|Lunar Limb Profiles from Occultation Observations|
During an occultation of a star by the Moon there is a specified relation between the position of the star, the observer and the Moon at the moment of the star's disappearance or reappearance on the lunar limb. Nowadays the positions of the stars and the Moon are known very accurately, so that a libration depending profile of the lunar limb can be derived from a sufficient number of occultation observations.
More than 300.000 observations...
348.750 observations of total and grazing occultations made worldwide in the period 1962 - 2003 were included into the MOONLIMB analysis:
First the theoretical distance of the star to the mean lunar limb for the moment of the occultation was calculated for each observation (= residual). For this the stellar positions from the Hipparcos catalogue in the ICRF system were used together with the lunar positions from the ephemerides DE/LE 405 created at the Jet Propulsion Laboratory.
After eliminating largely deviating observations the limb profile within an axis angle range of ±30° around the northern and southern lunar pole was derived from the residuals. The profiles are defined within a pattern of 323 combinations of longitude and latitude libration strips, where the residuals were averaged for a step width of 0.2° for the axis angle.
52.858 profile points on the northern and southern lunar limb were calculated from 48.435 observations. This is approx. 27 % of the maximum number of 193.800 profile points for the used pattern of librations and axis angles in both polar regions.
Compared to the previous version ML2002A the number of evaluated observations was enlarged by adding more recent observations. Besides this several new steps were applied to further improve the reliabilty and quality of the profile points. Due to a more strict assessment a larger number of observations was rejected from being used, so that the number of resulting profile points is lower.
Other lunar limb profiles
So far, the largest set of lunar limb profiles are the so called Watts Charts published by C. B. Watts at the US Naval Observatory (USNO) in 1963 and digitalized later at the RGO. Based on photographic exposures there are some significant gaps and errors, especially in the polar regions and beyond the poles.
A bit by bit improvement of the Watts Charts data in the polar regions is provided by the so called ACLPPP (Automatic Computer Lunar Profile Plotting Program), a project originally developed at the USNO and continuously upgraded by M. Soma at the Tokyo National Observatory. It is based on selected observations of grazing occultations.
MOONLIMB - ML2004B is now the most comprehensive set of lunar limb data based on occultations. It considers both grazing and total occultations and is much more extensive and consistent than the ACLPPP system. Both data sets largely correspond with each other, however there are some significant differences in a few cases.
MOONLIMB is an absolutely independant profile system. Its profile points are related to the mass centre of the Moon, whereas the Watts Charts and ACLPPP use the Moon's figure centre.
The improved lunar limb profiles permit a higher reliability of the grazing occultation predictions and graze observers have better chances to watch multiple contacts.
The new lunar limb data permit analyses of Baily Beads' observations during total or annular solar eclipses which are supposed to give important hints to the temporary variations of the solar radius.
During the last 20 years analyses of lunar occultations disappeared from the research programs of most professional astronomical institutions. Almost every prediction and analysis of occultations is now done by engaged amateur astronomers.
© 2006 by EVI Hummel
Last modified: Sun, Oct 29, 2006