AST 1002: "Other Information"

[Sections Taught by Prof. H.L. Cohen]

Site Map • Prof. Cohen • Department of Astronomy • University of Florida

Last updated October 22, 2002
Home
Page		 Course
Information		 Study
Guide		 Chaisson
Textbook		 Other
Resources		 Alachua
Astron. Club		 Undergrad.
AstroPhy. Soc.
Examples Figures Other Outlines PowerPt Tables
(The following article is an adaptation of an article that appeared in the October 2002 issue of FirstLight,
the monthly newsletter of the Alachua Astronomy Club, Inc.

Libration of the Moon

By Howard L. Cohen

The Moon's synchronous rotation causes the Moon to keep the same hemisphere turned toward Earth.
However, lunar librations allow us to see not 50% but 59% of the lunar surface

Llibration1 is a slow oscillation or wobble, real or apparent, of one body as viewed from another celestial body around which it revolves (from Latin, to oscillate).

Lunar librations are most familiar and allow observers from Earth to see more than one-half of a lunar hemisphere as the Moon orbits Earth.

The Moon very nearly keeps the same face (hemisphere) turned toward Earth. This occurs because the Moon's rotation period is synchronous with its revolution period around Earth. Synchronous rotation2 (or captured rotation) means the time for the Moon to spin once on its axis (about 27.3 days) is the same as its time to orbit once around Earth (about 27.3 days).3

However, about 59 percent rather than 50 percent of the lunar surface is visible from Earth due to libration.4 Causes of lunar librations are both physical and geometrical.

physical libration The Moon's rotation is not perfectly uniform so rotation and revolution are slightly out of sync causing a small libration (less than two arc min 5).

geometrical (optical) librations Three librations occur due to geometry (longitude, latitude and diurnal):

libration in longitude The Moon appears to swing side to side (east-west) by up to about 7.9 degrees in each direction. The Moon does not run at a constant rate in its elliptical orbit about the Earth. Because the Moon's orbital velocity is not constant, but its rotation is nearly so, the Moon's rotation is sometimes slightly ahead or behind its orbit position.

libration in latitude The Moon appears to nod up and down (north-south) by up to about 6.8 degrees in each direction. The Moon's axis of rotation is not perpendicular to its orbit plane.6 This allows observers from Earth to "peak" slightly beyond either lunar north or south limbs (edges) as the Moon orbits the Earth.

diurnal libration Observers on Earth can sometimes see slightly around the Moon's east or western limbs (by less than one degree) at different times of the day. This libration is thus caused by an observer's changing position relative to the Moon as the Earth spins (rotates). Thus, this libration is really a libration of the observer! The amount of diurnal libration depends on the observer's position and reaches its maximum value of about one degree at Earth's equator.

___________________
Foot Notes
  1. libration not libation!


  2. Synchronous rotation is a consequence of gravitational tidal action.


  3. Many people wrongly believe the Moon keeps the same face toward Earth because the Earth does not rotate. If the Moon did not rotate, all sides of the Moon would become visible during one orbit around the Earth. However, it is the Moon's synchronous rotation that keeps the same lunar hemisphere facing Earth.

    Note: A second common lunar myth holds the Moon has a perpetual dark side. It does not! The Moon has a day-night cycle as Earth except the Moon takes about 27.3 days to spin once while the Earth takes only about 23.9 hours


  4. The Soviet Union's Luna 3 Spacecraft first observed the "backside" or "far side" of the Moon in October, 1959! Until then, humans had never seen the 41% of the Moon's surface perpetually hidden from Earth.


  5. An arc minute (abbrev. arc min or ') is 1/60 of a degree.


  6. The Moon's axis tilt is about 6.7° from the perpendicular to its orbital plane. In contrast, the Earth's axis tilt is about 23.4° to its orbital plane.
Recommended web browser to view this site is Netscape Navigator 4.0 (or above) with video display supporting at least 256 colors.
High color (16 bit) or true color (24 bit) display is preferred.
Mailbox For comments and suggestions contact Howard L. Cohen at cohen@astro.ufl.edu
About this site (department disclaimer and recommended web browser)
© Copyright 1999-2003 Howard L. Cohen