Geographic Grid
Geographic grid: The series of latitudes and
longitudes that denote the unique physical
locations on the surface of the Earth.
Latitude
Latitude: the angular distance (arc-angle) of the
surface of the curved Earth north or south of the
Equator.
• Latitudes denote position on the Earth’s surface
relative to the Equator.
• Lines of latitude are called parallels; run as eastwest curved transects.
Equator: 0º latitude
• Serves as the reference position for all
latitudes (positions N and S).
• The imaginary great circle that lies
equidistance from the north and south
geographic poles.
• Bisects the Earth into the Northern and
Southern Hemispheres.
North Pole: 90º N
N
Arctic Circle: 66.5º N
Tropic of Cancer: 23.5º N
Equator: 0º
Tropic of Capricorn: 23.5º S
Antarctic Circle: 66.5º S
South Pole: 90º S
S
Notable Latitudes
Equator = 0º
North Geographic Pole = 90º N (+ 90º)
South Geographic Pole = 90º S (- 90º)
Tropic of Cancer = 23º30’ N (+ 23.5º)
Tropic of Capricorn = 23º30’ S (- 23.5º)
Arctic Circle = 66º30’ N (+ 66.5º)
Antarctic Circle = 66º30’ S (- 66.5º)
Geographic Poles: The points on the Earth at
which the Earth’s imaginary axis of rotation
emerges.
Earth’s rotation is from
West to East;
counterclockwise from
NP perspective
Longitude
Longitude: the angular distance (arc-angle)
of the surface of the curved Earth east or
west of the Prime Meridian.
• Longitude denotes position on the Earth’s
surface relative to the Prime Meridian.
• Lines of longitude are called meridians; run
as north-south curved transects
• All lines of longitude pass through the N
and S poles
Notable Longitudes
Prime Meridian (Greenwich meridian) = 0º
International Date Line = 180º
Prime Meridian 0º
Meridians
Latitude and Longitude
Earth’s Rotation
The Earth rotates on its axis every 24 hours—
completes one day-night cycle.
The Earth rotates on its axis from West to East
(clockwise from N pole perspective)
• The Sun rises at the eastern horizon, crosses
the sky, and sets at the western horizon.
Technically, the Earth rotates under the Sun allowing for
different parts of the Earth’s surface to be illuminated
(day) by the Sun and shadowed (night) facing away from
the Sun.
The Sun appears to move or traverse across the sky (from our
reference point), however, it is the Earth doing the moving.
Day-night sun cycle
•
•
•
•
•
•
•
•
•
Dawn
Sunrise
Morning
Solar noon
Zenith
Afternoon
Sunset
Dusk (twilight)
Night
Every 15º that the Earth rotates corresponds to
approximately 1 hour time difference between
corresponding position of the Sun in the sky.
Time Zones
• Time zones are global divisions of time based on
sequences of hour divisions.
• The Earth completes one rotation every 24 hours.
• Therefore, the Earth rotates 15º of longitude per
hour.

360 15

hr
24 hr
Time zones are first divided into ~ 15º longitude intervals
centered at multiples of 7.5º, then adjusted for international
political boundaries (small countries) or internal political
boundaries (larger countries).
The Continental United States (lower 48 states) has
four time zones.
• Eastern Time
• Central Time
• Mountain Time
• Pacific Time
Time Zones
The Continental United States (lower 48 states) has four time
zones.
• Eastern Time
• Central Time
• Mountain Time
• Pacific Time
Georgia lies at the westernmost edge of the Eastern Time
Zone.
Alabama lies at the easternmost edge of the Central Time
Zone.
Time Zones
Remember, the Earth rotates from west to east.
Eastern Time Zone is
5 hours behind the Mean Greenwich Time (London)
1 hour earlier than Central Time (e.g., Chicago)
2 hours earlier than Mountain Time (e.g., Salt Lake City)
3 hours earlier than Pacific Time (e.g., Los Angeles)
If it is 3 pm in Georgia,
= 8 pm London, = 2 pm Chicago, = 1 pm Salt Lake City, = 12
pm Los Angeles.
The Sun is the center of our solar system.
Heliocentric model.
All planets and small bodies orbit the sun
(revolve) clockwise as seen from the “top down”
view.
The Earth rotates 1 time per 24 hours (observed day-night
cycle)
Earth revolves around the sun 1 time per 365.24 calendar
days (observed year cycle)
The moon
revolves around
the Earth 1 time
every 27.8 days.
The Earth’s orbit lies in the plane of the ecliptic.
Plane of the ecliptic--the imaginary plane that
extends outward from the equator of the sun.
Earth’s orbit is an ellipse, not a perfect circle. The Earth
is sometimes closer to the sun and sometimes farther
from the sun.
Average distance: 150,000,000 km
Perihelion
1.477x108 km
~ Jan 3
Aphelion
1.521x108 km
~ July 4
The Earth has axial tilt (obliquity). The Earth is tilted by
23.5º from upright relative to the plane of the ecliptic
Zero Axial Tilt
No obliquity
23.5º
Axial Tilt = 23.5º
The line through the Earth is the Equator
The Earth has constant axial parallelism or polarity
regardless of it’s position around the sun. The North Pole
and the South Pole point in the same direction.
Notice that different parts of the Earth’s surface receive
different amounts of sunlight over the course of the
calendar year. This is called seasonality.
Vernal equinox
Winter solstice
Summer solstice
Autumnal equinox
Where is the Overhead Sun?
Mar 21
June 21
Sept 22
Dec 22
Vernal equinox
Summer solstice
Autumnal equinox
Winter solstice
Equator
Tropic of Cancer
Equator
Tropic of
Capricorn
During the equinoxes (equal day)
• Overhead sun over the equator
• All places on earth have 12 hours of light and 12
hours of dark
During summer solstice
• Overhead sun over the Tropic of Cancer
• All latitudes inside the Arctic circle = 24 hours
light
• All latitudes inside the Antarctic circle = 24 hours
darkness
During winter solstice
• Overhead sun over the Tropic of Capricorn
• All latitudes inside the Antarctic circle = 24 hours
light
• All latitudes inside the Arctic circle = 24 hours
darkness
The declination = latitude of the subsolar point. The sun’s
altitude above the horizon is based on latitude of the observer
relative to the subsolar point.
Diagram showing the distribution of incoming
solar radiation during the Summer and Winter
Solstices.