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Friday, 4th March 2011

The Moon

For people on earth, the Moon offers interesting titbits. For instance, because of gravity, the Moon always shows us the same face. In other words, to get a glimpse of the Moon's other face, you have to ride in a rocket. A Soviet spacecraft in 1959 did just that --- the first to successfully observe the Moon's other side.

Figure 1     The one and only face of the Moon that we can see.
Figure 1      The one and only face of the Moon that we can see.

Looking at the Moon, many of us earth-bound people would think we can only see 50% of its total surface. Actually we are able to see more than that. There are two reasons. One is that the Moon moves around the Earth in an elliptical orbit, i.e. not a perfect circle. As such its speed relative to the Earth tends to vary a little, i.e. not constant. This allows us to see 6% more along the edges of the Moon's surface. The second reason is that when the Moon is near the horizon, the line joining the centres of the Moon and Earth is at angle to us (a phenomenon called parallax), so that we are able to see about 1% more around the side of the Moon. These two factors, plus observations of the Moon at different times, make it possible for us to see a total 59% of the Moon.

Many of us also know that the Moon brings us roughly two high tides and two low tides every day. Again, it is because of gravity. On the side of the Earth closer to the Moon the gravitational attraction is greater on the oceans than that on the far side, resulting in a higher and lower bulge respectively (Figure 2). As the Earth rotates around itself once a day, so each place on Earth (e.g. Hong Kong) passes through the higher bulge and lower bulge during that time. This gives us two high tides per day, and of course, two low tides in between them. Because of its distance from us, the Sun's gravitational influence is only half as much as the Moon's.

Figure 2     Two bulges of water on the Earth, mainly due to the Moon's attraction.
Figure 2      Two bulges of water on the Earth, mainly due to the Moon's attraction.

No matter where and when, the Moon often brings poetic inspirations. There is no lack of mention of the Moon in Tang dynasty poetry. Here are a couple of titbits about Tang poems, not related to science. Partly a result of misunderstanding both poems are famous.

"I know you are sincere like the Sun and Moon,
 but I had vowed to serve my husband in sickness or health.
 The pearls are now returned with tears,
 I just wish we had met while I was still a maiden."

This is the latter half of a poem by Zhang Ji (circa 766-830). It is about a woman's revelation to a man. She knows very well his sincerity, but she has to stick with her husband for better or worse. Thus the pearls as a gift from him have to be returned with tears, and she regrets that they met each other too late. Interestingly, the verses do not actually allude to a love story. The poem's title indicates that it is addressed to a warlord named Li. Li had planned to secede from the central government and were enlisting supporters including Zhang. Zhang did not find this palatable and chose the subject of a woman's fidelity as a euphemism for his loyalty towards the country.

Let's take another poem, this time by the Late Tang poet Du Mu (803-852):

"The green mountains are faint and the waters far away,
 In this river region the grass is not yet cured even by autumn's end.
 The Twenty-Four Bridges are under clear moonnight,
 And where is the fair person who plays the flute."

Yangzhou in the river region was very prosperous during Tang period, with a total of 24 bridges (some say'Twenty-Four Bridge' was the name of a bridge). A clear moonlit night on the bridges, with music coming from a flute, would be alluring. However, the 'fair person' here does not necessarily refer to a girl, and back in Tang times was in fact applicable to both sexes. As the title of the poem is addressed to Magistrate Han Cho, an old friend and colleague of the poet while in Yangzhou, it could well be that the term refers to the magistrate and not to a lady. The poem remains popular today, and it may owe its popularity partly to the misunderstanding.

B.Y. Lee

Reference: Wikipedia.

Last revision date: <17 Jan 2013>