{ Practical astronomy | Astronomy | The Moon
  | Physical ephemeris | Libration }

Libration

Libration of the Moon.


Libration of the Moon, to the naked eye.

On average, the Moon always shows the same side to Earth. Over millennia, the Earth's tidal forces have adjusted the lunar rotation period to equal its revolution period. That is to say, the Moon rotates in the same time as it revolves once around the Earth. It then shows the same side to the Earth at all times.

While this lock is precise on a monthly or long-term basis, there are deviations during parts of each month. This is called libration.

• While the rotation rate is constant in time, the speed of revolution changes during the orbit, fast at perigee, slow at apogee. This leads to a phase shift, parts of the month we see a bit more of the left side, the opposite part of the month more of the right side. This is libration in longitude and expressed in the physical ephemeris as a non-zero central meridian.
• The rotation axis of the Moon does not coincide with the pole of the revolution. Part of the month we see more toward the northern pole, in the opposite part more toward the southern pole. This is libration in latitude and expressed in the physical ephemeris as a non-zero inclination of the axis.
• Due to the Earth's rotation (the diurnal motion of the observer) the aspect on the Moon for a given observer changes slightly from moon rise through culmination and to moon set. This causes diurnal libration. If central meridian and inclination are calculated w.r.t. the topocentre rather than the geocentre, then diurnal libration is included in them.

Galileo – one of the first astronomers to point a telescope at the Moon – denied the existence of libration in longitude, perhaps because he considered the lunar orbit circular. On the other hand, observers before Galileo might have noticed the libration even without a telescope. The second pair of images has been smoothed and re-sampled to a resolution similar to that of the naked eye.

Physical parameters:

• Average distance: 384,000 km
• Radius: 1740 km
• Mass: 1/81 M_Earth
• Surface gravity: 1/6 g_Earth

Image parameters:

• Camera: Canon EOS 300D
• Detector: 22 × 15 mm
• Focal length: 800 mm
• Field of view: 1 × 1°
• Aperture: f/12.6
• ISO: 400
• Exposure: 5 ms