Anthropologist - is a researcher who studies people, concentrating on their culture and sometimes on their history.

Arctic Circle - is a line of latitude at 66°33'N. On the shortest day of the year, the center of the sun does not rise above the horizon at this latitude.

Astronomical Polar Night - is a period of the arctic winter north of 84°33'N, when there is no trace of light in the southern sky, and even the faintest stars can be seen.

Astronomical Twilight - is the darkest period of evening twilight, before the only light in the sky is from stars and chemical reactions in the atmosphere. The Sun is over 12° below the horizon during astronomical twilight.

Atmosphere - is the thin blanket of gases around the earth,

Civil Polar Night - is a period of the arctic winter north of 72°33'N, when there is no civil twilight daily.

Civil Twilight - occurs while the Sun is below, but less than 6° below, the horizon. Most outdoor activities do not require artificial light during this period.

Equinox - is the time of year when the sun is overhead the equator and day and night are equal. Spring equinox is on about March 20 and fall equinox is on about September 22.

Geometry - is the arrangement of points, lines, areas, and volumes.

Horizon, or Skyline - is the line at which the earth and sky appear to meet.

Illumination - is the amount of light in the sky.

Lines of Latitude - are imaginary lines around the Earth, joining points of equal distance north or south of the equator Latitudes are given in degrees, indicating the angle between two lines drawn to the center of the Earth, one from the equator and one from the point of interest (see Fig. 5).

Light Intensity - refers to the amount of light in the sky. The sensitivity of the human eye to light varies as the intensity of light changes by powers of 10, i.e. by the logarithm of light intensity Logarithms measure the change by a factor often, so that, for example, as the light intensity increases a thousand times, the logarithm rises by three (ten x ten x ten).

Nautical Polar Night - is a period of the arctic winter north of 78°33'N, when the most light is a faint glow in the southern sky, but it is impossible for an observer to make out any horizon.

Nautical Twilight - occurs while the Sun is between 6° and 12° below the horizon. At the end of nautical twilight it is not possible to make out the horizon.

Northern Hemisphere - is the portion of the Earth which lies north of the equator.

Polar Night - is the period of winter acting more than 24 hours, when there is no twilight.

Poles - are the most northerly and southerly points on the Earth. Once each day the Earth spins around its axis, which is a straight line, through the Earth, between these poles.

Radiation - is electromagnetic energy which travels as a wave. Radios and TVs receive programs by radiation. Light is also transmitted by radiation.

Solstice - occurs when the tilt of the Earth is in line with the direction between the Earth and the Sun. Summer solstice occurs on the longest day of the year; winter solstice occurs on the shortest day.

Southern Hemisphere - is the portion of the Earth which lies south of the equator.

Twilight - is light from the sky which we receive each day just before the Sun rises or just after it sets. It is caused by the Sun's rays being reflected down from upper levels of the atmosphere.