I’ve been writing a lot about geostationary satellites lately. In December, SpaceX’s upgraded Falcon 9 rocket placed the SES-8 communications satellite into geostationary transfer orbit, and on Jan. 5, India’s Geosynchronous Satellite Launch Vehicle pulled off a similar feat with the GSAT-14 communications satellite.

As I wrote about the GSLV-D5 mission, I was tempted to include this standard informational line, punched directly out of the press kit:

The satellite was placed into a geostationary transfer orbit with a perigee of about 180 kilometers, an apogee of about 36,000 kilometers and an inclination of 19.3 degrees.

But unless you’re familiar with basic orbital mechanics, that sentence doesn’t have much meaning. What’s a transfer orbit? Is there a difference between geostationary and geosynchronous? Why is there such a wide range between the perigee and apogee?

For help explaining all of this, I turned to Mike Loucks of The Astrogator’s Guild. The Astrogator’s Guild uses a software package called STK/Astrogator from Analytical Graphics Inc. that is used to help spacecraft mission managers plan the trajectories of a variety of space missions. The software has been used on numerous NASA missions, including WMAP, LRO, LCROSS, New Horizons, Messenger, LADEE and MAVEN.

The first concept I want to tackle is the difference between a geosynchronous and geostationary orbit. Although these terms are often used interchangeably, they are not the same thing:

A Geosynchonous Orbit (GEO) takes a satellite around the Earth at a rate of once per day, keeping it roughly in the same area over the ground.

A Geostationary Orbit (GSO) is a geosynchronous orbit with an inclination of zero, meaning, it lies on the equator.

All geostationary satellites are geosynchronous. Not all geosynchronous satellites are geostationary.

Think of it like this: the “synchronous” part of geosynchronous describes the rate of the satellite’s orbit but says nothing about its inclination—the orbit’s angle with respect to the equator. A geosynchronous satellite with a non-zero inclination will trace out a figure eight in the sky as it dips above and below the equator.

The “stationary” part of geostationary describes how a satellite in this orbit remains fixed with respect to an observer on the ground. This is an ideal orbit for communications satellites, since ground-based antennas can remain pointed at the same spot in the sky.