- 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 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.

** **

**Introduction****I. Location of features on the surface of a planet: latitude and longitude****II. Crater formation, modification, and removal****III. Crater Image Interpretation****Conclusion**

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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.

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