Earth Science
Maps, Globes, Latitude and Longitude
Finding Your Location on a Sphere
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Coordinate Systems
When you are locating a point on a flat surface you can use Cartesian coordinates of x and y.
The point 2, 3 is plotted on the graph.
x-axis
y-axis
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Coordinate System for circle: Angles
For curved surface: sphere or circle, we use angles and reference points
One of our reference points is the center of the Earth.
A second reference is a line around
the middle of the earth. This is called
The equator
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The first coordinate: Latitude
Latitude is defined as the elevation of the location point above (or below) the equator.
The measurement is expressed in degrees (and, minutes, and seconds but for this class we won’t worry about that- Google Earth gives them however)
The equator is at 0°
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North Pole is at 90 degrees N (like a right angle to the equator)
North Pole
South Pole (-90 or 90oS)
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The Equator divides the Earth into
Northern Hemisphere and the Southern Hemisphere.
Reference point for latitude: The equator
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The word “hemisphere” means “half of a sphere”
Latitude
If the location point is north of the equator an N or + is added to the degrees.
If the point is to the south of the equator an S or – is added to the value.
52°N or +52°
24°S or -24°
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From 90N to 90S 180 total degrees of latitude on a globe
Latitude
On the Earth we designate lines of equal latitude as parallels.
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2nd coordinate: East vs. West
Longitude is defined as the measurement of the location point east or west of the Prime Meridian
The value is also expressed in degrees, minutes, and seconds.
Note: There are 60 minutes in a degree and 60 seconds in a minute.
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Reference for longitude
The Prime Meridian divides the Earth into the Western Hemisphere and the Eastern Hemisphere.
http://www.arcticice.org/images/long.gif
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Longitude
If the point is located to the west of the Prime Meridian we add a W to the value.
If the point is located east of the Prime Meridian we add an E after the value
30°W
80°E
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Longitude
On the Earth we designate lines of equal longitude as meridians.
The Prime Meridian has a value of 0° and no E or W. The longitude values increase up to 180° in both directions.
Longitude is trickier
Since it’s a full 360 degrees, it goes behind the earth
W E
E W
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Longitude is trickier- why?
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What’s wrong with this map?
(see demo + youtube to illustrate)
Mercator Projection
The Problem with maps
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https://www.youtube.com/watch?v=CPQZ7NcQ6YQ&feature=youtu.be
Compare appearance of Greenland vs. Africa on Mercator vs.
Google Earth
Compare Greenland to Africa in undistorted geometry
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This is better (“Robinson projection”)- why?
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What problem does this introduce?
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Equirectangular- meridians are straight.
Has area distortion- less than Mercator
Equal Earth projection- areas are
closest to reality. Meridians curved
Where are the North and South Poles???
Other flat projections
Longitude on maps
Either 0-180 E and W, or just 0-360
always adds up to 360o
Example: Longitude for MD is either:
-75o or 75oW or 285o
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Other bodies in the solar system have their own maps- same
coordinate system
Moon Map (0-180 from both directions)
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To 90E
To 90W
Back side
To 180
To 180
Lunar prime meridian
A Mars map: 360 degrees of longitude
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180 120 60 0 300 240 180
Longitude and Time
When the Sun crosses the meridian that pinpoints your location we say that the time is “local noon”. Shadows are shortest at this time.
http://www.nmm.ac.uk/upload/img/shadow-stick.gif
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Longitude and Time
Local noon poses a problem for travelers because it is noon at different times in different locations. That means that timetables are useless.
To solve this problem we have fixed time zones
Referenced to the Prime Meridian also called the Greenwich Meridian (since it passes through the Royal Observatory in Greenwich, England).
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Longitude and Time
If you place meridians 15° apart starting with the Prime Meridian, you will divide the Earth into 24 zones. These correspond to the 24 hours in a day.
http://www.mapsofworld.com/time-zone-map/maps/world-time-zone.jpg
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Longitude and Time
As you travel around the world to the east, you move your clock/watch ahead 1 hour in time for each zone you cross. If you travel to the west you move your clock/watch back 1 hour for each time zone.
http://www.astro.ufl.edu/~oliver/ast3722/lectures/CoordsNtime/timezon2.gif
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But…
If you traveled around the world to the east fast enough you would be a day older in a short time!
If you traveled west you could go back in the past!
To solve this problem it was agreed that the 180° meridian would signal the change of date point.
Travel east and the date goes back one day; travel west and you advance a day when crossing this meridian.
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But…
To keep all of a country in the same time zone, the International Date Line does not follow the 180° meridian exactly.
Other time zones make the same accommodations for country or state boundaries.
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How the date line works
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East coast to west coast: 3 hours time
zone difference.
Longitude of DC = 77W.
Longitude of San Francisco = 122W
Difference = 45 = 3 x 15 makes
sense!
Where are We?
Lat, Lon- Practice #1
First question: what is the interval in latitude and longitude
Lat, Lon- Practice #2
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Answers:
Interval is like spacing: here, 10 degrees in both lat and long. Other maps will differ- have to read the labels
Point 1 is 37°N, 98°W
Point 2 is 14°S, 25°E
Then: what is a likely time zone difference between point 1 and
point 2?
To calculate time zone, remember 15° of longitude is about 1 hour
First: need to calculate how many degrees of longitude separate the
two points above. What is the difference in longitude?
98W is like -98. So 25 - -98 = 25+ 98 =123 degrees total
123/15 = 8.2 so 8 hours is likely. Check slide 25.
Summary
Reference lines:
Equator,
Prime Meridian
International Date Line
Latitude lines for N/S latitudes
Meridians for E/W longitudes
Latitudes go -90 to +90
Longitudes go -180 to +180 or 0-360
Time Zones: each hour is about 15 degrees of long.
Problems with flat map projections (lab/hwk)
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