Tonight's Sky
Sun
Sun
Moon
Moon
Mercury
Mercury
Venus
Venus
Mars
Mars
Jupiter
Jupiter
Saturn
Saturn

Searching...

° '
° '

# How can the Sun's and Earth's gravities provide a Lagrangian point on the side of Earth opposite the Sun? Don't their combined gravities equal each other only at locations between the objects?

Stephen Smith, Charlotte, North Carolina
RELATED TOPICS: LAGRANGIAN POINTS
The future James Webb Space Telescope will orbit the Lagrangian point on the side of Earth opposite the Sun. Our planet blocks the Sun’s radiation from the craft.
Northrop Grumman

You often hear about Lagrangian points because they can be handy places to “park” a spacecraft where it doesn’t need much fuel to stay in the same position relative to Earth. If gravity were the only force in the equation that defines these points, you would indeed expect there to be only one such point — between the planet and the Sun, where the gravities from these objects balance. But it’s a bit more complicated: The Lagrangian points also involve centrifugal force.

This is the force you feel on your arm when you swing a heavy object around in circles. Scientists sometimes call it a “fictional” force to distinguish it from a fundamental force like gravity. But to a craft orbiting in the solar system, it is very real.

In an ordinary Keplerian orbit like the path of Earth around the Sun, the centrifugal force simply balances the gravitational force from our star. But for a satellite orbiting near one of Earth’s Lagrangian points, the centrifugal force approximately balances the sum of the gravitational forces from the Sun and Earth. That situation can occur in five different places in the solar system. For example, on the side of Earth opposite the Sun, the gravitational forces from both objects pull together in the same direction. So a body orbiting with slightly more centrifugal force than an ordinary Keplerian orbit will find its centrifugal force balanced by the combined gravity of the Sun and Earth — at the second Lagrangian point.

Marc Kuchner
NASA’s Goddard Space Flight Center, Greenbelt, Maryland
0

#### JOIN THE DISCUSSION

Read and share your comments on this article
Comment on this article
###### Want to leave a comment?
Only registered members of Astronomy.com are allowed to comment on this article. Registration is FREE and only takes a couple minutes.

Login or Register now.
0 comments
ADVERTISEMENT
FREE EMAIL NEWSLETTER

Receive news, sky-event information, observing tips, and more from Astronomy's weekly email newsletter. View our Privacy Policy.

### THE MAGAZINE

ADVERTISEMENT
ADVERTISEMENT
Observing the night sky is a fun and easy activity that anyone can do, but getting started can be daunting for beginners.
Find us on Facebook