This fantastic Hubble Space Telescope image of the week is of a galaxy called NGC 3156. It is a lenticular galaxy, that is, it is somewhere between an elliptical galaxy and a spiral galaxy. It is approximately 73 million light-years away from Earth and is located in the small equatorial constellation Sextants.
Galaxy NGC 3156 photographed by the Hubble Space Telescope. Image source: ESA/Hubble and NASA, R.Sharples, S.Kaviraj, W.Keel
The Sextant is a minor constellation that belongs to the Hercules constellation family. It is itself an astronomy-themed constellation, named after the instrument called the sextant. The sextant is generally considered a navigational instrument invented in the 18th century. However, the history of the sextant as an astronomical tool goes much further than that: Islamic scholars invented the astronomical sextant hundreds of years ago to measure angles in the sky.
UlughBeg Observatory in Samarkand, Uzbekistan. Built by the Timurid astronomer Ulugh Beg. Image source: ESA
A particularly striking example is the huge sextant with a radius of 36 meters developed by the Timurid Ulugh Beg in Samarkand (today's Uzbekistan) in the fifteenth century. These early sextants may have been a development of the quadrant, a measuring device proposed by Ptolemy. A sextant, as the name suggests, is shaped like one sixth of a circle, approximating the shape of a constellation.
Sextants are no longer used in modern astronomy, replaced by instruments that allow for more accurate and precise measurements of the positions of stars and celestial bodies. In addition to determining its precise location, NGC 3156 has been studied in many ways—from its population of globular star clusters, to its relatively recent star formation, to the supermassive black hole at its center that is destroying stars.
The sextant is a navigation instrument mainly used to measure the angle between celestial bodies and the horizon to help navigation at sea and in the air. This precision tool is shaped like one-sixth of a circle and utilizes a small telescope and reflector. By observing the altitude of a star, the sun, or the moon, navigators and pilots can determine their latitude and sometimes longitude relative to the Earth's surface. Historically, sextants have been useful to explorers and navigators, allowing them to accurately chart courses across vast expanses of ocean or air.