Lunar Eclipse Observing-The Penumbral Eclipse
Introduction. When a full moon
occurs close enough to the ascending or descending node of the
moon’s orbit, but not sufficiently close to make contact with
the earth’s dark umbral shadow, a penumbral eclipse takes
Unless the moon’s limb passes within
about 1120 km (700 mi) of the umbra, corresponding to a
penumbral magnitude of about 0.500, the unaided eye is unlikely
to detect any light reduction. However, there are a number of
interesting observations which may be undertaken by observers of
deep penumbral eclipses and for the penumbral phases of partial
and total lunar eclipses.
Detection timings. Timings of first
and last penumbral shadow detection are impossible, as there is
no clear–cut easily discernable shadow edge to be seen.
Instead, the penumbra appears as a somewhat dusky darkening on
the limb, which advances steadily across the surface.
Determining when you believe you see this
darkening is highly subjective and can vary greatly depending
upon whether observations are made by telescope, binoculars or
unaided eye. Don’t expect to detect the penumbra visually
until at least 30 minutes (and possibly as long as 45 minutes)
into the penumbral phase of a partial or total eclipse. For
shallow penumbral eclipses, the shadow may go undetected
throughout the eclipse. In making penumbral timings observers
are encouraged to estimate their probable accuracy as well.
Some observers tape record a continuous
narrative of the moon’s appearance, including times when
contacts or any dimmings are first suspected and finally
certain. Others have reported some advantage by racking the
moon’s image slightly out of focus to blur the lunar surface
features, which vary significantly in albedo (reflectivity),
making the moon’s brightness more uniform.
Shadow position. Contact timings
should also include an indication of the position along the
lunar limb where the shadow is first detected. This may be done
by position angle, using either an eyepiece reticle or lunar
map, or by reference to a known feature near the lunar limb.
Umbral separation. Penumbral
observations during eclipses which will be partial or total
should include estimates of the greatest distance the penumbra
may be detected from the umbra. Estimate the separation for both
the leading and trailing penumbral shadow. This is easier to
determine than penumbral detection timings, and can provide a
reasonable check on detection timing reliability. Use a linear
or bull’s–eye eyepiece reticle, or note the positions of the
umbra and penumbra with respect to lunar features, then measure
their angular or actual separation from a lunar map.
The use of neutral density or paired
polarizing filters to significantly dim the moon may be helpful
in detecting the penumbra; color filters may be used to enhance
the shadow as well.
Shading variations. Penumbral
shading is not uniform; it becomes darker the closer it gets to
the umbra. Care should be taken not to confuse variations in the
penumbral shadow with differences in the albedos of lunar
surface features. Photocopies of simple lunar maps may be used
to shade in variations.
Color variations. While not as
spectacular as umbral colors, the deeper penumbra can present a
variety of dusky brown or yellowish–brown colors. Note
variations on photocopies of lunar maps; numbers might be
assigned to different colors and written inside sketched
boundary lines on maps. Use of a variety of color filters at the
eyepiece is recommended.
Photoelectric photometry and intensity
variations. Penumbral theory is not well established. There
is a distortion in the solar disk, provided by the intrusion of
the earth’s atmosphere. This results in wavelength–dependent
differences in the illumination of the penumbra from what would
be expected from an earth having no atmosphere. These
differences are not completely understood, making photometry of
features within the penumbra a worthwhile undertaking (see
Chapters 14 and 15 also).
The brightness of different parts of the
penumbra can be estimated in terms of a graduated scale based
upon the brightness of different lunar features, for those
lacking photoelectric equipment.
Other observations. The penumbral
eclipse or penumbral stages of a partial or total eclipse, when
the full moon’s brightness is diminished, are good times to
look for lunar transient phenomena (LTP); lunar surface
materials may exhibit fluorescence, caused by solar wind
interactions. Chapter 14 discusses LTP observing in greater
detail and recommends specific lunar features for special
Timing when you can last and first see
shadows of lunar features on the limb opposite the penumbral
shadow’s center is another interesting project to conduct.
Photography. Frequent exposures
offer a possible alternative to visual estimates for contact
timings. Using slow films (around ISO 32) and deliberately
underexposing them, the full moon can be dimmed sufficiently on
initial exposures to show a detectable change some 30 minutes
after the penumbra begins its advance (note, however, that this
method will not give pleasing images of the penumbral eclipse
itself). Shoot at six–second intervals, beginning around 30
minutes after the predicted time of penumbral contact, using an
exposure previously determined to barely show the full moon’s
disk with your system. Be certain to finish and change to a
faster film and exposure setting well before first umbral
similar approach might be employed with video cameras equipped
with time–lapse features which can produce a speeded–up
eclipse sequence to accent the penumbral advance.
Photograph 13-2 Penumbral eclipses can
often be recorded only through the use of a photometer.
This is a simple photocell photometer for penumbral brightness
measurements. Photograph taken by Francis Graham.