The graph represents the projection of the lunar limb on the surface of the Earth at different times.
The event will be observed from all points located on the limb at the time associated to that limb.
The limb drawn in red is called "central limb", and its associated time is indicated in the text box,
also in red (this particular limb is just a reference limb and does not enjoy any special significance).
The temporal separation between two consecutive limbs is plus or minus 5 minutes. The sign can be
guessed if we bear in mind that the Moon moves in general from W o E on the surface of the Earth.
Labels along the projected limbs correspond to the values of γ (position angle along the lunar
limb, from the North Celestial Pole towards E), in degrees, for each event and observer.
Note that, in some cases, two or more families of limbs are represented in the graph. Limbs in each
family are arranged in a regular fashion, more or less parallel to each other. However, limbs
that belong to different families are at angle. Limbs associated to the event of interest correspond
to the dark limb and contain the central limb (in red), while other limbs in different families
correspond to the illuminated limb, which is not observationally interesting.
Note that for an event to be observed from some specific location, the latter must lie
in between two limbs; in case the location lies in an area free from limbs, the event will not be
visible, because the Moon is not occulting the star, or the occultation is grazing, or simply because
the Moon is two low on the horizon.
The best arrangement for observers participating in a particular occultation is that for which
the γ values are spread in an arc as long as possible. If an imaginary curve joining points
with the same value of γ in different limbs is drawn, observers should be located at different
distances from that curve, in directions perpendicular to the curve (depending on the case,
optimal directions can be north-south, east-west, northeast-southwest, and northwest-southest).
Nevertheless, all observations, whatever the value of γ, are valuable.