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

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 place.

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 attention.

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 contact!

A 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.