Uses of the Astrolabe

In the 10th century, Abd al-Rahmn b. Umar al-Suf (d. A.H. 376/A.D. 986-7) wrote a detailed treatise on the astrolabe consisting of 386 chapters in which he described 1000 uses for the astrolabe. al-Sufi perhaps overstated the flexibility of the astrolabe, but astrolabes can be used to solve many astronomical problems that would otherwise require rather sophisticated mathematics. All of the everyday uses of the astrolabe are not known, but they were certainly used to tell time during the day or night, to find the time of sunrise and sunset and, thus, the length of the day, to locate celestial objects in the sky, as a handy reference of celestial positions and, as astrology was a deeply embedded element of the cultures that used astrolabes, to determine aspects of horoscopes. Islamic prayer times are astronomically determined, and the astrolabe could be used to determine the required times. Modern astrolabes, such as the Personal Astrolabe, can be used to solve astronomy problems involving sidereal time and can be used with modern civil time. Following are two examples of astrolabe uses:

Finding the time of day.

The time of day is found in the following steps:

  1. The altitude of the Sun or a bright star is determined using the back of the instrument. The astrolabe is held above eye level from the suspension. The astrolabe is oriented so the Sun or star is lined up with the back of the astrolabe. The alidade is rotated until the Sun's shadow or the star itself is visible through the sights on the alidade. The altitude is noted from the altitude scale on the back of the instrument.
  2. The Sun's position on the ecliptic is found by setting the alidade on the date and reading the Sun's longitude on the zodiac scale.
  3. On the front of the astrolabe, the rule is rotated until is crosses the ecliptic at the Sun's current longitude. The point where the rule crosses the ecliptic is the Sun's current position.
  4. The rete and rule are rotated together until the Sun or star pointer is at the measured altitude.
  5. The rule points to the apparent solar time on the limb. Apparent solar time is the time as shown on a sundial and is different for each longitude. In modern use, apparent solar time must be corrected to zone time by compensating for the equation of time and the difference in longitude from the center of the time zone. The appropriate scales for this correction are on the back of the Modern Edition of the Personal Astrolabe.

    It should also be noted that in the middle ages the time of day was usually expressed as the part of the day or night that had passed. That is, sunrise was the beginning of the 1st hour of the day, noon was the end of the 6th hour and sunset was the end of the 12th hour of the day and the beginning of the 1st hour of the night. The length of the hour changed during the year with the amount of change depending on the latitude. An "hour" was longer in the summer than in the winter. These hours are called, "Unequal Hours" and many astrolabes had curves on the plate for determining the unequal hour of the day or night. The use of unequal hours for civil time keeping gradually declined as reliable clocks and watches became available in the 17th and 18th centuries although their use continued in parts of the world well into the 19th century. The use of unequal hours is not as awkward as it might sound. The unequal hour of the day is the percent of the day that has passed. The convention is quite easy to get used to and is quite valuable for some environments.

Finding the time of a celestial event.

The time of a celestial event such as sunrise, sunset or the culmination of a star is found by setting the astrolabe to the circumstances of the event and reading the time:

  1. Determine the Sun's position on the ecliptic (longitude).
  2. Set the rule to that position on the ecliptic on the front of the astrolabe.
  3. Rotate the rete and rule together until the desired event is in position. For example, to find the time of sunrise, rotate the rete and rule until their intersection is right on the eastern horizon.
  4. Read the time from the rule's position on the limb.

The length of the day can be found by finding the time of sunrise and sunset and calulating the difference. Similarly, the time until sunrise and sunset can be found as the difference with the current time.

Other uses.

The rule on many astrolabes was divided by declination. The declination of a celestial object could be found by placing the rule over the object and reading the declination directly. This function is particularly useful for the Sun. Right ascensions are found by rotating the rete until the celestial object is on the meridian and reading the sidereal time from the position of the First Point of Aries on the rete. A special plate for the latitude where the horizon is the ecliptic (90 deg - obliquity of the ecliptic) was sometimes provided for finding celestial latitudes and longitudes.

Sidereal time is easily found on an astrolabe. Local sidereal time is the hour angle of the vernal equinox at Aries 0 (the so-called, "First Point of Aries"). To find the current local sidereal time, the rete is rotated to the current position of the ecliptic and the sidereal time is read directly from the position of the vernal equinox. Similarly, the right ascension of any celestial object is the sidereal time when the object is on the meridian. To read the right ascension of a star, set the rete so the star is on the meridian and read the right ascension from the position of the vernal equinox.

Islamic instruments often had special scales for finding the direction to Mecca (qibla) and for determining prayer times. For example, the time of the asr prayer is defined as the time when the shadow of vertical gnomen is equal to the length of the shadow at noon plus the length of the gnomen. A scale of cotangents was sometimes provided to determine when this would occur.

Astrolabes can be used to solve problems for the moon and planets such as rising and setting times, meridian passage and finding sky positions in conjunction with an almanac. The moon or planet's declination and right ascension are found in the almanac and a mark is made at its location and the problem is solved using the normal procedures.

The The Personal Astrolabe is in regular use by hikers to find the amount of daylight remaining, sailors to find navigational stars, architects to determine shadow positions and astronomers to plan observations. It is also in use at several innovative colleges and universities as an aid in teaching basic positional astronomy.

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This page constructed by James E. Morrison, Janus.