User:Fenixels/Astrolabe

Astrolabe
Astrolabe is a scientific instrument used in astronomy that was later made into a hobby. In its simplest form it is a metal disc with a pattern of wires, cutouts, and perforations that allows you to calculate astronomical positions precisely. It is most commonly used to measure the altitude and azimuth of celestial bodies such as stars or planets. A less common use for an astrolabe is as a sundial. The charge plate provides the time indication which makes this instrument a heat-beat type sundial with an hour hand usually found on many other sundials. During the middle ages, the astrolabe was known as one of the most widely popular and used scientific instruments. However, many Greek texts and other documents from great societies testify that similar instruments to the astrolabe have been in development ever since second century BCE.

History
An early astrolabe was invented in Greece by Hipparchus of Nicaea in the 2nd century BC. It is a device that allows a star or planet to be observed from any point on Earth at any given time, using the position of the sun as its reference point. The astrolabe, which can be used to determine local time from celestial bodies, also functions as a solar and lunar dial and a tide-predicting machine. Invented in Mesopotamia and Persia before it was known in western countries, it was first described by Ptolemy (90-168 AD).

The first Renaissance treatises dealing with scientific problems were based on earlier classical works and were often concerned with Ptolemaic doctrines. Among these, the most influential was the astronomical section of Regiomontanus' Almanacium, written in 1472 and published in Venice in 1506.

Astrolabes were widely avidly used by astronomers and navigators throughout the 17th and 18th centuries for plotting latitude and longitude on charts and marine atlases. They were still used as early as the 18th century for such purposes as measuring land surfaces such as harbors or canals. The use of astrolabes declined after the widespread availability of sextants by 1770.

Applications
Supposedly, the astrolabe has over 100 functions. These functions include: 1. Altimeter 2. Azimuth 3. Altitude 4. Sun/Moon Rise/Set 5. Solar Noon.

According to The History of the Astrolabe, it is a timekeeping tool, a device for locating a specific star, a navigational tool, and an instrument for measuring altitudes and distances between objects above the horizon. On the other hand, according to Astrolabe – Definition and History, the astrolabe “…was used as an aid for telling time, as well as an instrument to determine latitude (or north-south position) by observing the height of stars above the horizon”. Today, the astrolabe is a gold, silver, or bronze plate with a circle of different colored jewels around the periphery. The astrolabe has a protractor at its center used to measure angles. The astrolabe marks the time by the shadows made by marks on the ground.

In 2010, Jens Finkenstaedt, Silke Wenzel, and Thomas Zair from the Research Institute for Astronomy/Astrophysics (IFA) at University of Hawaii discovered that shear motion within clusters of stars leads to a change in their angular velocity with time. Their results were published in Astronomy & Astrophysics on January 16th, 2011.

Construction/Parts
Parts of the astrolabe consisted of a circular disk, a sighting tube, an alidade, and arms that held graduated scales. The circular disk is the main part of the astrolabe. It was mainly used to view figures in the sky. The sighting tube is placed on top of the circular disk and was used to observe stars or planets. The alidade had vertical and horizontal cross-hairs which plots locations on an azimuthal ring called an almucantar (altitude-distance circle). An arm called a radius connects from the center of the astrolabe to the optical axis which is parallel with another arm also called a radius. The other radius contains graduations of altitude and distance measurements.

The instrument and it's parts measure time and distance by using the shadow cast by the gnomon which is the alidade. The gnomon casts a shadow from a conical shaped slot on the edge of the circular disk. This causes an angle that is equal to the difference of time and altitude for that specific time of day. In order to use this tool for altitude measurement, one must know their latitude. One must also decide on a star as a reference point, as well as determine any other optical corrections. The first person to record personal observations with an astrolabe was Edwin Hubble in 1916 after he had seen Halley's Comet for himself.