Understanding Earth's Rotation and Geographic Coordinate System

1. Myneni Lecture 03: Rotating Sphere Jan-26-05 (1 of 17) Further Reading: Chapter 03 of text book Outline - Introduction - Latitudes and Longitudes Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University - Map Projections - Time

2. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 03: Rotating Sphere Jan-26-05 (2 of 17) Earth as Rotating Sphere • Let us begin to lay the foundation for the first part of the course, which is - • Energy balance of the earth system • that is, What energy comes in, how it changes form, what goes out • The energy source for the earth is the sun. Therefore, we need to look at the earth-sun • “astronomical relationship” • We begin by looking at the earth as a Rotating, Orbiting Sphere • From this we will be able to answer many questions about the basic climate of the earth • - Why are there seasons? • - Why is there such a temperature difference between the equator and poles? • - What effect does this temperature difference have on the circulation of the • atmosphere and oceans

3. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 03: Rotating Sphere Jan-26-05 (3 of 17) Shape of the Earth • We begin by looking at the earth as a Rotating, Orbiting Sphere N • Approximately spherical • Actually an “oblate ellipsoid” • Slightly compressed from north to south • Slightly bulging from east to west • - But, we treat it as a sphere 12756 km 12714 km S The Blue Marble

4. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 03: Rotating Sphere Jan-26-05 (4 of 17) Great and Small Circles • Because earth is effectively a sphere, the geometry, • (meaning, how we define where we are on the sphere) • is more difficult than if the earth was flat. • We introduce two concepts for drawing lines on the surface - Great Circles Small Circles

5. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 03: Rotating Sphere Jan-26-05 (5 of 17) Parallels and Meridians • From these concepts we can draw systematic set of coordinates on the earth’s surface called • “Meridians and Parallels” • Parallels • Parallel to one another • - Intersect meridians at 90-degree angles • Meridians • - Not parallel to one another • - Intersect at the poles

6. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 03: Rotating Sphere Jan-26-05 (6 of 17) Geographical Coordinate System: Latitude • From these sets of lines, we can define a “geographic coordinate system” based on the relation • of our position on the globe to the fixed meridians and parallels Fixed Meridian Parallel Equator Latitude Latitude Position measured in degrees of arc (along a fixed meridian) from the Equator

7. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 03: Rotating Sphere Jan-26-05 (7 of 17) Geographical Coordinate System: Longitude Meridian Fixed Parallel Prime Meridian Longitude Longitude Position measured in degrees of arc (along a fixed parallel) from a fixed meridian Called the “Prime Meridian” - passes through Greenwhich, England and is defined as 0-degrees longitude

8. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 03: Rotating Sphere Jan-26-05 (8 of 17) Geographical Coordinate System: Example • Location of point P is: • 50 degrees North, 60 degrees West • So P would be located….?

9. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 03: Rotating Sphere Jan-26-05 (9 of 17) Maps and Projections • To make life easier, cartographers usually represent three-dimensional objects in two • dimensions using cartographic projection systems, that is, maps • But, such transformations introduce various • types of distortions. • - e.g., Polar Projection – areas towards the • edges appear larger! • Typically, selection of a projection requires • trade-off between direction preserving vs. • area preserving maps.

10. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 03: Rotating Sphere Jan-26-05 (10 of 17) Common Projections: Lat-Long Equal Angle, Un-equal Areas

11. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 03: Rotating Sphere Jan-26-05 (11 of 17) Common Projections: Mercator Preserves direction (equal meridians) but not scale (area)

12. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 03: Rotating Sphere Jan-26-05 (12 of 17) Common Projections: Goodes Preserves area but not shape

13. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 03: Rotating Sphere Jan-26-05 (13 of 17) Earth’s Rotation • Rotation of the earth produces “time” • We count time with respect to the position of the sun either over a: • - Fixed point, which is “solar time” - will discuss later • - Imaginary point, which is “standard time” - will discuss later • Remember, the Earth spins in a counter-clockwise direction when looking down on the • North pole (one revolution or 360 degrees defines 1 day)

14. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 03: Rotating Sphere Jan-26-05 (14 of 17) Solar Time Solar Time: time relative to position of sun over a fixed point Midpoint of the day (i.e. when the sun is highest overhead) called “solar noon” Midpoint of night (i.e. when the earth has rotated 180-degrees from solar noon) Sunrise and sunset = time when earth rotates into and out of illumination Sunrise Sun Solar noon Solarnight Sunset

15. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 03: Rotating Sphere Jan-26-05 (15 of 17) Problem with Solar Time • Define time at a point on Earth’s surface relative to passage of Sun • Angular rate of rotation = 360 degrees/ 24 hours • Therefore, 15 degrees/hour 11:00 am 15 West Solar noon 1:00 pm • Problem: Does not provide fixed/universal time system!! • Local time varies continuously with longitude

16. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 03: Rotating Sphere Jan-26-05 (16 of 17) Standard Time • Therefore, we create something called ‘Standard Time” • Define “time zones” - swaths of approximately 15-degree longitude where we • define time to be the same everywhere • - “Standard time” - time as defined by a given time zone • - “Standard meridian” - imaginary longitude whose solar time is defined to • be the standard time for an entire time zone

17. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 03: Rotating Sphere Jan-26-05 (17 of 17) U.S. Time Zones • US Time Zones • Eastern time ~ 75W • Central time ~ 90W • Mountain time ~105W • Pacific time ~ 120W • Note – within any time zone • Local solar time > standard • time E of standard meridian, • and vice versa “Daylight savings time” is a political construct which relates to an arbitrary selection of the time for a given time zone. Hawaii and Arizona follow Standard Time all year long.