Lunar Eclipse Observing-The Partial Phases
Introduction. Partial lunar eclipses
occur when the moon enters the earth’s umbral shadow, but not
centrally enough to become completely immersed. This chapter
suggests projects which are suitable for partial eclipses and
the partial phases of total lunar eclipses.
Photograph 14-1 A partial eclipse of the
moon is a spectacular sight even if a total eclipse will not
be visible. Photograph taken by Mike Reynolds.
Program management. The partial
phases are busier than totality for lunar eclipses; as busy as
the time around totality for solar eclipses. If your observing
program is particularly packed, you may want to consider
pre–recording a carefully–timed countdown tape which directs
your attention to each of the things you want to do during the
eclipse. Produce the tape from a script with a timeline having a
built–in 15– to 30–second safety margin. These have proven
very helpful at solar eclipses (see Chapter 4: Instrument), but
it’s a good idea to make a backup copy or carry the original
script along if you’re going to rely completely on this (or
any) device.
Tape–recording observations and timings
has become a popular alternative to frequent note–taking in
the field, but it is important to remember that tapes can run
out or jam, and recorder batteries can fail; check your
equipment frequently to avoid losing valuable data!
Contact timings. First contact, when
the earth’s umbra first touches the moon’s celestial eastern
(lunar western) or leading limb, and fourth contact, when it
last departs the moon’s celestial western (lunar eastern) or
trailing limb, should be timed to an accuracy of 0.1 minute (6
seconds). Neutral–density filters are recommended to enhance
the umbra’s appearance if glare is a problem.
Instruments having apertures between 10 and
40 cm (4 and 16 in) and very low magnifications (between 40 and
100X) are preferred for contact timings, although smaller
telescopes, binoculars and even the naked–eye may be used. The
important consideration is that most or all of the moon fit into
your field of view at one time. The darker penumbral shadow is
often dark enough to merge with the umbra when large–aperture
instruments are used at high power, complicating accurate
timings; beginners should especially be alerted to this
possibility.
Recording your verbal descriptions against
WWV or CHU radio time signals is the most reliable method of
obtaining good timings. Begin your narrative several minutes
prior to the predicted times of the two contacts.
If you are without a short–wave radio,
you might be able to calibrate your timepiece to another’s
radio before and after the eclipse, and apply an average
correction to your timings if necessary. WWV also offers a long
distance telephone time service; the number is (303) 499–7111,
but as this signal is being transmitted through telephone lines,
only assume accuracy to a second or so.
If none of these timing methods are
available to you, time the interval between first and fourth
contact as accurately as possible with a sweep second–hand,
digital wristwatch or a stopwatch.
Photograph 14-2 The twenty craters
recommended for lunar eclipse contact timings by the Association
of Lunar and Planetary Observers. See Table 14-1, below,
for identification of craters 1 through 20. Illustration
by John Westfall, A.L.P.O. Lunar Section. Lick Observatory
photograph.
Table 14-1
Recommended Craters for Lunar Eclipse Contact
Timings
|
1. Grimaldi
|
6.
Timocharis
|
11. Manilius
|
16. Gassendi
|
21. Billy
|
2. Aristarchus
|
7.
Tycho
|
12. Menelaus
|
17. Birt
|
22. Campanus
|
3. Kepler
|
8.
Plato
|
13. Plinius
|
18. Abulfeda E
|
23. Dionysius
|
4. Copernicus
|
9.
Aristoteles
|
14. Taruntius
|
19. Nicolai A
|
24. Goclenius
|
5. Pytheas
|
10. Eudoxus
|
15. Proclus
|
20. Stevinus A
|
25. Langrenus
|
You should not anticipate or allow your
observations to be unduly influenced by the predicted contact
times. The predictions are determined by the diameter of the
earth’s umbra. However, due to the earth’s thick atmosphere,
the umbral diameter is routinely larger by approximately two
percent than the simple geometry would expect. Since the
umbra’s diameter, and even its shape, varies from one eclipse
to another, careful contact timings can be instrumental in
determining both its diameter and degree of ellipticity.
Crater contact timings. As the umbra
sweeps across the full moon’s face, its leading edge
encounters many surface features. These encounters are easier to
time than first and fourth contacts, and are equally capable of
yielding information on the size and shape of the umbra.
Timings are best accomplished using
small–aperture instruments; larger telescopes should be
“stopped down” by covering the front of their tubes with
cardboard masks in which off–center circular openings of 8 to
10 cm (3 to 4 in) in diameter have been cut. This will reduce
the brightness of the lunar surface sufficiently for reliable
crater contact timings. Neutral–density filters may also be
employed to further darken the lunar surface and to enhance the
umbra. As with contact timings, use voice over radio time
signals and aim for an accuracy of 0.1 min (6 sec).
The Association of Lunar and Planetary
Observers (A.L.P.O.) recommends 20 craters for special attention
during lunar eclipses; the Royal Astronomical Society of Canada
(R.A.S.C.) adds five more to that list, although any
well–defined craters are fair game. Table 14–1 on the
previous page compiles the recommendations of both organizations
(craters numbered 21–25 are the R.A.S.C. additions).
Small craters are fairly easy to time, for
the umbral shadow engulfs them relatively quickly. For larger
craters (i.e., Copernicus and Tycho), time the umbra’s arrival
at first one rim and then at the opposite rim of the crater;
later, average the two timings to determine a mean crater
contact time.
Observers who do not observe the moon
regularly are encouraged to familiarize themselves with the
appearances and locations of these recommended craters on
several full–moon nights prior to the eclipse; a good lunar
map suitable for field use is an indispensable tool. Consider
laminating the map to protect it from dew and to allow for the
use of grease pencils or transparency–marking pens.
Umbral characteristics. The general
appearance of the umbra varies from one eclipse to the next, and
may vary during the course of a single eclipse. Observers should
note the sharpness (or diffuseness) of the umbra’s leading and
trailing edges frequently during the partial phases. Note the
time when specific variations are detected, and pinpoint their
proximity to lunar features on your map.
Be also attentive to the brightness and
color of the umbra’s interior, and whether any variations are
noted as the eclipse progresses. Standard color charts might be
employed for the latter, while the former lends itself well to
comparisons with the brightness of lunar mare, rilles or rays on
the part of the surface still outside the umbra. Failing in
that, an arbitrary numerical scale may be developed at the
telescope and calibrated for several familiar features already
inside the umbra (0=darkest to 5=brightest works well). The
brightness of other features within the shadow can be quickly
compared to the calibration features, using intermediate decimal
values if significant variations are detected.
Apparent magnitude estimates. While
generally reserved for observations of totality, estimates can
be made of the moon’s apparent magnitude throughout the
partial phases by comparing its brightness to the magnitudes of
bright surrounding stars and planets.
One approach recommended by the A.L.P.O. is
to reduce the apparent size and brightness of the moon with a
highly–polished convex reflector. A hemispherical hub–cap
from an automobile (old Volkswagen “Beetle” ones are ideal),
an silver Christmas–tree ball ornament, or a convex bicycle
mirror works well.
View the moon in the reflective surface
while viewing several stars of known magnitudes with the unaided
eye. Adjust your eye’s distance from the reflector (the
further the distance the greater the dimming) until the
brightness of the reduced lunar image matches that of one of the
stars, then measure that distance. The following equation yields
the moon’s apparent dimming or change in magnitude, ∆M
(pronounced “delta–M”), from that of the normal full moon:
∆M = K – 5 log R,
where R is the distance from the eye to the
reflector’s surface, and K is a constant which needs to be
determined just before or immediately following the eclipse.
When ∆M is subtracted from the full moon’s normal
–12.7 magnitude, the result is the magnitude of the
eclipse–dimmed moon. To find K, use the same observing method
on the uneclipsed full moon (magnitude –12.7) and an
equivalent comparison star, set ∆M equal to their
magnitude difference, and solve the equation for K.
Example: Shortly before an eclipse begins, the planet
Jupiter (magnitude –2.1) appears to equal the
brightness of the full moon’s image in a bicycle
mirror 254 cm (100 in) from your eye (the full moon’s
magnitude is –12.7). Setting ∆M equal to the
magnitude difference between the two objects, we find
K = ∆M + 5 log
254
K = –10.6 + 5
(2.40) = 1.4.
At maximum umbral phase, the
moon’s reflected image appears to equal the brightness
of a star of magnitude 0.4 when you are 35 cm (14 in)
from the mirror. Applying the constant K already
determined, we find that
∆M = 1.4 – 5
log 35 = –6.3,
and
the moon’s eclipsed magnitude is –12.7 – (–6.3),
or –6.4.
|
A.L.P.O. Eclipse Recorder Francis G. Graham
suggests using a pivoted convex mirror and a pivoted plane
mirror side–by–side. His “geoumbrascope” device allows
the mirrors to be adjusted so that the moon and comparison
object are seen together; the math is identical.
Another A.L.P.O. method reduces the
moon’s size and brightness to simplify comparisons with stars.
You view the moon through the objective end of a pair of
reversed binoculars with one eye while the other eye, without
optical aid, seeks out a suitable comparison star of equal
apparent brightness.
The magnification of the binocular is used
to determine the amount of dimming in stellar magnitudes. Table
14–2 provides the dimming factors for the most common
binocular magnifications.
Table 14-2
Reversed–Binocular
Magnitude Dimming Factors (F)
|
6x
|
=
|
4.20 mag.
|
7x
|
=
|
4.54 mag.
|
8x
|
=
|
4.83 mag
|
9x
|
=
|
5.08 mag.
|
10x
|
=
|
5.31 mag.
|
11x
|
=
|
5.52 mag.
|
12x
|
=
|
5.71 mag.
|
16x
|
=
|
6.33 mag.
|
Factors for magnifications not given in
Table 14–2 may be calculated from the expression
F = 5 log P + 0.31,
where P is the “power” (magnification)
of the reversed binocular (the constant 0.31 assumes some
absorption in the optics).
The magnitude M of the moon is readily
found from the naked–eye comparison star’s magnitude m and
this factor F, by
M = m – F.
With this method no full–moon calibration
is required.
Example: A partially eclipsed moon viewed through
reversed 10x binoculars (dimming the moon by 5.3
magnitudes) resembles a star of magnitude 1.3; its
magnitude M would be 1.3 – 5.3 = –4.0.
|
A third A.L.P.O. approach employs two small
identical telescopes placed close together. Inexpensive imports
from the same manufacturer are ideal. One is focused on the moon
while the other is trained on a comparison star and racked out
of focus to make the star’s image size equal the moon’s.
When the brightness of a star’s unfocused image through one
instrument equals that of the moon’s focused image in the
other, the moon’s magnitude equals the star’s (one telescope
will suffice if you calibrate your setup; look at the moon
naked–eye and at the star with the out–of–focus
telescope). To be effective for the partial phases, however, the
moon must be at least as dim as the brightest comparison planet
or star available (the brightest planet, Venus, ranges from
–3.9 to –4.7; the brightest star, Sirius, is –1.4).
Consult monthly magazine or annual handbook sky calendars for
planet magnitudes around the eclipse date; for star magnitudes,
annual astronomical handbooks and observing guidebooks are good
sources.
Table 14-3
Atmospheric Extinction Magnitude Corrections
|
Elevation
of Lower Object
|
Elevation
of Higher
Object
|
|
70˚
|
60˚
|
50˚
|
40˚
|
30˚
|
25˚
|
20˚
|
15˚
|
70˚
|
0.0
|
0.0
|
0.0
|
0.1
|
0.2
|
0.3
|
0.4
|
0.6
|
60˚
|
—
|
0.0
|
0.0
|
0.1
|
0.2
|
0.3
|
0.4
|
0.6
|
50˚
|
—
|
—
|
0.0
|
0.1
|
0.2
|
0.3
|
0.4
|
0.6
|
40˚
|
—
|
—
|
—
|
0.0
|
0.1
|
0.2
|
0.3
|
0.5
|
30˚
|
—
|
—
|
—
|
—
|
0.0
|
0.1
|
0.2
|
0.4
|
25˚
|
—
|
—
|
—
|
—
|
—
|
0.0
|
0.1
|
0.3
|
20˚
|
—
|
—
|
—
|
—
|
—
|
—
|
0.0
|
0.2
|
15˚
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
0.0
|
If you are nearsighted, you have still
another approach available. Remove your glasses and,
naked–eye, compare the moon with a bright star!
Finally, unless both the moon and your
chosen comparison star are at the same elevation, the magnitudes
derived by each of these methods must be corrected for
differences in atmospheric extinctions for the two objects.
Table 14–3 provides rough atmospheric extinction correction
values for the moon’s previously–estimated magnitude. Add
the correction value if the moon is the higher object in the
sky, and subtract it if the moon is the lower object.
Lunar transient phenomena (LTP).
During an eclipse, the moon undergoes a significant reduction in
the amount of sunlight falling on its surface.
During the penumbral phases, shorter
wavelengths of solar radiation bathe the moon’s surface and
may cause some lunar rocks to fluoresce. During the umbra’s
passage, the surface is subjected to very sudden changes in
temperatures which might cause stresses, strains and surface
cracking. Such structural deformations could trigger a mild form
of lunar vulcanism, releasing trapped subsurface gases and
stirring up lunar dust possibly visible from the earth.
Observers should monitor areas before,
during and after umbral passage which have historically been
suspected of exhibiting lunar transient phenomena (LTP). Table
14–4 on the next page lists nine of the most suspect lunar
features and the nature of their events.
LTP have also been reported in Byrgius,
Censorinus, Delambre, Euler, Kepler, Linné, Manilius, Menelaus,
Messier and Messier A, Pickering, Plinius, Proclus, Pytheas, Römer,
Schickard, and other features.
Table 14-4
Suspected LTP Features
|
Alphonsus
|
Central peak emissions, dark areas
on floor
|
Aristarchus
|
Wall bands, “red glow” areas in
& nearby
|
Atlas
|
Dark areas on floor
|
Conon
|
White area on floor
|
Eratosthenes
|
Dark areas on floor and walls
|
Grimaldi
|
Tone of floor; bright spots on E
& NE
|
Plato
|
Light spots on floor
|
Riccioli
|
Dark areas on floor
|
Stöfler
|
Dark areas on floor
|
Tycho
|
Glow near central peak
|
Photoelectric photometry. Amateurs
so equipped may wish to make direct photometric measurements of
the brightness of select areas of the lunar surface within the
umbra and penumbra. Such measurements are useful for determining
the darkness and uniformity of the umbral and penumbral shadows,
especially if done at several wavelengths.
Photography. Umbral contacts with
the moon’s limb are attractive photographic targets for still
and video cameras, as are umbral crater contacts and the
shadow’s progress across larger craters and seas.
Capturing the colors and subtle shading
differences of the umbra are worthy goals; try a variety of
exposure times with your favorite color films.
You might want to compile a photographic
portfolio of favorite lunar features as they appear at frequent
intervals throughout the eclipse. Don’t neglect including
prospective LTP features; you might get lucky and capture an
event (a photograph or photo sequence is much more valuable than
a subjective LTP visual sighting). Video and CCD imaging are
especially encouraged for these projects.
Multiple exposures on a single frame, taken
at five or ten minute intervals, can capture an impressive
sequence of the partial eclipse’s progress. Multiple exposures
spaced about an hour apart, taken with a camera driven at a
solar (or at least a sidereal) rate, will “freeze” the
earth’s shadow; the moon’s multiple limbs will nearly touch,
creating an impressive light–map of the earth’s umbral
shadow limb and its position in space. Practice on the
uneclipsed moon to determine the best exposure interval to
prevent overlap of lunar images.
Finally, time–exposures with slow films
and small f/–stops result in attractive light–trails of the
moon’s dwindling, then growing illumination.
Photograph 14-3 A "moon trail"
of the 6 Jly 1982 total lunar eclipse. A camera, locked
into position on a tripod, is set for a time exposure.
As the earth rotates, the moon moves across the
photograph. And as the eclipse progresses, the light of
the moon diminishes. Photograph taken by Jay Anderson.
|