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Total Eclipse Of The Moon July 16 / 17

Royal Astronomical Society of New Zealand


The light of the Moon is caused by the Sun shining on it. Since the Sun can only light one side of the Moon, we see the fully lit Moon when it is on the opposite side of the Earth to the Sun. Once or twice a year when it is full the Moon may pass through the Earth's shadow cast by the Sun so cutting off the light from the Sun. When this happens a total eclipse of the Moon occurs. Usually when it is full, the Moon passes either above or below the Earth's shadow so no eclipse takes place.
Two total eclipses of the Moon occur during 2000. The first was on January 21. It was visible from much of Europe, Africa and the Americas, but was invisible from New Zealand.

The second eclipse is on the night of July 16 / 17 and will be well placed for viewing from New Zealand and Australia.

During a total eclipse the Moon is not completely dark, a little light from the Sun is refracted by the Earth's atmosphere and bent onto the Moon. This is just like the way some light is bent round the Earth's horizon at sunset and sunrise producing twilight. And just as the sky is often reddish at sunset so the light bent onto the Moon is also reddish giving the faintly lit eclipsed Moon a reddish colour.

On July 17 (NZ time) the Moon will pass almost centrally through the Earth's shadow. As a result the total eclipse will be quite long, totality lasting for just over an hour and three quarters from 1:02 am NZST until 2:49 am NZST on the morning of July 17. The equivalent UTC times are July 16, 13:02 to 14:49.

Since the Moon will pass through the centre of the Earth's shadow it may be uniformly dark. How dark depends on how clear the Earth's atmosphere is. Large amounts of dust from a volcanic erruption would lead to a darker eclipse. During the January eclipse the Moon did not pass centrally through the Earth's shadow and so was brighter on one side. Also since the Earth's atmosphere was clear, there having been no recet large volcanic erruptions, the eclipsed Moon was relatively bright.

During the July eclipse, the Moon will first touch the penumbral part of the Earth's shadow at about 10:47 pm (NZST) on the evening of July 16. During this stage if there was a person watching from the Moon, he or she would see the silhouette of the Earth partly covering the Sun. For us on Earth there will be little dimming in the bright light of the full Moon, although as time progresses the right hand side especially will get noticeably duller.

Three minutes before midnight the first part of the Moon will enter the Earth's umbra, that is the total shadow. A very dark region will appear on the right hand side of the Moon. Our observer, if on this part of the Moon, would now see the Earth completely hiding the Sun.

Gradually, during the next hour and five minutes, the dark region will spread over the face of the Moon until it is totally eclipsed at 2 minutes past 1 am. During this phase careful observation will show the edge of the dark shadow is curved. This is due to the circular outline of the Earth's shadow and is a demonstration that the Earth really is round.

During the total eclipse quite faint stars will be visible close to the Moon. If you watch from a really dark site away from town lights, you may also be able to see Vesta, the brightest of the Asteroids, or Minor Planets, which mostly move round the Sun between the orbits of Mars and Jupiter. If you look a little above the Moon you will see a small and fairly faint kite shaped group of stars. Vesta, quite a bit fainter than the stars, will be about half way between the Moon and this group.

An hour and three quarters after the Moon is first totally eclipsed the right side of it will begin to brighten and then emerge out of the Moon's umbra back into the penumbra. This part starts at 2:49 am. The faint stars which will have been visible will rapidly disappear again swamped by moonlight. The whole Moon will be completely out of the umbra just over an hour later at 6 minute to 4. It will take more than another hour to completely leave the penumbra.

Observers in New Zealand should make the most of this total eclipse of the Moon. There is not another visible from New Zealand until 2007 on August 28.


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