Total Eclipse Of Moon This Month

Press, Volume CVI, Issue 31333, 1 April 1967, Page 16

Total Eclipse Of Moon This Month

On the night of April 24-25 there will be a total eclipse of the moon, all phases of which will be visible from New Zealand. The beginning of the total phase will be at 11.28 p.m. on April 24 although the moon- will have commenced to enter the main part of the earth’s shadow an hour earlier,

The total phase will finish at 12.46 a.m. on April 25 after which, for another hour, it will be possible to see the moon gradually coming out of the earth’s shadow, or umbra. This umbral zone marks the tapering shadow cast by the earth and in which all sunlight is excluded except for rays refracted, or bent, by the atmosphere of the earth. These refracted rays make the umbra less perfectly dark with the result that the moon can almost always be seen during the total phase.

Between 1601 and 1913 there were eleven very dark eclipses. In each case observers described the moon as very dark and colourless and in a few instances as invisible. It is perhaps significant that of these eleven eclipses at least eight took place within a year or two of a major volcanic eruption. This suggested that the volcanic dust in the atmosphere prevented light being refracted into the umbral zone and as a result there was a dark eclipse. With this knowledge, astronomers were not surprised when a total eclipse of the moon on December 30, 1963, proved to be exceptionally dark. In fact after the event it was referred to as a “black eclipse’* in which the moon became almost invisible. Earlier in the same

year a vast quantity of volcanic dust from the eruption of Mount Agung, in Bali, had commenced to circulate around the earth. The dust caused some very brilliant sunsets and sunrises throughout the southern hemisphere. The following total eclipse of the moon on June 24-25, 1964, also proved to be a particularly dark one, comparable in many ways to that of the previous December.

It is unlikely that the forthcoming lunar eclipse on April 24-25 will be a dark one. For almost an hour, before and after the moon enters the umbra, it will be in what is termed the penumbral zone. This will cause a slight darkening of the moon's limb and to the layman the interesting phase Is when the umbra commences to encroach on the moon.