The applet is computing and visualizing the geocentric optical libration of the Moon.
The red dot is located at 0° longitude and 0° latitude (selenographic).
There are three types of libration:
(From Wikipedia, the free encyclopedia)
The parallactic angle (position angle) q of a celestial body (X) is the angle of the body's hour circle (XP) and its vertical circle (XZ), or between the declination circle (the path of the body) and the local horizon.
Apparent view of the Moon
Animation of lunar libration
Libration in latitude 2007.
The maximum value is:
1.54° (tilt of the Moon's equator to the ecliptic)
+ 5.15° (tilt of the Moon's orbital plane in respect to the ecliptic)
Libration in longitude 2007.
The libration in longitude creates an east-west displacement of +/- 8.0°.
The "total" libration is computed by SQRT(L*L+B*B).
The curves below are showing the motion of the 'zero-point' of the lunar globe around its mean position, due to the librations in longitude and in latitude (Select "Lib. Path Month" from the Details menu).
The curves repeat after a period of almost exactly 6 years, when the major axis has performed one complete revolution with respect to the line of nodes (cf. Meeus).
The 0ptical libration of the Moon from 2006, Jan 1 to 2009, Jan 1.
Select "Lib. Path Year" from the Details menu:
0ptical libration of the Moon 2006
0ptical libration of the Moon 2007
0ptical libration of the Moon 2008
0ptical libration of the Moon 2009
Select "Data Lib." from the Details menu:
Scroll down to see extreme librations in longitude.
The mean periode of the libration in longitude is the anomalistic month, 27.5545 days, the time between perigee passages. The result computed by the MoonLibration applet (2006-2024) is 27.526 days.
Diagram of minimum libration in longitude, 2006-2010.
The mean periode of the libration in latitude is the draconic month, 27.212 days, the time between node passages. The result computed by the MoonLibration applet (2006-2024) is 27.215 days.
|Jean Meeus: Mathematical
Willmann-Bell, Richmond, Virginia, 2nd Printing 2000,
Chapter 6: The librations of the Moon.
Jean Meeus: Astronomical Algorithms,
Willmann-Bell, Richmond, Virginia, 1rst English Edition 1991,
Chapter 51: Ephemeris for Physical Observations of the Moon.
modified: 2013, Feb 03