote:Some supernovae have been bright enough to see during the day, so they'd surely be bright enough to cast a shadow.

See http://en.wikipedia.org/wiki/List_of_supernovae
You'd probably want to pick a definition of "celestial body" as well. Supernovas have been bright, but none currently are (from Earth). Comets are dim most of the time, but brighter when they're near the sun, and (presumably) even brighter when they're near the Earth. Spud nick was visible from earth, but was neither permeate or reoccurring.

For the definition of casting a shadow, I'd go with: Bright enough to create a detectible difference of brightness on reflection or a non-mirrored surface, with no light pollution, during a frequently occurring celestial configuration. i.e. :
New moon, night, Venus not visible.
A typical human observer looking at the ground, has to be able to tell if the object is occluded or not.

As for the actual quiz: It's probably WAY past being a fair question when you can't look it up, or if common sources like Wikipedia disagree with your answer.
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Re: Magnitude of celestial body bright enough to cast shadow

Postby The Geoff » Tue Nov 12, 2013 2:36 pm UTC

If I look up at, say, Rigel and then put my hand in front of my eyes I can no longer see the star. Does that not imply that I can't see it because I'm in the shadow my hand is creating?

A shadow is simply photons being blocked from whatever the shadow is being cast on at the end of the day. If you can't see the luminous object because something is in the way then it's casting a shadow, surely?
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Re: Magnitude of celestial body bright enough to cast shadow

Postby davidstarlingm » Tue Nov 12, 2013 4:20 pm UTC

The Geoff wrote:If I look up at, say, Rigel and then put my hand in front of my eyes I can no longer see the star. Does that not imply that I can't see it because I'm in the shadow my hand is creating?

A shadow is simply photons being blocked from whatever the shadow is being cast on at the end of the day. If you can't see the luminous object because something is in the way then it's casting a shadow, surely?

I think the definition of a shadow needs to factor in reflectance at some point. Though you could put a mirror on the ground and do the same thing.
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