Observe Eclipses! 
Excerpts from book by Dr. Michael D. Reynolds and Richard A. Sweetsir

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