I wanted to tell fortunes, but I saw the stars

Friends who like to study horoscopes must have heard of the horoscope, and will more or less use the horoscope to understand their love, career, and other fortunes.

In fact, in astronomy, the astrolabe is used to observe the sky and the earth. Today we will talk to you about this instrument.

As early as the ancient Greek period, astronomers invented a small astronomical instrument - the astrolabe.

It is usually in the shape of a metal disk and can be held in the hand. Its functions include measuring the height of celestial bodies, identifying stars or planets, determining their latitude according to local time, and performing triangulation. It can be said that the astrolabe is an instrument with a very wide range of applications and plays an important role in ancient astronomical measurements.

It should be noted that in ancient times, astrolabes could be used for fortune-telling in addition to astrology.

Ancient Greek astrolabe

The first person known to have recorded the astrolabe was Theon, a scholar from Alexandria in the 4th century AD, who made a relatively detailed record in his book On the Astrolabe.

But Theon was not the first person to invent a star disk. According to Synesius, a contemporary of Theon, the astronomer Hipparchus had used an instrument to measure celestial bodies hundreds of years ago. This instrument only had a disk and a peep tube, which should be the original appearance of the star disk.

Ptolemy also mentioned that he used an instrument similar to Hipparchus' to observe celestial bodies. Based on these data, it is currently believed that the earliest astrolabe was invented by Hipparchus in the 2nd century BC.

Ancient book introducing the astrolabe. Image source: wiki

The Development of the Astrolabe in the Arab World

Although the astrolabe originated in ancient Greece, it was further developed after it was introduced to the Arab world in the Middle Ages. The eighth-century mathematician Muhammad al-Fazari was the first person in the Arab world to make an astrolabe, and soon the Arabs fell in love with this small and lightweight instrument.

Due to the requirements of religious doctrine, Arabs had to perform worship ceremonies on time, accurately grasp the time of the sun and moon, and also know the direction of religious holy places, so the emergence of the astrolabe just met their needs. In order to achieve better use, the Arabs modified the astrolabe and added some new structures and functions.

The most important change is the introduction of an angle scale and the addition of a horizon circle to indicate the position of the earth. In this way, in addition to calculating the time of the rising of the sun and stars, it is also possible to determine the specific time and direction of the Qiblah, which is very important for Muslims.

An astrolabe with Arabic inscriptions made in 1282 (Source: Wiki)

Later, the astrolabe developed into many different forms in the Arab world. For example, a spherical astrolabe was recorded at the end of the 9th century, which was a combination of an astrolabe and an armillary sphere. In the 12th century, someone invented a linear astrolabe, which was a wooden stick with scales but no sight, equipped with a plumb line and a pointer. In 1235, Abi Bakr of Isfahan invented a mechanical gear astrolabe.

The Arabs developed the style and function of the astrolabe almost to the extreme.

The structure of the astrolabe

The main body of the astrolabe is a circular disc, usually made of brass, with a recessed middle to form a cavity. There are scales marked on the circumference of the disc to represent the horizon, from which the azimuth and angle values ​​can be read.

The back of the disk is usually engraved with scales required for various applications, including time conversion curves, calendars that convert months to the position of the sun on the ecliptic, trigonometric scales, etc.

Disassembled star disk image source: wiki

In the concave part of the disc, there are usually several thin discs with different scales, engraved with circles representing the zodiac, the names of constellations, the brightest stars, etc. These thin discs can be replaced according to different needs.

There is another hollow disk on the outside of the thin disk, which shows the range of the celestial sphere and the visible sky, and usually has scales and decorative patterns.

The outermost part is a ruler with scales, also called an alidade, with small holes at both ends for looking at celestial bodies to measure height and azimuth. The alidade can be installed on the outermost part or on the back.

These parts are connected together through the central axis to form a complete astrolabe.

Disassembled star disk image source: wiki

How to use the astrolabe

When using the star disk, different thin discs can be installed according to different needs. The collimator can also be installed in the front or back, and then the collimator can be rotated to get the desired result based on the reading. Below we list two examples of practical applications.

Example 1: To measure the height of a celestial body: Place the star disk upright and check whether the horizontal and vertical scales on the star disk are consistent with the upright state. Then find the celestial body to be measured through the small hole on the alidade so that the celestial body can be seen through the small holes at both ends of the alidade. At this time, read the scale values ​​pointed to by the two ends of the alidade to know the height of the celestial body.

Using an astrolabe to measure building heights - source wiki

Example 2: Confirm the zodiacal constellation where the sun is located: When the sun rises, observe the position of the sun through the small hole on the alidade, and obtain the trajectory of the sun in the sky through continuous observation. Then rotate the thin disk with the zodiac belt so that the engraved lines on it coincide with the observed trajectory of the sun. The constellation pattern corresponding to the engraved lines is the zodiacal constellation where the sun is located.

The above is just a simple example of the application of the astrolabe. Through a series of observations and calculations, more information can be obtained. Of course, the accuracy of this kind of observation cannot be compared with modern astronomical instruments, but in ancient society, it can fully meet the needs of people at that time.

In addition to its application in astronomy, the astrolabe also played an important role in religion and daily applications. Before the invention of the sextant, the astrolabe should be the most widely used small scientific instrument in ancient times.

There are many more instruments related to astronomy. If you are interested, I will explain them to you later.

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Review expert: Lu Yangxiaoyi, PhD in planetary physics

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