Libration of the Moon

May 31, 2014

Libration1730Tx (2)

Photographer: Rob Ratkowski
Summary Authors: Rob Ratkowski; Jim Foster

As Earth rotates on its axis, we're able to observe a little more of the Moon than we could if the Earth wasn't spinning -- up to about 59 percent of the lunar surface. This is referred to as libration. The libration of the Moon when rotated to the west exposes hidden eastern seas (mare). Maria Humboldtianum, Marginus, Smythii and Australe are exposed above (at right) showing a region of the Moon normally hidden from our view -- just beyond its eastern limb. Captured from Pukalani, Maui, Hawaii, these photos illustrate the most western libration of the past eight months. Note that longitude and latitude librations can reach, respectively, 7 degrees, 54 minutes and 6 degrees 50 minutes -- for a geocentric observer. These are due to eccentricity of the Moon's orbit and to the tilt of the Moon's rotation axis. By contrast, diurnal motion results in librations of less than 1 degree. Photos taken on June 16, 2013 and January 9, 2014.

Photo details: Nikon D700; ISO 400; 1/500 - 1/750 sec. exposure; taken thru a Celestron C9.25 OTA telescope; processed in Photoshop.