Dream Pool Essays - Quotes - Astronomy


When the Director of the Astronomical Observatory asked Shen Kuo if the shapes of the sun and moon were round like balls or flat like fans, Shen Kuo explained his reasoning for the former:

If they were like balls they would surely obstruct each other when they met. I replied that these celestial bodies were certainly like balls. How do we know this? By the waxing and waning of the moon. The moon itself gives forth no light, but is like a ball of silver; the light is the light of the sun (reflected). When the brightness is first seen, the sun(-light passes almost) alongside, so the side only is illuminated and looks like a crescent. When the sun gradually gets further away, the light shines slanting, and the moon is full, round like a bullet. If half of a sphere is covered with (white) powder and looked at from the side, the covered part will look like a crescent; if looked at from the front, it will appear round. Thus we know that the celestial bodies are spherical.

When the director of the astronomical observatory asked Shen Kuo why eclipses occurred only on an occasional basis while in conjunction and opposition once a day, Shen Kuo wrote:

I answered that the ecliptic and the moon's path are like two rings, lying one over the other, but distant by a small amount. (If this obliquity did not exist), the sun would be eclipsed whenever the two bodies were in conjunction, and the moon would be eclipsed whenever they were exactly in position. But (in fact) though they may occupy the same degree, the two paths are not (always) near (each other), and so naturally the bodies do not (intrude) upon one another.

On the use of the sighting tube to fix the position of the pole star, Shen Kuo wrote:

Before Han times it was believed that the pole star was in the center of the sky, so it was called Jixing (Summit star). Zi Geng(-zhi) found out with the help of the sighting tube that the point in the sky which really does not move was a little more than 1 degree away from the summit star. In the Xining reign-period (1068-1077) I accepted the order of the emperor to take charge of the Bureau of the Calendar. I then tried to find the true pole by means of the tube. On the very first night I noticed that the star which could be seen through the tube moved after a while outside the field of view. I realized, therefore, that the tube was too small, so I increased the size of the tube by stages. After three months' trials I adjusted it so that the star would go round and round within the field of view without disappearing. In this way I found that the pole star was distant from the true pole somewhat more than 3 degrees. We used to make the diagrams of the field, plotting the positions of the star from the time when it entered the field of view, observing after nightfall, at midnight, and early in the morning before dawn. Two hundred of such diagrams showed that the 'pole star' was really a circumpolar star. And this I stated in my detailed report to the emperor.

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