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All rights reserved. Natural Light - Is it a factor? by Patrick McCarthy The natural lighting conditions are an overlooked issue in many civil and criminal matters. The most common situations where natural lighting is an issue are automobile accidents, slip and fall claims or identification issues concerning a suspected criminal. Was there sufficient light and contrast for a witness to properly identify the suspect? Could a driver see the pedestrian? Was there enough light to see an obstacle? This data is particularly important when producing a trial exhibit that demonstrates what a witness or driver could or could not see, under identical conditions, using still photographs or videotape. Determining what effect ambient lighting has on a criminal or accident scene requires a basic understanding of astronomical events and definition of the relevant terms. The Basics In the course of a single day, the Earth rotates once on its axis, presenting the phenomena of sunrise and sunset as viewed from a particular location. Most celestial bodies, the stars and planets, appear in the sky at the horizon to the east of any particular place, then cross the sky, then disappear at the horizon to the west. The most significant of these events, in regard to ordinary affairs, is the rise and set of the sun. Many witness statements include phrases such as; "it was just getting dark." "A little after sunset." "It was dusk." These statements can be translated into more precise "facts" by asking the right question. Time The definitions of relevant timekeeping and astronomical terms were first established at the International Meridian Conference in 1884. For precision timekeeping, the International Bureau of Weights and Measures in Paris, France synchronizes a number of atomic clocks maintained around the world. This precision time coordination, Coordinated Universal Time (UTC) is extremely important to a number of applications including the military, banking, airlines and communications services where fractions of a second make a difference. For the crime scene and accident site reconstruction the nearest whole minute is an acceptable time frame for the analysis of ambient lighting conditions. Sun Under good atmospheric conditions, the apparent diameter of the sun is 0.51° or 32 [arc] minutes. The apparent size of the sun can be greater when the sun is at a low angle, due to atmospheric refraction. Since the sun is observed as a disk, not a small point of light, it is not seen to rise or set all at once. Sunrise and sunset occurs when the upper edge of the sun is at the horizon, unobstructed to the observer. To be exact, sunrise or sunset occurs when the center of the sun is at 90.8333° from the observer's vertical, or 0.8333° below the horizon. Twilight Before sunrise and after sunset, light from the sun is reflected from the upper atmosphere onto the Earth. These are the periods of twilight. There are specific time periods of twilight and specific times for the occurrence of sunrise and sunset. Civil twilight, sunrise and sunset occur at different times throughout the year. "Civil Twilight" is a reference commonly used to establish when vehicle headlights and community streetlights are required to be illuminated. For example, a local law may require that vehicle lights be illuminated between the end of civil twilight in the evening and the beginning of civil twilight in the morning, or a variation of this concept. Civil twilight is defined to begin in the morning and to end in the evening when the center of the sun is 6° below the horizon. Civil twilight is the period during which ambient illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished; the horizon is clearly defined and the brightest stars are visible. In the morning, before, and in the evening, after civil twilight, artificial illumination is required to conduct ordinary activities. There are definitions for "Nautical Twilight" and "Astronomical Twilight," as well. These events are not of great concern in civil litigation; it may be useful to be aware of their existence. Nautical twilight, most often used in Admiralty Law, begins and ends when the center of the sun is 12° below the horizon. During the period between the start of nautical twilight and the start of civil twilight, general outlines of ground objects may be distinguishable, but detailed outdoor operations are not possible and the horizon is indistinct. Astronomical twilight begins and ends when the center of the sun is 18° below the horizon. During the period between astronomical twilight and nautical twilight, illumination is so faint that it is practically imperceptible. Geography 101 Horizon is another term that requires definition. Wherever one is located, the Earth can be perceived as a flat plane. The sky resembles one-half of a sphere centered over the observer. If there are no visual obstructions, such as buildings, mountains or trees, the apparent intersection of the sky with the Earth is the horizon. For astronomical computations, the observer is considered to be on the Earth and the horizon is a level line 90° from the observer's vertical direction. The unobstructed visible distance to the horizon, in statute miles, is calculated by taking the square root of the observer's eye height and multiplying it by 1.322. For kilometers, multiply the square root of the observer's eye height by 2.119. For a six-foot tall person, their eye is approximately 67 inches above the surface. Using the formula above one can calculate that the visible horizon is about 3.1 miles distant. Every point on Earth has an address, expressed as latitude and longitude. The first part of the address is latitude, the distance north or south of the equator (0° latitude). The second part of the address is longitude, the distance east or west of a line going from the North Pole to the South Pole through Greenwich, England (Greenwich Meridian, 0° longitude). Since the Earth is a sphere, these distances are expressed as angles. There are two ways to express latitude and longitude. The invention of the first method is credited to the Babylonians, about 5000 years ago. It divides angles into 360 degrees (°), each degree into 60 [arc] minutes ('), and each minute into 60 [arc] seconds ("). The Washington Monument, in Washington, DC, is located 38°53'21.5" north of the equator, and 77°02'08.0" west of the Greenwich Meridian. The second method is to express the angles in decimal form. North latitudes and east longitudes are positive (+), while south latitudes and west longitudes are negative (-). For example, the Washington Monument is at +38.8893° latitude and -77.0356° longitude. To determine local astronomical event times, the latitude and longitude must be known with some precision. Many roadway-engineering drawings indicate latitude and longitude as base reference points. A portable global positioning system (GPS) receiver will provide latitude and longitude with sufficient accuracy provided they are properly set to the appropriate datum (WGS-84 is most commonly used in the U.S). At median latitudes (covering most of the U.S.), an error of just 1/4th of a degree [15 arc minutes] can result in a sunrise/sunset time error of one minute. For incidents that occur in a time period near these rise or set events, the error could be unacceptable in determining the facts of the matter. At lower latitudes, greater precision than illustrated above may be required. Height & Direction The position of the sun in transit, stated as azimuth and elevation, can be determined in one-minute (time) intervals and correlated to the observer's or driver's direction and field of view. If the initial research indicates the sun was between 1° below and 70° above the horizon, and on an azimuth within 60° of either side of the center of the individual's field of view, then a more detailed analysis may be warranted. Consideration must be given for variables such as the position of the item of interest relative to the observer, atmosphere, and the individual's inherent propensity to have their visual acuity diminished by the situation. There are various conventions used to express the position of the sun. Some information providers use the terms of altitude, elevation and angle, all referring to the sun's position relative to the horizon, interchangeably. Azimuth is the direction expressed in degrees, from true north. Weather More than just astronomical event time is required to complete an analysis of the situation. Local weather conditions and terrain may affect ambient light. At any location, ambient light can be limited by surrounding objects. It is necessary to observe lighting conditions at the accident or incident site, near the same astronomical time, to determine if such obstructions existed and what influence they may have on the situation. It is entirely possible, that the observer may have been in a location where the functional equivalent of civil twilight in an unobstructed area was not available due to the terrain. An inspection of the site, at the appropriate time and under similar conditions, will reveal what natural lighting was available. Weather can also play an important role in ambient light levels. The previous descriptions of twilight assumed no adverse atmospheric factors. Consideration must be given to cloud cover, haze or fog. Recent weather data may be obtained from a local newspaper, but these reports do not contain sufficient information and are seldom acceptable as evidence in court. Detailed historical weather data is available from the regional climate centers. Return to the Scene It is a mistake to return to the accident site or crime scene on the same calendar day, a year or more later, at the same time as the incident. The altitude and azimuth of the sun may be slightly different but the phase, altitude and azimuth of the moon will not be the same it was on the day of the incident in question. The orbit and rotation of the Earth undergoes constant, albeit small, variations thus the civil calendar will not match the solar or lunar year precisely. In some instances, the phase and position of the moon may be a factor. Illumination by the moon in rural areas can be notable, but is less so in urban areas with abundant artificial illumination. The rising and setting of the moon is not synchronized to the solar day. Due to an eccentric orbit, the time from one moonrise to the next moonrise can vary between 24.5 and 26 hours. On any particular day the moon may rise before it sets, set before it rises, set only or rise only. To recreate conditions, the variables require an analysis of the moon's phase and position on the date of the incident and then identifying a corresponding astronomical date and time. For any given date the phase, altitude and azimuth of the Moon will not repeat for nearly two decades. Odds & Ends The time from the beginning of civil twilight to sunrise can be very short. At median latitudes, the sun will appear to move a distance equal to its own diameter in about three minutes. The response time of the first police investigator to the scene may be of such length that the lighting situation, when they arrive, is entirely different from the conditions that existed at the time of the incident. Using police dispatch records, indicating when the first call was received, and then extrapolating back, allowing the caller time to comprehend the situation and make the call, could provide a more accurate estimate of the time of the accident than what is indicated in the police report. When the ability of a driver to perceive a hazard is at issue, an ambient lighting analysis, using reliable data sources and meticulous attention to detail, can reveal environmental factors previously overlooked. The same holds true in dealing with the observations of witnesses. Summary Though often overlooked by legal professionals, astronomical data can help clarify the conditions that existed when an incident occurred. Phrases like "It was getting dark." is changed to "between 5:25 PM and 5:50 PM." "It was dusk." becomes "approximately 5:55 PM" and "It was just after sunset." changes to "5:20 PM." Do you want the jury to deal with precise statements of the time of an event, or statements containing general terms or phrases that are open to personal interpretation? Mr. McCarthy is an expert in the calculation of astronomical data with over 20 years of experience as a Marine Navigator including six years on the staff of the Nautical Almanac Office of the U.S. Naval Observatory in Washington DC. He recently retired from the U.S. Naval Service and is President of Astronomical Facts, a technical service company providing calculations of astronomical information. Contact Information