Backyard Astronomy


German Equatorial Mount Movement

Amateur astronomers accustomed to fork mounts or Dobsonians sometimes have trouble visualizing the movement and orientation of German equatorial mount (GEM) as it tracks across the sky. The photos on this page might help. They show a telescope on a Losmandy G-11 mount viewed from the north looking south. Red arrows show the movement direction at various points.

Looking Due East

Here the telescope is looking east at declination 0. The telescope is horizontal, the counterweight shaft is pointing downward, and the whole assembly is rotating slowly clockwise as the mount's RA axis tracks westward. As this rotation continues, the telescope "falls backward" on the west side of the mount, still looking eastward.

Looking Due South Before Meridian Flip

From the due-east position shown above, the telescope has tracked almost to the meridian, due south. The telescope is on the west side of the mount (still looking slightly east of south), and the counterweight shaft is horizontal on the east side.

As the left-pointing arrow at the bottom of the photo indicates, if telescope were pointing higher in the sky (it's still at declination 0) further rotation would cause the camera equipment hanging off the rear to collide with the concrete pier. At or near this point, the mount must perform a "meridian flip" to swap the position of the telescope and the counterweight so the telescope can continue to track the target without hitting anything. You (or automation software) must initiate this explicitly; the mount does not "know" when to flip.

Doing the Flip

During the meridian flip, the RA axis rotates counter-clockwise and the declination axis rotates the telescope through north. Here the scope has just passed through north, the counterweight is traveling upward on the west side of the mount, and the scope is moving toward the east side.

Looking Due South After Meridian Flip

After the meridian flip, the telescope is once again pointing due south at the same target as it was before the flip (the target transited the meridian while the mount was performing the flip). Notice that the telescope and counterweight have swapped positions. The telescope now is on the east side of the mount and the counterweight is on the west side.

As the arrows in this photo indicate, the mount continues to track westward (rotate clockwise) on the RA axis, with the telescope out of danger of hitting anything beneath the mount. As the rotation continues, the telescope ramains on the east side of the mount, looking westward.

Looking Due West

Tracking continues from the position shown above until finally we reach this position, where the telescope is looking due west at declination 0. The telescope is once again horizontal, and the counterweight shaft is pointing downward. Continued clockwise rotation from this point won't hurt anything, but the telescope then would be pointing below the horizon (or, by increasing the declination, would be looking northward).


Most mounts have provisions to define a "Park" position. This is my G-11's Park position. It was carefully chosen to ensure that all parts of the telescope are well below the roof structure when it is rolled closed, but also out of the way of people walking into the observing area.

Updated September 7, 2016