Daytime Starlight

What is the threshold of brightness for seeing an astronomical object
in the daytime Earth sky?

Specifically, would a star with 70% of Sol's absolute brightness be
visible at 45 AU?

Mike Miller, Materials Engineer

(Mike Miller) wrote in message . com...
What is the threshold of brightness for seeing an astronomical object
in the daytime Earth sky?

Specifically, would a star with 70% of Sol's absolute brightness be
visible at 45 AU?

Yes. It would be hundreds of times brighter than a
full moon.

(Christopher M. Jones) wrote in message . com...

Yes. It would be hundreds of times brighter than a
full moon.

Now that I'm more awake than when I first asked this, I thought of a
different approach to this:

Sunlight is 1400W/m^2
Moonlight is about 0.0001W/m^2 (0.1 milliwatt per square meter)

This star would add (at 45AU)...(1400x0.7)/(45^2) = 0.4W/m^2.

Or 4839x moonlight levels.

That's pretty bright. Something like one 100-watt bulb per 16m x 16m
(50ft x 50ft) area.

Mike Miller, Materials Engineer

(Mike Miller) wrote in message . com...
What is the threshold of brightness for seeing an astronomical object
in the daytime Earth sky?
With good transparency (deep blue sky, not haze whitish), and knowing
precisely where to look(important!), you should be able to see Venus
(vis. magnitude -4).

Specifically, would a star with 70% of Sol's absolute brightness be
visible at 45 AU?
That would be .7*(45^2) = cca 1420 times weaker then Sun.

Sun is visual magnitude -26.7, 10x is 2.5 magnitudes, so 1420 should
be about 8 magnitudes (+-0.5 mag). That star would be visible
magnitude 18-19.

For comparison, the full Moon is -12, so the light of few hundred full
Moons would be concentrated in a very small area. Clearly visible, and
very conspicuous even during broad daylight.

Stefan

Mike Miller, Materials Engineer

JRS: In article , seen
in news:sci.space.tech, Christopher M. Jones
posted at Sat, 8 May 2004 15:23:32 :
(Mike Miller) wrote in message news:5dcb47db.0405061609.78da
...
What is the threshold of brightness for seeing an astronomical object
in the daytime Earth sky?

Specifically, would a star with 70% of Sol's absolute brightness be
visible at 45 AU?

Yes. It would be hundreds of times brighter than a
full moon.


70% is not much different from 100%.

The Sun, at 1 AU, is magnitude -26.8. At 45 times the distance, it
would be 45^2 ~ 2000 times less bright. Since 5 magnitudes is a factor
of 100, a factor of 2000 must be about 8 magnitudes, giving the star at
about Mag -19. Since the Full Moon is about -12.6, the star would
indeed be over a hundred times brighter - though perhaps (remembering
that 70%) not quite as much as hundreds.

This differs from my previous answer (not yet seen in News), in which
four zeroes escaped; 10000 should be 100000000.

--
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Web URL:http://www.merlyn.demon.co.uk/ - FAQish topics, acronyms, & links.
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Mike Miller wrote:
What is the threshold of brightness for seeing an astronomical object
in the daytime Earth sky?

Specifically, would a star with 70% of Sol's absolute brightness be
visible at 45 AU?

Is the question meant to ask if one would see both "Suns" in the sky
if you had a Earth-style world orbiting the brighter star of Alpha
Centauri during the closes approach of the two?

Unless i'm grossly mis-remembering it is always bright enough to be
seen in the daytime sky. But teh amount of warmth (and how big it is
in teh scky) would vary quite a bit.


Mike Miller, Materials Engineer

--
Sander

+++ Out of cheese error +++

In article , (Christopher M. Jones) writes:
| (Mike Miller) wrote in message . com...
| What is the threshold of brightness for seeing an astronomical object
| in the daytime Earth sky?

This part of your question isn't well phrased, I am afraid. It
depends immensely what you mean by 'the daytime Earth sky'.
Depending on conditions, Venus may be visible, or the sun may
be invisible. There is a lot of variation even in cloudless
skies - it isn't rare to be unable to feel the heat of an
unobscured sun, for example.

| Specifically, would a star with 70% of Sol's absolute brightness be
| visible at 45 AU?
|
| Yes. It would be hundreds of times brighter than a
| full moon.

In almost all cloudless skies, yes, but I have seen conditions
hazy enough that a full moon was effectively invisible. It is
possible that they can be much more extreme.


Regards,
Nick Maclaren.

(Nick Maclaren) wrote in message ...

This part of your question isn't well phrased, I am afraid. It
depends immensely what you mean by 'the daytime Earth sky'.

You know, the sky during the day.

Mike Miller, Materials Engineer

(Mike Miller) wrote in message . com...
What is the threshold of brightness for seeing an astronomical object
in the daytime Earth sky?

As someone else points out, you need to define the sky conditions and
also what optical aid (if any) you allow. Venus at magnitude -4 or so
is easily visible to the naked eye *if you know exactly where to
look*, but it's very difficult to find. I would *guess* that
something ten times brighter would be fairly easy to find, but there
aren't a lot of examples, so it's impossible to be sure. (Actually,
come to think of it, aircraft make good experimental examples.
Anybody know the visual magnitude of an aircraft when it's easily
visible?)

Specifically, would a star with 70% of Sol's absolute brightness be
visible at 45 AU?

I calculate an apparent magnitude of -18 or 100 times brighter than
the full moon, and the light would be concentrated in a point rather
than spread over half a degree. Should be impossible to miss, even
through moderate clouds.

(Nick Maclaren) wrote in message ...
In article , (Christopher M. Jones) writes:
| (Mike Miller) wrote in message . com...
| What is the threshold of brightness for seeing an astronomical object
| in the daytime Earth sky?

This part of your question isn't well phrased, I am afraid. It
depends immensely what you mean by 'the daytime Earth sky'.
Depending on conditions, Venus may be visible, or the sun may
be invisible. There is a lot of variation even in cloudless
skies - it isn't rare to be unable to feel the heat of an
unobscured sun, for example.

My answer originally started out with a more generalized
portion analyzing brightness of distributed and point
like objects relative to the brightness of the daytime
sky as observed by the human eye. But it got long
winded and I was having trouble finding adequate data on
the surface brightness of the daytime sky so I snipped it
and just posted the specific answer since that was a far
simpler case.