Light levels on planet of Alpha Centauri-A at night?

Hi group!

Let's say

- there was a habitable planet around Alpha Centauri-A
- at 1,2 AU
- A & B are at closest approach (11,2 AU)
- B is at opposition, getting the planet as close as
10 AU to B
- A is 90° below the horizon, B is 90° above the horizon

At 10 AU Alpha Centauri-B's apparent magnitude is -20,9,
which makes it 223 times dimmer than our sun.

My question now is what are the light levels like at that
time? I mean, we all know what light levels a full moon
produces, but what is "223 times dimmer than our sun" like?

I witnessed a partial eclipse back in 1999, when the moon
covered 99% of the sun where I live. With the sun at ~55°
above the horizon it was still pretty bright (clear skies).

Compared to that light levels on the planet around Alpha
Centauri A would still be 2 times dimmer (magnitude-light levels?),
but I guess this means "no night" during that time of the
year nevertheless.

What do you think (and feel free to correct any mistakes/wrong
assumptions)?

(Phillip Gabero) wrote in message
om:

Hi group!

Let's say

- there was a habitable planet around Alpha Centauri-A
- at 1,2 AU
- A & B are at closest approach (11,2 AU)
- B is at opposition, getting the planet as close as
10 AU to B
- A is 90° below the horizon, B is 90° above the horizon

At 10 AU Alpha Centauri-B's apparent magnitude is -20,9,
which makes it 223 times dimmer than our sun.

My question now is what are the light levels like at that
time? I mean, we all know what light levels a full moon
produces, but what is "223 times dimmer than our sun" like?

Well, maybe like night in a modern city which has lots of street lights.

I witnessed a partial eclipse back in 1999, when the moon
covered 99% of the sun where I live. With the sun at ~55°
above the horizon it was still pretty bright (clear skies).

Compared to that light levels on the planet around Alpha
Centauri A would still be 2 times dimmer (magnitude-light levels?),
but I guess this means "no night" during that time of the
year nevertheless.

What do you think (and feel free to correct any mistakes/wrong
assumptions)?

Could there be periods on such a planet when none of the Alpha Centauri
stars is visible? If yes then it would look very much like here on Earth.

In article , Mr. 4X wrote:
time? I mean, we all know what light levels a full moon
produces, but what is "223 times dimmer than our sun" like?

Well, maybe like night in a modern city which has lots of street lights.

I heard a statistic recently that normal office lighting is around
1/500th of the brightness of midday sunshine. You could estimate the
difference by comparing the settings that an auto-exposure camera gives
you.

--
Aidan Karley,
Aberdeen, Scotland,
Location: 57°10'11" N, 02°08'43" W (sub-tropical Aberdeen), 0.021233

In article ,
(Phillip Gabero) writes:
Let's say
- there was a habitable planet around Alpha Centauri-A
....
- B is at opposition, getting the planet as close as 10 AU to B
[and ignoring A]

At 10 AU Alpha Centauri-B's apparent magnitude is -20,9,
which makes it 223 times dimmer than our sun.

My question now is what are the light levels like at that
time?

I haven't checked your apparent magnitude value, but it looks
plausible.

You have pretty much answered your own question: the illumination
levels are like those at a 99.6% partial solar eclipse. That is
indistinguishable by eye from the 99% eclipse you observed, though of
course a camera exposure meter could tell the difference (about 1
f-stop). Shadows would be about 10 times sharper than normal solar
shadows, similar to (but not as sharp as) those at the partial
eclipse. Color vision should be near normal, and reading should be
very easy. (Isn't this brightness level about 1000 times that of the
full moon?)

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
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