John Pazmino
 NYSkies Astronomy Inc
 2015 July 15

    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
    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 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
    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!
    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
    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
    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.

    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! 

    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.