The motion of a massive body around another body, governed by the force of gravity. Planets in our solar system follow an orbit around the Sun, as first noted by Johannes Kepler, in the shape of an ellipse. An orbit is actually composed of two motions: one directly toward the other body (planet or star...) and the another that points away. In the case of a circular orbit, these two component motions are orthogonal (90 degrees apart, or at right angles). Einstein would say that an orbit is actually a geodisc in space -- time, formed by the interaction between space and the mass of the two bodies.

Radio Programs

Earliest Sunsets Stretching out the sunsets Monday, December 15, 2014
Mars at Perihelion Mars gets close to the Sun Saturday, December 13, 2014
Loopy Planet A planet gets a little loopy Sunday, April 27, 2014
Getting Close Snuggling close to two giants Friday, January 3, 2014
Moon and Jupiter Centering up a bright duo Wednesday, November 20, 2013
Mars and Regulus Wandering across the night sky Sunday, October 13, 2013
Juno at Earth Getting a boost from home Tuesday, October 8, 2013
Jupiter and Mars Jogging into the morning sky Sunday, July 21, 2013
Distant Sun Moving away from the Sun Friday, July 5, 2013
Moon and Planets Lining up the “evening star” Monday, June 10, 2013
Moon and Saturn A leisurely trek through the stars Wednesday, May 22, 2013
Moon and Saturn Sharing tracks around a giant planet Friday, March 29, 2013
Moon and Companions The Moon slides across the sky Sunday, March 17, 2013
Apophis Keeping an eye on a troublemaker Tuesday, January 8, 2013
Moon, Venus, Mercury Getting close to the Sun Monday, August 13, 2012
Distant Sun Slipping away from the Sun Wednesday, July 4, 2012
Zippy Planets Getting a big kick from a black hole Thursday, June 28, 2012
Racing Ahead Racing the Red Planet around the Sun Friday, March 16, 2012
Mars at Opposition Drawing a map of the solar system Saturday, March 3, 2012
Sirius B A close companion for a bright star Tuesday, January 31, 2012
Earth at Perihelion Snuggling close to the Sun Wednesday, January 4, 2012
Mars and Regulus Two old friends in the morning sky Thursday, November 10, 2011
Doomed Moons Recycling on a giant scale Saturday, September 24, 2011
ARTEMIS Teaching old spacecraft new tricks Monday, September 19, 2011
Earliest Sunrise Getting ready for some early sunshine Sunday, June 12, 2011
Urbain Leverrier A "calculated" planetary discovery Friday, March 11, 2011
More Moon and Jupiter Disappearing in the sunlight Monday, March 7, 2011
Upsilon Andromedae An out-of-whack solar system Wednesday, February 9, 2011
Mars in Conjunction Hiding in the bright sunshine Thursday, February 3, 2011
Earth at Perihelion Soaking up some extra sunlight Monday, January 3, 2011
Close Targets Grabbing a lingering visitor Friday, November 19, 2010
Asteroid Thruster Getting a push from day and night Thursday, September 2, 2010
Moon and Companions Drying out some lunar folklore Tuesday, July 13, 2010
Earth at Aphelion Our "eccentric" orbit around the Sun Monday, July 5, 2010
Mars and Regulus A planet speeds across the sky Sunday, June 6, 2010
Moon and Spica Following the rhythms of the night Tuesday, March 30, 2010
Moon and Venus Harmonizing with a neighbor world Tuesday, March 16, 2010
Simon Newcomb Stepping across the solar system Friday, March 12, 2010
Mercury in the Morning A planet with double-dip sunrises Saturday, January 23, 2010
Dodging a Bullet Earth dodges a cosmic bullet Friday, January 22, 2010
Backwards Mars Mars heads the wrong direction Sunday, December 27, 2009
More Moon, Mars, and Gemini Catching up to a neighbor Sunday, October 11, 2009
Jupiter at Opposition Speeding past a bright giant Thursday, August 13, 2009
Moving Right Along Speeding up our trip through the galaxy Wednesday, July 8, 2009
Earth at Aphelion A long snooze for the Sun Wednesday, July 1, 2009
L2 Getting away without losing touch Tuesday, April 7, 2009
Moon and Companions Hanging around with the lion Saturday, April 4, 2009
Shifting Seasons Charging away from the celestial ram Saturday, March 7, 2009
Moon and Venus The Moon scurries on Thursday, February 26, 2009
Kepler's Laws Adding up the orbits of the planets Thursday, February 19, 2009
Venus Stands High The "evening star" rides high Sunday, January 11, 2009

Featured Images

The Sun as seen from space
Close to the Sun Monday, January 3, 2011
Kidnapped? Friday, August 4, 2006


©2014 The University of Texas McDonald Observatory