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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

Moon and Companions Circling through the stars March 20, 2016

Moon and Venus Tracing a path for the morning star March 6, 2016

Moon and Mars A neighbor world gets brighter February 28, 2016

Delta Cygni Spreading out a stellar trio July 30, 2015

Earth at Aphelion Stretching out the summer July 4, 2015

Living in Circles The intelligent choice for planetary systems June 9, 2015

Moon and Leo “Weighing” two giant stars May 23, 2015

Close to the Sun Snuggling close to the Sun January 2, 2015

Earliest Sunsets Stretching out the sunsets December 14, 2014

Mars at Perihelion Mars gets close to the Sun December 12, 2014

Loopy Planet A planet gets a little loopy April 26, 2014

Getting Close Snuggling close to two giants January 2, 2014

Moon and Jupiter Centering up a bright duo November 19, 2013

Mars and Regulus Wandering across the night sky October 12, 2013

Juno at Earth Getting a boost from home October 7, 2013

Jupiter and Mars Jogging into the morning sky July 20, 2013

Distant Sun Moving away from the Sun July 4, 2013

Moon and Planets Lining up the “evening star” June 9, 2013

Moon and Saturn A leisurely trek through the stars May 21, 2013

Moon and Saturn Sharing tracks around a giant planet March 28, 2013

Moon and Companions The Moon slides across the sky March 16, 2013

Apophis Keeping an eye on a troublemaker January 7, 2013

Moon, Venus, Mercury Getting close to the Sun August 12, 2012

Distant Sun Slipping away from the Sun July 3, 2012

Zippy Planets Getting a big kick from a black hole June 27, 2012

Racing Ahead Racing the Red Planet around the Sun March 15, 2012

Mars at Opposition Drawing a map of the solar system March 2, 2012

Sirius B A close companion for a bright star January 30, 2012

Earth at Perihelion Snuggling close to the Sun January 3, 2012

Mars and Regulus Two old friends in the morning sky November 9, 2011

Doomed Moons Recycling on a giant scale September 23, 2011

ARTEMIS Teaching old spacecraft new tricks September 18, 2011

Earliest Sunrise Getting ready for some early sunshine June 11, 2011

Urbain Leverrier A "calculated" planetary discovery... March 10, 2011

More Moon and Jupiter Disappearing in the sunlight March 6, 2011

Upsilon Andromedae An out-of-whack solar system February 8, 2011

Mars in Conjunction Hiding in the bright sunshine February 2, 2011

Earth at Perihelion Soaking up some extra sunlight January 3, 2011

Featured Images

The Sun as seen from space

Close to the Sun January 3, 2011