DAYLIGHT ON PLUTO --------------- John Pazmino NYSkies Astronomy Inc www.nyskies.org email@example.com 2015 July 15 Introduction ---------- 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 ------ 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. Complications ----------- 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 -------- 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.