12-Hour Format
The time format used in the USA.  This format uses AM and PM designations for morning / day and afternoon / night.  (e.g., 10:00AM day,  10:00PM night)

24-Hour Format
24-hour time is widely used in scientific work throughout the world.  In the United States it is used by the Armed Forces.  In Europe it is frequently used by transportation systems.  The time of day starts at midnight and goes from 00:00 to 23:59.   (e.g., 10:00 day,  22:00 night)

Altitude (ALT)
The height of an object above sea level.  This program gives altitude readings for the sun’s position during the day.  You should use a Clinometer to accurately measure these angles.

Azimuth (AZ, Bearing)
The angular distance of the horizon measured from “True North” to a position or object.  Measured in degrees from the North, clockwise to 360 degrees.  This program gives azimuth bearing for the sun’s rise, set and positions during the day.  You should use a compass to accurately measure these bearings.  If the “Magnetic North” option is selected in the Preferences section, this program will give azimuth bearings from “Magnetic North.”

Bar Scale
The proportion between a distance on the map and the actual distance in the field.  Use the Map’s Bar Scale to determine the distance on a map.

Bearing
Direction, especially angular direction measured from one position to another using geographical or celestial reference lines.  A bearing used by sunPATH is the horizontal angle in degrees from North.  North being zero degrees on a compass with a clockwise scale of 360 degrees.

Clinometer
A device that measures the angular distance in degrees from the horizon to an object above the horizon.  This device that looks similar to a compass, is needed for accurate measurements of an object’s height above the horizon.  (e.g., height of the sun)

Compass
A device used to determine geographic direction, usually consisting of a magnetic needle horizontally mounted or suspended and free to pivot until aligned with the magnetic field of the Earth.

Date Range
Start date and end date for the desired range of sunrise and sunset calculations.

Dawn  (as used by sunPATH)
The time each morning at which daylight first begins, the period of increasing light between dark and sunrise.  This program calculates the approximate point in time when there should be enough light from the sky for an exposure (T1.3 @ 500ASA).  This is considered by some as “Magic Hour.”  Terrain features and weather conditions can limit the available skylight.  (This is not astronomical, civil or nautical twilight.)

Day Length
(Daylight hours)  The length in hours and minutes that the sun is above the horizon.

Daylight Savings Time (DST, D.S.T.)
Time during which clocks are set one hour or more ahead of Standard Time to provide more daylight at the end of the working day during late spring, summer, and early fall.  Daylight Savings Time in other countries varies from that of the USA.  Most countries near the equator do not deviate from Standard Time.  (Also called Summer Time, Daylight Time or Advanced Time)

Declination
(See Magnetic Declination)

Degrees
A unit of latitude or longitude, equal to 1/360 of a great circle.

Deviation
A false reading from the compass due to outside forces.  Steel and Iron objects as well as electric circuits close to the compass may cause deviation.  Large structures like buildings will cause deviation at some distance.  When using the compass, keep it well clear of metallic objects (knifes, wristwatches, light meters, lighters, steel rimmed eyeglasses and belt buckles) they can cause serious errors.

Dusk (as used by sunPATH)
(Twilight) The evening period of waning light between sunset and dark, when the sun is below the horizon. This program calculates the approximate point in time when there should be enough light from the sky for an exposure (T1.3 @ 500ASA).  This is considered by some as “Magic Hour.”  Terrain features and weather conditions can limit the available skylight.  (This is not astronomical, civil or nautical twilight.)

GMT (Greenwich Mean Time)
The mean solar time of the meridian of Greenwich, used as the prime basis of standard time throughout the world.  Greenwich, England.  Also known as Universal Coordinated Time (UTC) and Zulu time.

GPS (Global Positioning System)
A receiver that uses signals from GPS satellites to determine your position on earth.

Latitude
The angular distance measured North or South of the earth's equator, expressed in degrees, minutes, and seconds.  Measured by means of parallels that are imaginary circles parallel with the equator.

Location
A city or place where sun calculations are desired.

Longitude
The angular distance measured East or West from the prime meridian at Greenwich, England, to the meridian passing through a position, expressed in degrees, minutes, and seconds.  Measured by means of meridians that are imaginary lines drawn around the earth through the poles.

“Magic Hour”
"Magic Hour" is very subjective and depends on the shot.  It may start when shadows come over your location, which could be twenty minutes before the sun actually sets over the "True Horizon."  If you are shooting into the sky, it may last twenty minutes after the sun sets.  With sunPATH you can make a “guesstimate” on when you will “lose the light“ (no longer have enough light for a "normal" exposure).

Magnetic Declination (Magnetic Variation)
In most places compasses do not point to True North they will point toward Magnetic North. The difference  between True North and Magnetic North is the Magnetic Declination.  The magnetic North Pole moves slowly therefore the magnetic declination changes slowly from year to year, but not enough to be of concern for our purposes.  For example at present the change in the contiguous 48 states varies from a minimum of 5’ (minutes) E or W per year (meaning it’ll take 12 years to change the declination by one degree) to a maximum of 12° W (meaning it changes one degree in five years).  Hawaii, where the declination is 11°E to 12°E, has virtually no annual change now.  (Sailors use the term “variation” for Magnetic Declination.)

Allowing for Declination:  If your Preferences have been set for Magnetic North, all the calculations made by the program will make the necessary adjustments for the magnetic declination.  This allows you to take direct reading with your compass without any compensation.  If you select True North in the Preferences, you must make declination corrections on your compass for a true reading.

Magnetic North
The direction of the earth's magnetic pole, to which the north-seeking pole of a magnetic needle points when free from local magnetic influence.  If you are using a magnetic compass to read the bearings on the horizon, you should choose Magnetic North from the Preferences menu.  You may then use the program’s bearings directly on your compass without correction for the magnetic declination.  (also called compass bearing)

Meridian
An imaginary great circle on the earth's surface passing through the North and South geographic poles. All points on the same meridian have the same longitude.  The Greenwich meridian is 0° longitude.

Minutes
 A unit of angular measurement equal to one sixtieth of a degree, or 60 seconds. Also called minute of arc.

Standard Time (ST, S.T.)
The time in any of 24 time zones, usually the mean solar time at the central meridian of each zone. In the continental United States, there are four standard time zones: Eastern, using the 75th meridian Central, using the 90th meridian Mountain, using the 105th meridian and Pacific, using the 120th meridian.

Sun’s Position (Altitude and Azimuth)
The center of the sun is at this Altitude and Azimuth for the stated time of day.  The position calculations for sunrise and sunset are for the upper edge of the sun passing through “true horizon,” accounting for standard atmospheric refraction at sea level, ignoring terrain features.  The azimuth and altitude calculations during the day are for the center of the sun.

Sunrise
The time of day the sun’s apparent upper edge first appears at the eastern horizon (“true horizon”).  All horizons are considered at sea level. This program calculates the time and azimuth when the TOP of the sun first appears at the “true horizon,” accounting for standard atmospheric refraction at sea level and ignoring terrain features.  (The azimuth and altitude calculations during the day are for the CENTER of the sun.)

Sunset
The time of day the sun’s apparent upper edge disappears below the western horizon (“true horizon”).  All horizons are considered at sea level.  This program calculates the time and azimuth when the TOP of the sun finally disappears at the “true horizon,” accounting for standard atmospheric refraction at sea level and ignoring terrain features.  (The azimuth and altitude calculations during the day are for the CENTER of the sun.)

Time Zone
Any of the 24 longitudinal divisions of the Earth's surface in which a standard time is kept, the primary division being that bisected by the Greenwich meridian. Each zone is 15° of longitude in width, with local variations, and observes a clock time one hour earlier than the zone immediately to the East.  Standard Time maintained in many countries does not coincide with the zone time.

True Horizon
A “true horizon” is the equivalent of the sea level horizon on the coasts.  All sunrise and sunset times are based on a “true horizon.”  The altitude measurements are from a “true horizon” or sea level.

True North
The geographic North Pole, from which all meridians radiate, gives the direction of True North anywhere on Earth.

Universal Coordinated Time  (UTC)
Also known as GMT or Zulu time.