Introduction to Earth’s Solar System Setting and Latitude and Longitude

 

I. The Earth in its Solar System setting

A. The objective of this introduction is to give you a frame of reference in which to consider your study of Earth and to give you some key facts and concepts about the nature of the Earth and the forces acting on it.  I'll also introduce you to some terms such as longitude, latitude and equator that will be used again and again when the Earth's oceans are described.

B. Our Solar System was formed about 4.6 billion years ago when a huge rotating cloud of interstellar gas began to collapse. Our Sun, a star, then began to form at the center of this disk-shaped cloud.  Continued collision of particles within the outer portion of this cloud led to the formation of ever larger particles, which eventually formed the planets. After a few million years the Sun was orbited by 9 planets.

C. The planets in our solar system revolve around the Sun from east to west in nearly the same plane forming a system with a diameter that may exceed 14 X 1012 kilometers. The Sun, like all other stars, is a hot, shiny ball of gas (mainly H and He). It is the main source of the solar system's light and heat. Its gravitational force holds the system together.

D. A planet is a celestial body that appears to wander about the sky, in contrast to the seemingly fixed stars. This appearance results from the planets orbiting around their suns.    

1. There are 9 planets in our solar system and in order of distance from the Sun the Earth is the third. The first four planets (Mercury, Venus, Earth, Mars) are called the terrestrial planets because of their similarity to Earth in size, density and composition.  The other five planets are Jupiter, Saturn, Uranus, Neptune and Pluto.

2. Like the other planets except for Venus, Uranus, and Pluto, the Earth rotates on its axis from west to east.

E. Some time during the first few hundred million years after the Earth formed, events occurred which raised the Earth's temperature enough to melt Fe and Ni which then migrated towards the center of the Earth. This migration generated heat and displaced lighter substances upwards.  As lighter materials continued to move upwards and out onto the surface they cooled and solidified. The Earth emerged from this period completely reorganized and differentiated into a layered system we'll discuss in more detail later.

F. Sometime after this first few hundred million years of Earth history the Earth cooled enough for light materials such as water to condense and remain at the surface to form the oceans. These and other light materials outgassed from the interior of the Earth were also the source of Earth's atmosphere.

II. Shape of the Earth

A.  The shape of the Earth is the same as that of the other planets that is spherical. This shape is largely due to the force of gravity that encourages material to flow downhill and causes irregularities to be smoothed out.  Under the influence of gravity, the Earth, like other astronomical bodies pulled itself into a spherical shape soon after it formed. However, due to the fact that it spins it has a slightly larger radius at the equator than at the Poles. This is referred to as the equatorial bulge.

1.  Polar radius  =  6357 km

2. Average radius =  6371 km

3. Equatorial radius =  6378 km

B. The highs (mountains, Everest = 8.8 km) and lows (Oceanic trenches, Mariana trench = 11km) on the surface of Earth are actually minor features when compared to the size of Earth itself.

 

Latitude and Longitude

 

I. During the course of the semester I'll often be talking about different parts of Earth by refer­ring to their latitude, because the latitude of a particular region is largely responsible for determining its climate. Therefore, I'm going to spend some time talking about the system of latitude and longitude by which we locate the positions of various features on a spherical object such as the Earth. You are also starting your study of maps in 1501 lab and I want to give you a little different way of thinking about longitude and latitude that might help you as you work on topographic maps.

 

II. Basic terminology and reference points

A. The equator is the imaginary circle that can be traced on the surface of the Earth equidistant from the poles.

B. The poles are the points at which Earth’s imaginary axis of rotation emerges at the top and bottom of the Earth. Earth rotates around its axis of rotation from west to east.

 

III. Rationale for using latitude and longitude

A. If the Earth were flat it would be a simple matter to lay off a series of grid lines, perpendicular to each other and equally spaced, to help us locate our position. This system is like the one you use for graphing in mathematics.  The “origin”, or reference point, could be located in the lower, left-hand corner of our flat Earth and we could draw a series of horizontal and vertical lines to serve as the grid lines.  By numbering these lines consecutively and assigning a fixed distance to the space separating them, we could determine any position on the flat Earth as shown in class. That is, we could assign a set of coordinates to each location.

 

B. To make the example more like the actual coordinates used in some mathematics and on maps, we could expand it so that there were 4 quadrants instead of one (i.e., NE, SE, NW, SW) and the origin, or (0,0) point could be located at the center.

 

C. If this grid system were on transparent cloth and you tried to wrap it around the spherical Earth, it wouldn't fit, and there would be places where the grid lines would overlap.  So for the Earth we need a different system. However, we would like to preserve the most useful features of the system just described:

1. A reference point that will be assigned a distance value of “0”.

2. Grid lines

3. Equal spacing of grid lines making it easy to calculate distances.

 

IV. The system of latitude and longitude

 

A. On the spherical Earth, consider the E/W grid lines as a series of circles. (It may help you to imagine passing a plane through the Earth, parallel to the equator, and noting the shape created by the intersection points of this plane with the surface of the Earth) Set one circle (it will be the largest) at the Earth's widest point, the equator, and designate this the zero line.  Since the Earth is approximately a sphere, this largest circle can be divided into 360o, like all circles can. A half circle is 180 o, a quarter circle is 90 o, and so on.

 

B. If we drew 90 circles parallel to our largest circle and if these circles were separated by equal distances, could we calculate how far apart these circles (these are the parallels or lines of latitude) would be?

1. We can approximate the Earth as a sphere, which has a circumference of 24,902 miles. Any plane cutting through the center of the Earth and intersecting the surface of the Earth would form a great circle with a circumference of 24,902 miles.  A quarter of that circle, or 360 o /4 = 90 o, would have an arc length of 24,902/4 = 6225 statute miles. So, 90o or ¼ the distance around Earth is 6225 miles.(Remember, the “arc length” is the distance you walk around the circumference of the circle.

 

a. If 90 o  = 6225 miles then 1 o  = 69 miles.

2. Now 69 miles is a difficult number to work with, especially when you consider that early navigators didn’t have calculators. Also, from your study of mathematics you will remember that degrees can be divided up into 60 minutes, and each minute can be divided into 60 seconds.  So map makers and navigators came up with a new measurement of length called the nautical mile.  In this case, a degree is equal to 60 nautical miles, so each minute of arc is equal to 1 nautical mile. A nautical mile is larger than a statute mile by about 800 feet.

a. 1 degree  =  60 nautical miles  =  60 min. of arc

 

C. Another way to think of latitude is merely as the angular distance of a given point from the equator, either north or south.

 

D. By means of latitude we can fix the location of a point north or south of the equator.

1. The circumference of a circle of latitude gets smaller as you approach the poles, but all these circles are parallel.

E. Analogously to latitude, longitude is the angular distance of a point east or west of what is called the Prime Meridian that runs through Greenwich, England. (The Prime Meridian is a Great Circle, just like the equator, only it is oriented north-south. The equator is the only Great Circle oriented east-west, whereas all the meridians, or circles of longitude are Great Circles.)

 

1. Longitude is a bit more complex because, unlike the equator, there is no obvious choice for a reference point.  All the Great Circles running from N to S around the globe have the same circumference.  After many decades of disagreement, the Great Circle running through Greenwich, England was arbitrarily chosen as the zero line of longitude. 

2. One of the main differences between meridians and parallels is that the meridians converge north to south, while the parallels remain equidistant.  This means, in practical terms, that a degree of longitude at the equator is 60 nautical miles, however, at 45o north latitude that same degree of longitude is closer to 30 nautical miles, because the meridians or longitude lines are converging.