### I. LATITUDE AND LONGITUDE

Concepts:

• Geography
• Coordinate systems
• Origin
• Angles
• 2-d and 3-d geometry

In order to discuss features on the surfaces of planets, we first need a way to describe where they're located. On the Earth, we use a system of latitude and longitude, lines which divide up the spherical surface of the Earth into a grid, based on angles.

Latitude represents how far north or south of the equator a point is. The latitude of a point is the number of degrees in an angle made by the equator, the center of the Earth, and the point. The latitude of Tucson [or insert your city here], for example, is 32 degrees north, meaning that Tucson is 32 degrees north of the equator. The latitude of points on the equator is 0 degrees (north or south).

While with latitude, the obvious place on Earth from which to measure north or south is the equator, there is no similarly obvious choice from which to measure east or west. An imaginary line passing through Greenwich, England was arbitrarily defined, and longitude represents how far east or west a point is from this line. So Tucson, at longitude 111 degrees W, is therefore 111 degrees west of Greenwich, England.

The origin of a coordinate system is the starting point for measurements. The origin for the latitude-longitude coordinate system on Earth is where the equator and the line through Greenwich, England, meet, at zero degrees latitude, zero degrees longitude.

Interpretation:

• Given a map or globe, find the origin (0 degrees,0 degrees).

• Given a list of cities and their latitude and longitude, find them on a globe.

• What is the latitude of the north pole? the south pole? What is the longitude of the north pole?

• Trace a circle of constant latitude on a globe.

Q: Where is such a circle the largest?

A: It is largest at the equator.

Q: What is the radius of a circle of constant latitude at the equator?

A: It is the radius of the Earth.

• Trace a circle of constant longitude.

Q: Where is such a circle largest?

A: They are all the same size.

Q: What points do all such circles go through?

A: They go through the north and south poles.

• Such a circle on the surface of the Earth, which goes through the equator or the north and south poles, is called a great circle.

Scientific context: A latitude-longitude system is fundamental for the reliable location of surface features. With an origin, any place on a planet can be located with only two numbers: how many degrees north or south of the origin, and how many degrees east or west of the origin. The hard part is choosing an origin. On other planets, the zero of longitude is chosen once the surface of the planet has been mapped in enough detail to pick one. On some planets, the origin is defined as the point on the equator known as the "sub-Earth" point, which is the point on the surface that faces the Earth at the time when the two planets are closest in their orbits. On other planets, the choice is much more arbitrary. The most important criterion is that the origin must be a point that is easy for everyone to find, whether it's the center of a crater or some other obvious permanent geologic feature.

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This module was written by Cynthia Phillips, Dept. of Planetary Sciences, University of Arizona, Tucson AZ, and funded in part by the NASA Spacegrant program.

Galileo Solid State Imaging Team Leader: Dr. Michael J. S. Belton

The SSI Education and Public Outreach webpages were originally created and managed by Matthew Fishburn and Elizabeth Alvarez with significant assistance from Kelly Bender, Ross Beyer, Detrick Branston, Stephanie Lyons, Eileen Ryan, and Nalin Samarasinha.

Last updated: September 17, 1999, by Matthew Fishburn