By . Alex Finnis . An 18-carat gold timepiece considered 'the Holy Grail of watches' is expected to sell for a record-breaking $16.5m at auction. The celebrated Henry Graves Supercomplication is a masterpiece of horology - the science of measuring time - and took eight years to make by hand. Swiss watchmaker Patek Philippe was commissioned to make it by New York banker Henry Graves in 1925, and it is considered to be the most complicated watch ever made. An 18-carat gold Henry Graves Supercomplication timepiece considered 'the Holy Grail of watches' is expected to sell for more than $16.5m at auction . Tim Bourne, Sotheby's Worldwide Head of Watches, said: 'The list of superlatives which can be attached to this icon of the 20th century is truly extraordinary. 'Indisputably the "Holy Grail" of watches, The Henry Graves Supercomplication combines the Renaissance ideal of the unity of beauty and craftsmanship with the apogee of science.' The 18-carat gold openface watch was the culmination of three years of research and two years of painstaking craftsmanship. The watch last sold at Sotheby's in 1999 for $11m and became the most expensive of all time to sell at auction. Now it looks set to break its own record . The watch features a perpetual calendar, moon phases, sidereal time, power reserve, and indications for time of sunset and sunrise and the night sky of New York City. It has 24 horological complications and was considered the 'world's most complicated watch' for 56 years, according to Sotheby's. A horological complication is any feature on  that indicates something other than hours or minutes. Only with the advent of computer-assisted machines to aid technical perfection in watch making today has any timepiece surpassed the Henry Graves' ingenuity. This is the second time it is being sold at auction, after being bought in 1999 for a record-breaking $11m. It became the most expensive timepiece ever sold at auction, but is now set to enter the record books for a second time. The sale will come among Sotheby's lots of important watches in Geneva in November, to coincide with celebrations for Patek Philippe's 175th anniversary. Unsurprisingly, the Henry Graves Supercomplication, which is a minute repeating chronograph clockwatch with Westminster chimes, is the highlight. Sotheby's first sold the watch in New York in December 1999, as part of a sale of 81 masterpieces from the world-renowned Time Museum. Daryn Schnipper, chairman of Sotheby's Watch Division, said: 'Our offering of this horological work of art in 1999 was unquestionably the highlight of our professional careers and set a world record which has held until today. 'We are extremely privileged to be offering it once again.' It will go under the hammer on November 11.
Henry Graves Supercomplication going under hammer at Sotheby's .
Features 24 horological complications and is 18-carat gold .
Set to break own record for most expensive watch sold at auction .
By . Alex Finnis . An 18-carat gold timepiece considered 'the Holy Grail of watches' is expected to sell for a record-breaking $16.5m at auction. The celebrated Henry Graves Supercomplication is a masterpiece of horology - the science of measuring time - and took eight years to make by hand. Swiss watchmaker Patek Philippe was commissioned to make it by New York banker Henry Graves in 1925, and it is considered to be the most complicated watch ever made. An 18-carat gold Henry Graves Supercomplication timepiece considered 'the Holy Grail of watches' is expected to sell for more than $16.5m at auction . Tim Bourne, Sotheby's Worldwide Head of Watches, said: 'The list of superlatives which can be attached to this icon of the 20th century is truly extraordinary. 'Indisputably the "Holy Grail" of watches, The Henry Graves Supercomplication combines the Renaissance ideal of the unity of beauty and craftsmanship with the apogee of science.' The 18-carat gold openface watch was the culmination of three years of research and two years of painstaking craftsmanship. The watch last sold at Sotheby's in 1999 for $11m and became the most expensive of all time to sell at auction. Now it looks set to break its own record . The watch features a perpetual calendar, moon phases, sidereal time, power reserve, and indications for time of sunset and sunrise and the night sky of New York City. It has 24 horological complications and was considered the 'world's most complicated watch' for 56 years, according to Sotheby's. A horological complication is any feature on  that indicates something other than hours or minutes. Only with the advent of computer-assisted machines to aid technical perfection in watch making today has any timepiece surpassed the Henry Graves' ingenuity. This is the second time it is being sold at auction, after being bought in 1999 for a record-breaking $11m. It became the most expensive timepiece ever sold at auction, but is now set to enter the record books for a second time. The sale will come among Sotheby's lots of important watches in Geneva in November, to coincide with celebrations for Patek Philippe's 175th anniversary. Unsurprisingly, the Henry Graves Supercomplication, which is a minute repeating chronograph clockwatch with Westminster chimes, is the highlight. Sotheby's first sold the watch in New York in December 1999, as part of a sale of 81 masterpieces from the world-renowned Time Museum. Daryn Schnipper, chairman of Sotheby's Watch Division, said: 'Our offering of this horological work of art in 1999 was unquestionably the highlight of our professional careers and set a world record which has held until today. 'We are extremely privileged to be offering it once again.' It will go under the hammer on November 11.
18-carat gold Henry Graves Supercomplication craftsmanship. The watch last sold at Sotheby's break its own record . The watch features a It has 24 horological complications and was most expensive timepiece ever sold at auction,