Jan Oort: Father of the Oort Cloud<\/h3>\n
Jan Hendrick Oort was born on today\u2019s date \u2013 April 28, 1900 \u2013 in Franeker, Netherlands. We know his name today because he theorized the existence of the Oort Cloud, a vast comet cloud in the outermost reaches of our solar system. <\/p>\n
As early as 1932, Oort also became one of the first to use the term dark matter. <\/p>\n
And, when it came to expertise about our home galaxy, the Milky Way, few astronomers in the 20th century were more knowledgeable than Jan Oort.<\/p>\n
Jan Oort and the Oort Cloud<\/h3>\n
1950 was a key year for Oort. That was the year he proposed the theory of the Oort Cloud.<\/p>\n
The Oort Cloud is also known as the \u00d6pik-Oort Cloud in honor of Ernst \u00d6pik, an Estonian astronomer. \u00d6pik had independently postulated the existence of a cloud of comets encircling our solar system in 1932.<\/p>\n
The theory of this comet reservoir stemmed from astronomers\u2019 observations. They noticed that two types of comets travel into the inner solar system to round the sun that binds them in orbit. Some have relatively short orbital periods, on the order of about 200 years or less. And some comets require much longer, thousands of years, to orbit the sun once.<\/p>\n
But where do these comets come from? Oort proposed a reservoir of comets at the outer limits of our solar system. He said that long-period comets are sometimes knocked from their very distant orbits (perhaps by passing stars) to orbits that bring them near our sun.<\/p>\n
If it exists, this cloud of comets \u2013 the Oort Cloud \u2013 contains material leftover from the formation of our solar system, 4 1\/2 billion years ago. The comets within it lie as close as about 5,000 times up to about 100,000 times the Earth-sun distance. That\u2019s a distance of up to 93 trillion miles (150 trillion km) away.<\/p>\n
The Oort Cloud of comets is not an observed fact. It\u2019s still a theory. But it\u2019s a well-accepted theory by astronomers that has stood the test of time. And it\u2019s thought to explain the origin of long-period comets such as Comet Hale-Bopp.<\/p>\n Prior to Oort\u2019s work on the Oort Cloud, astronomers wondered for hundreds of years (or thousands of years, if you count history\u2019s earliest watchers of the skies) where comets originate. Astronomers in the 20th century knew that comets collide with other celestial bodies. They knew comets vaporize when they pass too near the sun. And sometimes those close encounters eject them from our solar system. <\/p>\n And yet there are always new comets coming to our part of the solar system. Why? Where do they come from?<\/p>\n The Oort Cloud answers this paradox of comets that seem to appear out of nowhere.<\/p>\n Oort was one of five children. His father, Abraham Hendrikus Oort, was a psychiatrist. Oort\u2019s parents always encouraged him to follow his passions. And so he decided to study physics at the University of Groningen in 1917.<\/p>\n Attending the lectures of astronomer Jacobus Kapteyn was a turning point for Oort. In fact, Kapteyn\u2019s research greatly inspired him and he switched to studying astronomy.<\/p>\n Later, in 1924, Leiden Observatory welcomed Oort, where he began studying high-velocity stars. Two years later, he defended his doctoral thesis on that subject. This was, additionally, four years after the death of his friend and mentor, Professor Kapteyn.<\/p>\n In 1926, astronomer Bertil Lindblad explained the stellar motion properties studied by Kapteyn to be the result of the rotation of the Milky Way. He explained it by proposing that stars closer to the center of the galaxy revolve around the galaxy\u2019s center faster than stars farther away from the center. Subsequently, Jan Oort successfully proved and modified Lindblad\u2019s theory in 1927 after observing the velocities of many stars.<\/p>\n During Oort\u2019s studies of star motions in 1932, he noticed that many stars move faster than expected, given their location within the Milky Way. With this in mind, he then used the term dark matter<\/em> \u2013 not as we use it today \u2013 but in the sense of ordinary stars that are either dim (or dark) or hidden from us behind other stars.<\/p>\n Read more about Fritz Zwicky, Jan Oort and dark matter here<\/p>\n Oort continued developing the Lindblad theory. It eventually came to be known as the Lindblad-Oort theory because of his contributions.<\/p>\n Later, Oort became a professor at the University of Leiden in 1935. Among other major accomplishments, the young professor determined that our sun is some 30,000 light-years from the center of our Milky Way galaxy. This is still the number we use today. He also calculated that the sun orbits around the center of the galaxy once every 225 million years.<\/p>\n In 1945, the Observatory of Leiden appointed Oort as their Director. <\/p>\n He maintained this position until 1970.<\/p>\n Oort died in 1992, at 92 years old. But his contributions to astronomy live on.<\/p>\n Bottom line: Dutch astronomer Jan Oort was born on April 28, 1900. He visualized a vast reservoir of icy comets on the outskirts of our solar system, which now bears his name.<\/p>\n <\/div>\n![\"Diagram](\"https:\/\/earthsky.org\/upl\/2016\/04\/Oort_Cloud_1400-e1461767642983.jpg\")
Jan Oort solved the comet puzzle<\/h3>\n
![\"Diagram](\"https:\/\/earthsky.org\/upl\/2014\/03\/solar-system--e1395672935279.jpg\")
In school, he followed his passions<\/h3>\n
![\"Black](\"https:\/\/earthsky.org\/upl\/2016\/04\/jan-oort-young-e1461770550372.jpg\")
Jan Oort\u2019s early work<\/h3>\n
![\"Old](\"https:\/\/earthsky.org\/upl\/2016\/04\/jan-oort-beret-leiden-e1461770466556.jpg\")