In order to discuss what the first observatory was, we first have to define observatory. An observatory is a building housing an astronomical telescope and other various scientific equipment to observe celestial bodies. Since a telescope is a requirement, the first observatory could not have predated the invention of the telescope in 1608. Before this, some civilizations had primitive forms of celestial tracking using megalith structures. Places like Stonehenge in Europe and the Pyramid of the Moon in Mexico once functioned this way. What we would now consider an observatory would not appear until Galileo Galilei in 1609.
Though Galileo is often accredited with the first telescope, it was actually invented by a Dutch lensmaker named Hans Lippershey in 1608. Galileo was, however, the first to use the advancement in technology to create an observatory. The Dutch telescope was used to see features on the Earth's surface, mainly for the use of tactics in battle. It had a magnification rate of three times, so objects appeared three times larger than they otherwise would. Galileo was a part of a small group of scientists who pointed the telescope toward the sky. He took the Dutch telescope and modified it, eventually reaching a magnification rate of x30, so objects appeared 30 times larger.
Founded in 1609, Galileo’s observatory housed a telescope that only rotated in one direction, along the meridian. The meridian is a figurative circle around the Earth passing through both the North and South poles. By centering his telescope along the meridian, Galileo was able to more accurately position measurements of celestial bodies. Other observatories were built in Europe the same way, in the 18th and 19th centuries.
Other Notable Observatories
In the 18th century, an observatory was built in Slough, England housing the largest telescope in the world at the time, known as the Great Forty-Foot Telescope. It housed a mirror 48-inches in diameter with a 40-foot focal length. The Observatory House discovered the 6th and 7th moons of Saturn. This was the largest telescope in the world for 50 years until it was dismantled due to safety constraints.
Now we have many observatories with bigger, more advanced technology than the Observatory House in England. One of our largest Earth-based observatories is now in Mauna Kea, Hawaii. This observatory, known as the Keck Observatory, houses two telescopes with mirrors 33-feet (10-meters) in diameter.