OUT GOES THE MOON.

Hawera Star, Volume XLVI, 15 June 1927, Page 4

OUT GOES THE MOON.

A LUNAR ECLIPSE. TO-NIGHT’S THEN OMEN ON. A total eclipse or the moon .is due to. take place xilns evening, and it will be visibi© throughout !Vew Zealand. Such m phenomenon is not very rare —• there is on an average a total eclipse or the moon every two years, but tonight’s eclipse will be interesting from the fact that the phase of totality is of comparatively long duration, and fromi the fact that the eclipse takes place at a very convenient hour tor Observation, not necessitating late hours or early rising on a cold and frosty morning.

The moon will enter the penumbra at 5.4 p.m., and the umbra at 6.13. Totality begins at 7.44 p.m., and ends at 8.5. The moon will leave umbra at 9.36 p.m. land the penumbra at 10.45. Tne total phase therefore lasts for about 20 minutes, and tile whole phenomenon of the eclipse occupies five and three-quarter hours. It is a popular, but wrong, belief that eclipses of the moon are far commoner than eclipses of the sun. It arises from the tact that an eclipse of the moon When it occurs can be- seen from every place where the moon happens at the time to be above the horizon. This is not tire case when the sun is eclipsed, for then only a very small area of the -earth’s surface is affected: In another point ail eclipse of the moon differs from one of the sun; the moon may remain totally eclipsed for nearly two hours, whereas a total eclipse of the sun is usually of very brief duration —often a matter of only a few seconds. Tire total phase of the eclipse of the isun which will be visible in England at the end of this month is of only about half a minute’s duration.

FACTORS IN LUNAR ECLIPSES Everyone knows that an eclipse of the moon as claused' by the eantn getting directly between the sun and the moon, the earth’s shadow thereby being projected on to the moon’s disc. Such an eclipse can, therefore, only happen when the moon is full, but it does not happen dt every full moon, as the moon’s orbit isjinolined at an angle of a little over five degrees to the ecliptic, on which the centre of the 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 m towards the axis of the earth’s shadow, these which pass through the lowest strata of tlj© air being most refracted' land converging to a point at a distance from the earth’— centre of about 42 radii of the earth. As tire moon’s mean distance from the earth is about 60 radii, it follows that that luminary never enters the part of the shladow which is completely dark. The moon therefore during an eclipse never doses her light entirely, but appears of a reddish colour resembling tarnished copper, an appearance caused by the atmospheric absorption, which is very great for blue 'Light, in much the same way tes is produced the ruddy colour of the clouds at sunset.

Another factor entering into the phenomenon of iam .eclipse of the moon is that every shadow cast by the sun's rays, including the 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 enters the real shadow the contrast is not between one part of it an Shade and the other in full brilliance, but between a part in shade and a .part in paritdail shade. Upon emergence there is the same contrast, part- in the umbra 1 and part in the penumbra. So softly l do Hie 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 Hie umbra.

The western side, that is the left hand iside, 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 pant of the world. This is because the moon’s motion As swifter than that of the earth’s shadow; the moon overtakes the shadow, passes through it. and leaves it ‘behind.