卷六、卷七·第 322–325 问　顺逆与任意时刻的星位、星盘及其部件

§322. How are their direct and retrograde motions known?

第三百二十二问　怎样知道它们是顺行还是逆行？

The Sun and the Moon need no looking into for this, for there is no retrogradation in their course; likewise the Head, for there is no direct motion in its movement. It is needed only for the five planets. So one looks at the place of Saturn on the morrow of the day we assumed in the example, and we find it less than its place on our day by 3 minutes, its degree being the same on both days; so it is retrograde on this day, travelling the 3 minutes. As for Jupiter, we find it greater by 50 minutes, and Mars by 16 minutes; so they are both direct. Likewise Venus, for it increases by 1 degree and 14 minutes; and plainer than that, it is passing on that day from Virgo to Libra, which is the mark of direct motion. Mercury too is direct, because it increases by 1 degree and 28 minutes. When we consider this state in the hours, they are less on the morrow by 2 minutes, so the daytime is decreasing; and besides, it is shorter than the night, because it is less than 12 hours. The state of the midday altitude is like it, decreasing until the time of the winter solstice.

太阳和月亮在这件事上不必查看，因为它们的运行中没有逆行；龙头也是如此，因为它的运动中没有顺行。只有五颗行星才需要查看。于是我们看土星在例中所设那一日的次日的位置，发现它比我们这一日的位置少三分，两日的度数相同；所以它在这一日是逆行的，走了这三分。至于木星，我们发现它多了五十分，火星多了十六分，所以二者都是顺行。金星也是如此，因为它增加了一度十四分；比这更明显的是，它在那一日正从处女座进入天秤座，这是顺行的标志。水星也是顺行，因为它增加了一度二十八分。我们若在时数上考察这种情形，次日的时数少了二分，所以白昼在缩短；此外白昼也比黑夜短，因为它不足十二小时。正午高度的情形与此相同，一直减少到冬至之时。

§323. How is the place of a planet known for an assumed time other than midday?

第三百二十三问　在正午以外的某个指定时刻，怎样知道一颗行星的位置？

Let us make the given time, for example, 7⅔ hours elapsed of our day. We halve the hours of the daytime, and half of them is 5 hours and 39 minutes. We take the difference between this and the given hours, and it is 2 hours and 1 minute; we drop the minute for ease, and there remain 2 hours. These are the hours elapsed from midday to that time, because the hours of half the daytime are fewer than the given ones; had they been more, these 2 hours would be what remains from the time to midday. They are, of the whole hours of the day with its night, a half of a sixth. Then we come to the Sun and take its portion, which is what it travels from midday of this day to midday of the morrow; that is 1 whole degree, and a half of a sixth of it is 5 minutes. We add them to the place of the Sun at midday, and it comes to 19 degrees and 59 minutes of Libra, which is its place for 7⅔ hours elapsed of the daytime. Had the time been 2 hours before midday, we would have subtracted these 5 minutes from the place of the Sun and arrived at 19 degrees and 49 minutes of Libra, which is the place of the Sun for 3⅔ hours elapsed of the daytime. We vary the example for the Moon by making the time 2⅓ hours elapsed of the night of Wednesday. We add to them the hours of half of Tuesday's daytime, so that the total becomes 8 hours, which are those elapsed after midday of Tuesday; they are, of the hours of the day with its night, a third. Then we take the buht of the Moon, which is 14 degrees and 39 minutes, and its third is 4 degrees and 53 minutes; added to its place at midday this gives 12 degrees and 17 minutes of Virgo [corrected: MS has 4 degrees and 53 minutes of Virgo], which is the place of the Moon for 2⅓ hours elapsed of the night of Wednesday [corrected: MS has '2 hours']. On this pattern they have worked it for every direct planet and for everything that increases in its column and does not decrease. As for the retrograde planets, let the example at this time be Saturn: its buht is 3 minutes, and its third is 1 minute, which it travels in the 8 hours between midday and the time. But Saturn is retrograde, travelling against the order of the signs, so we subtract this minute from the place of Saturn at midday, and it comes to 2 degrees and 45 minutes of Gemini [corrected: MS has 2 degrees and 25], which is the place of Saturn for 2⅓ hours elapsed of the night of Wednesday [corrected: MS has '2 hours']. On this pattern is worked everything else that decreases in its column and goes back.

我们设所给的时刻，比如说，是我们这一日白昼已过七又三分之二小时。我们把白昼的时数折半，其半是五小时三十九分。取它与所给时数之差，是两小时一分；为求简便去掉这一分，剩下两小时。这就是从正午到那一时刻所经过的时数，因为半个白昼的时数少于所给的时数；如果多于所给的时数，这两小时就是从那一时刻到正午所剩的时数。它们是一整日连同其夜的全部时数的六分之一的一半。然后我们来看太阳，取它的份额，也就是它从这一日正午到次日正午所走的路程，为整整一度，其六分之一的一半是五分。把它加到太阳在正午的位置上，得天秤座十九度五十九分，这就是白昼已过七又三分之二小时时太阳的位置。如果那一时刻是在正午之前两小时，我们就从太阳的位置减去这五分，得天秤座十九度四十九分，这就是白昼已过三又三分之二小时时太阳的位置。对月亮，我们把例子换一下，设时刻为星期三之夜已过二又三分之一小时。我们把星期二白昼一半的时数加上去，总共成为八小时，这是星期二正午以后已过的时数；它们是一整日连同其夜的时数的三分之一。然后取月亮的布赫特，为十四度三十九分，其三分之一是四度五十三分；加到它正午的位置上，得处女座十二度十七分〔抄本作“处女座四度五十三分”，据算改正〕，这就是星期三之夜已过二又三分之一小时时月亮的位置〔抄本作“两小时”，据算改正〕。人们就照这个样式，对每一颗顺行的行星、对一切在其栏中只增不减的数来计算。至于逆行的行星，此时的例子就以土星为例：它的布赫特是三分，其三分之一是一分，这是它在正午与该时刻之间的八小时里所走的。但土星是逆行的，逆着星座的次序走，所以我们从土星在正午的位置减去这一分，得双子座二度四十五分〔抄本作“二度二十五分”，据算改正〕，这就是星期三之夜已过二又三分之一小时时土星的位置〔抄本作“两小时”，据算改正〕。其余一切在其栏中递减、倒退的数，都照这个样式计算。

§324. What is the astrolabe?

第三百二十四问　何谓星盘？

It is an instrument of the Greeks; its name is asṭurlābūn, that is, the mirror of the stars, and for this reason Ḥamza al-Iṣfahānī derived for it from Persian that it is sitāra-yāb ('star-finder'). With this instrument the times are found, and what has passed of the daytime and the night is known with the least trouble and the greatest correctness; and further operations are obtained with it that almost pass counting for their multitude. It has a back and a front, and separate members that a pin in the middle holds together; on them is a pattern of lines distinguished by names and titles agreed upon for identification.

它是希腊人的一种仪器，名叫“阿斯图尔拉本”，意思是“星之镜”；因此哈姆扎·伊斯法哈尼从波斯语给它找了出处，说它是“西塔拉亚卜”（寻星器）。用这件仪器可以求得时刻，以最少的麻烦、最高的准确知道白昼与黑夜已过了多少；还能用它得出别的许多操作，多得几乎数不清。它有背面和正面，还有几件分开的部件，由中间的一根轴钉把它们连在一起；部件上有一套线条图样，按约定的名称和称号加以区分，以便辨认。

§325. What are the members of the astrolabe?

第三百二十五问　星盘有哪些部件？

The astrolabe as a whole is round. From its rim projects an additional piece called the throne (kursī), in which there is a hole holding the shackle (ʿilāqa) and the ring (ḥalqa). At the centre of the astrolabe is a hole in which turns a pin (quṭb), into which enters a 'horse' (faras) that fastens to it and holds the pin in place. On its back is a long piece like a ruler, turning on the pin, called the alidade (ʿiḍāda). At its two ends are two sharp tips called the pointers of the alidade, and inside them are two square pieces standing upright on the alidade, called the two 'bricks' (labinatān) and the two sights (hadafān); in the middle of each of them is a narrow hole called the hole of the ray, and it would be permissible to call it the point of sight. The face of the astrolabe is the other side from the back. Round it runs a wall called the ḥujra ('enclosure'), and inside the ḥujra, on the face, is a pierced plate called the spider (ʿankabūt) and the net (shabaka). On it are projecting pieces bearing the names of the twelve signs, which are called the zodiac. From the head of Capricorn on it projects a small tongue called the pointer of the ascendant, or simply the pointer, without qualification by anything; when the spider is turned, the pointer keeps touching the ḥujra. Round the point are pointed tips projecting from cut-out pieces of a certain shape [text uncertain], on which are written the names of fixed stars; these are called the pointers of the stars. When the horse is taken out of the pin, the spider and the plates beneath it, made for the climes and the latitudes of towns, come apart, each face of them being for one of these. Above the plate is the ḥujra, and its lines are divided into 360, either by fives or otherwise; there is a line for every 15, which is an equal hour, these being the equal hours, and the one that passes over them is the pointer, which is the head of Capricorn on it; and for what does not make up a whole hour, each time-degree is 4 minutes. This is the figure of that.

星盘整体是圆的。从它的边缘伸出一块附加的部分，叫作“宝座”，上面有一个孔，挂着挂扣和吊环。星盘中心有一个孔，里面转着一根轴钉，轴钉上插进一个“马”，把它扣住、固定轴钉。背面有一件像直尺的长条，绕轴钉转动，叫作照准尺。它两端有两个尖头，叫作照准尺的指针；指针以内有两块方片，直立在照准尺上，叫作两块“砖”，也叫两块觇板；每块中间有一个窄孔，叫作光孔，也不妨称为视点。星盘的正面是与背面相对的另一面。正面四周围着一道墙，叫作“胡吉拉”（围垣）；围垣以内的正面上，有一块镂空的盘，叫作“蛛网”，也叫“网”。它上面有一些突出的部分，写着十二星座的名称，称为黄道带。从其上摩羯座的起点伸出一个小舌，叫作上升指针，也可不加任何限定，径称指针；蛛网转动时，指针一直贴着围垣。在那一点的周围，有一些尖头，从某种形状的镂空部分伸出〔原文疑有讹误〕，上面写着恒星的名称，称为星针。把“马”从轴钉上抽出，蛛网和它下面的盘片就分开了；盘片是为各气候带和各城的纬度而做的，每一面各对应其中之一。盘片之上是围垣，它的线分成三百六十，或每五一格，或用别的分法；每十五有一条线，即一个等时，这些就是等时；从这些线上经过的是指针，也就是蛛网上的摩羯座起点；不满一整小时的，每一时度合四分钟。这就是它的图。