LUNAR ECLIPSE.

Grey River Argus, 15 June 1927, Page 6

LUNAR ECLIPSE.

ON TO-NIGHT. Long Totality. To-night a total eclipse of tlie moon is due to take place, and it will be visible throughout New Zealand. There is on an average a total eclipse o.t the moon every two years, but this eclipse will be interesting from the fact that the phase of totality is of comparatively long duration, and from the fact that the eclipse takes place at a very convenient hour for observation, not .necessitating late hours or early rising.

The moon will enter the penumbra at 5.4 p.m., and Hie umbra at G. 13. /totality begins at 7.1-1 p.m., and ends at 8.5. ’1 he moon will leave umbra at 9.36 p.m., and the penumbra at 10.45. The total phase therefore lasts for about 120 minutes, and the whole phenomena of the eeplise oeeupii's live and three-quarter hours.

It is a popular, but wrong, belief that eclipses of the moon arc far commoner than eclipses of the sun, It arises from the fact that an eclipse of the moon when it occurs can be seen from every place where the moon- li:i| ■ pens at the time, to be above the horizon. This is not the case whim the sun is eeplipsed, for then only a very small area of the earth s surface- is affected. In another point an eclipse of the moon differs froin one of Hie sun; the moon may remain totally eclipsed for nearly two hours, whereas a total eclipse of tl.e sun is usually of very brief duration —often a matter of only a few seconds. The total phase or the eclipse of the sun which will be visible in England at the end of this month is of only about bait a minute’s duration.

Everyone knows that an eclipse of the moon is caused by the earth getting directly between the sun and the moon, the earth’s shadow thereby being projected on to the moons disc. Such an eclipse can, therefore, only happen when tie moon is lull, but it does not happen at every full moon, as the moon's orbit is inclined at an angle of a little over five degrees tti the ecliptic, on which Hie centre of tin earth's shadow moves. Were it not for the inclination, there would be an eclipse at every full moon. Various factors combine to make an eclipse of the moon a less striking phenomenon than might be anticipated. The effect of atmospheric refraction is to bend the rays which are incident on the atmosphere in towards the axis ol the earth's shadow, those which pass i through the lowest strata id the air being most refracted and converging to a point at a distance from the earth's centre of about 42 radii of the earth. As the moon's mean distance from the earth is about 60 radii, it follows that (hat luminary never enters the part of the shadow which is completely dark The moon therefore during an eclipse never loses her light entirely, but ap pears of a reddish colour resembling tarnished copper, an appearance caus-e-l by the atmospheric. absorption, which is very great for blue light, in much the same way as is produced the ru.idv colur of the clouds at sunset. Another factor entering into the phenomenon of an eclipse of rhe moon is that every shadow cast by the sun s ravs including Hie shadow of the earth, necessarily has a penumbra on each side of the true shadow or umbra. When the moon is about to suffer eclipse it first loses its brightness on entering this penumbra, so that when it enter’s the r. al shadow, the contrast! is not between one part of it in shade and the other in full brilliance, but between a part in shade and a part in partial shade. Upon emergence there is the same contrast, part in the umbra and part in the penumbra. So softly do the degrees of light merge into one another that it is impossible to tell exactly by eye when any one part leaves the penumbra to pass into the umbra. ’ The western side, that is the left hand side, of the. moon, is the first portion to be obscured and is the first to emerge from the shadow when an eclipse is observed from this part of the world. This is because the moon’s motion is swifter than that of the earth’s shadow: the moon overtakes the shadow, passes through it, and 'leaves it behind.