You are here


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

Martian Summer Heading into a long, moderate summer August 25, 2021

Jupiter at Opposition “Wandering” across the night sky August 19, 2021

Saturn at Opposition A superior showing for a superior planet August 1, 2021

Far From the Sun The changing rhythms of the Sun July 3, 2021

Moon and Planets Rounding off Jupiter’s orbit May 30, 2021

Beyond Neptune Denizens of the deep solar system May 26, 2021

Space Rocks Digging into the history of the solar system May 14, 2021

Close to the Sun Rounding out Earth’s orbit January 1, 2021

Mars Stands Still The Red Planet comes to a halt November 14, 2020

Mars at Opposition II Figuring out a “loopy” problem October 13, 2020

Sagittarius A high-speed ride ’round the galaxy August 15, 2020

Mars at Perihelion Mars gets close to the Sun August 3, 2020

Hunting by Gravity Using gravity to track down planets July 24, 2020

Saturn at Opposition II Planets head into reverse July 20, 2020

Close to the Sun Snuggling up to the Sun January 5, 2020

Getting Closer A small gift for the planet Venus December 24, 2019

Far From the Sun Changing the distance to the Sun July 3, 2019

Moon and Companions Planets that pump up Earth’s orbit September 12, 2018

More Moon and Venus Putting Venus in its place August 14, 2018

Mars at Opposition The Red Planet shines bright July 23, 2018

Saturn at Opposition Racing past three bright planets June 24, 2018

Moon and Aldebaran Moving the Moon across the sky January 26, 2018

Sun Time Clocking some early sunsets December 3, 2017

Far from the Sun Circling far from the Sun July 3, 2017

Alexis Bouvard Inklings of an eighth planet June 26, 2017

On the Move Racing across the universe June 10, 2017

A ‘Proximate’ Orbit Making three stars a family April 25, 2017

Moon and Regulus Sneaking up on the celestial lion — after this. February 10, 2017

Earth at Perihelion Soaking up some extra sunlight January 4, 2017

Moon and Planets Earth and the Moon move around January 3, 2017

Winter Solstice Winter and the laws of planetary motion December 20, 2016

Moon and Aldebaran Going nose to nose with the bull November 14, 2016

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

Featured Images

Juno at Jupiter

Keeping Track May 30, 2021

Summertime! October 8, 2019