DAYLIGHT ON PLUTO
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John Pazmino
NYSkies Astronomy Inc
www.nyskies.org
nyskies@nyskies.org
2015 July 15
Introduction
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Dueinf the NYSkies Astronomy Seminar of 2015 July 10 we chated up
the new Horizons project. This space mission would fly by planet Pluto
on July 14th for the first really good detailed look at this world.
We took a break to watch the Stonehenge sunset, occurring on days
bracketting the meeting. Our meeting hall is on 14th Street on City
being a mid-rise district.
After the viewing we bantered a bit more about new Horizon. A
couple of the astronomers mentioned that the new Horizons web had a
way of appreciating how dark the planet is so far from the Sun. For
July 10th in new York City, the ground illumination at 8:33PM is the
same as that on Pluto at its mid day.
Others of us were a bit doubtful since twilight at that hour was
still rather bright, as we actually experienced from the street or
looking out the meeting room's window.
The situation
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The web was trying to give a personal scene to simulate the
illumination at Pluto by finding the moment of local twilight that
achieved that same illumination.
The idea was plausible because the ground level of lighting does
fall off from dayylight to night during twilight. Surely in full night
the Earth ground is orders darker than Pluto, so there could be a way
of determinating an hur in dusk that made the grounds of the two
places equal.
Twilight
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Twilight is the interval after sunset, or before sunrise, when the
sky shifts between fully lighted by the Sun and fully darkened by the
removal of sunlight. The illuminnation comes from the sunlight
scattering off of the air at elevations farther and farther above the
ground as the Sun sinks farther below the horizon. More of the
shadowed air is above us until the the Earth's shadow ascends to such
height that the air is too thin to scatter sunlight.
We can model twilight by assuming the composition and clarity of
the air. In reality, each instance of twilight is different due to the
action of weather in the lower air, in the troposphere. Haze,
aerosols, dust, moisture, and more can distort the gradation of
lighting from sunset to nightfall.
New Horizons had to use some particular model of twilight which at
the moment I didn't obtain from it. An reater distortion is that from
14th Street in front of our hall we do not have an open sky like in
manhattan, Kansas. Buildings, already daek in shadow, line the
street,m nlocking light from the sky. Even in the ideal air quality we
would achieve equality of street and Pluto sooner than 8:33PM.
Daylight on Pluto
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Just how dark is it on Pluto? The answer is simple to campute by
the inverse square law of illumination. At Earth, full sunlight shines
about 130,000 lux on the ground. This is at perpendicular incidence in
clean day air. When the air is hazy, dusty, humid, sunlight is
dispersed into the open sky away grom the Sun. The total is about the
same but for the anlge the light eays strike the ground.
We can remove the atmospheric modulation by using the solar
lighting on the Moon, which is entirely from the Sun's disc.
Pluto during the flyby of New Horizons is 32 times farther from
Sun than Earth, 32 AU or solar distances. At that location the Sun
appears in Pluto's sky as barely 1 arcminute diameter!
The light from the Sun dilutes over a sphere 32 times largers in
diameter than at Earth or 32^2 = 1,024 times area. The 130,000 lux at
Earth dilutes to, rounded, 130 lux at Pluto.
This is bright!
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130 lux at first seems awfully low. It's avtually remarkably good
lighting for many purposes on Earth. For example, this is within the
realm of illumination on the floor of a house room lighted by ordinary
lamps. It's the lighting level of most public spaces where people must
walk and interaction casually with others.
From the astronomy side, the 130 lux equals 130 full Moons! Yes,
the full Moon, overhead, shines quite only 1 lux on the ground. Yet we
can, with some care, go about under this lighting.
By way of comparison, a business office or school classtoom if
fitted with a few hundred lumen/meter2 for close interaction with
others, jhhandling tools and props, reading letterpress.
On the other side of the 130 lux we have roadway lighting of only
a couple tens of lumen/meter2. The same level prevails in stock rooms,
workrooms, nonpublic corridors.
It's hard to believe, but if the folk on Pluto have an vision
mechansism like humans, they would do quite well on their planet. In
fact, there probably would be mo real distinction between indoor and
ambient lighting, specially if they did not develop an industry for
artificial illumination.
Civil twilght
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By long convention we rexognize three levels of twilight after
sunset. A reverse order of twilight obtains before sunrise. Sunset on
July 10th was at 20:29 PM EDST. For Manhattanhenge it was a minute or
so earlier because the Sun sank behind low hills in New Jersey and not
a geometric horizon.
At the quitting of sunlight from the local ground, the
illumination begins to decline amazingly rapidly.Civil twilight lasts
from sunset to when the Sun's altitude is -6 degrees. On July 10th in
New York this was at 9:00 PM EDST. This moment is nominally when the
ambient lighting from the open sky is too low for confident outdoor
activity such as sports or sightseeing.
Nautical tqilight picks up after civil twilight and lasts until
the solar altitude is -12 degrees. It ended at 9:41 OM EDST. After
then the sky and ground are too dark to distinguish the horizon for
sextant observations.
Astronomical twilight ends when the Sun is -18 degree altitude,
occurring at 10:29 PM EDST. At this moment the night sky reaches full
darkness until the following dawn. In New York we do not experience
astronomical twilight due to the grayish skies over us. The sky is as
dark as it will gt for the night at the end of nautical twilight.
From nautical twilight thru the night to the nsawn's nautical
twilight we need artificial lighting to go about under the open sky.
However for some low-risk tasks, like starviewng, we can make do with
the minimal light received from the sky.
Boxing in the Pluto
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Depending on the peculiar model of twilight to hand the equality
of Pluto and Earth twlight illumination occurs in early civil
twilight. The decline of lighting level is so steep that a small shift
of the lux vs hour curve among models causes a large shift in the
hour. So I can not duplicate the new Horizons stated hout of 8:33 PM,
but I'm satisfied that it's in the warly end of civil twilight. It did
seem about right in the sense that from inside out room with its
lights on the outdoor scene was still plainly visible. Shortly later
in time the scene was definitely 'dark' and most of the view out the
window was either eflection from the interior lights or luminous
sources, like storefronts and cars.
Complications
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A complication was that older works use the 'foot-candle' for
illumination. Happily for all purposes in lighting work we can equate
one lux to ten foot-cndles, because there are quite closely ten square
foot in a square meter.
An other glitch is that some authors measure the emitted light
from the sky in limens/degree2 or similar. And that could be from the
whole sky or only a cap in the zenith.
Yet an other problem is that mesurements were made in various
parts of the visual spectrum and at different band widths. As a
further monkey wtench in the gears, the spectral distribution of light
shifts suring twlight!
Conclusion
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It is not a simple definite calculation of Earth ground lighting
under a twilight sky. The assorted maodels I inspected, from a variety
of studies gave a nasty spread of ours. These studies related to such
matters as nocturnal wildlife activity, fish migration, ophthalmic
cases, montioring air pollution, tracing chem-trails.