The possibility of intelligent life on other planets has been a subject for speculation for a very long time.
As noted above, there was considerable excitement in the 1890s over sightings of a mysterious “airship” over the United States. Such reports appear to have started in California and then worked their way progressively eastward.
Numerous stories were published at the time of how the ship had been revealed to be the invention of a mysterious genius, or of a secret society of inventors. Other stories said that it conveyed visitors from another world. Still others identified individual sightings as simple phenomena such as the planet Venus during an unusually bright appearance or as hoaxes.
Daniel Cohen wrote a very good survey of the phenomenon called The Great Airship Mystery. He concluded that the sightings were divided among honest mistakes of natural phenomena and hoaxes.
While Cohen suggested that Jules Verne’s two novels about Robur could have had an influence on people, he admits that the books had been published in a previous decade. An even more likely influence, I suspect, is Mark Twain’s little-remembered novel Tom Sawyer Abroad. It was still being serialized in the children’s magazine St. Nicholas when the sightings started. This story tells how Tom, Huck and Jim go to St. Louis to see an airship being displayed there by its inventor. It takes off accidentally after they sneak on board. The story was profusely illustrated, and many of the drawings show a fantastic craft which looks to be part tall-masted ship and part blimp.
In the years leading up to World War I there were a series of airship sightings over Scotland. It has been suggested that war anxieties could help account for these. During World War II a number of Allied flight crews over Europe reported seeing “Foo Fighters”. This name derives from the nonsense language of the then-polular comic strip Smokey Stover. These "fighters’ were glowing shapes which seemed to pace aircraft. Many of them are believed to have been natural phenomena such as St. Elmo’s Fire.
During the early 1930s there were a number of sightings of an airship over Canada. Shortly before his disappearance, W. D. Fard, founder of The Nation of Islam, announced that this was “The Mother Ship”, a secret weapon of the Japanese which was going to lay waste to The United States until only 144,000 people were left.
Following World War II there were numerous sightings of “phantom rockets” over Sweden. War anxieties could again be a cause, as the rockets seem to have always have come from the direction of The Soviet Union.
And then, of course, came Kenneth Arnold’s classic sighting. As noted above, he said that the things he saw seemed to move like saucers skipped on water. A wire service editor in a hurry had written the phrase “flying saucers” in a story which was sent out.
While UFO spotting is largely a modern phenomenon, the possibility of life on other worlds has been speculated about for a very long time.
In the late 19th Century a number of Spritualist mediums claimed to have obtained information about life on Mars and other planets while in trances.
In the early 19th Century a New York paper published the notorious “Moon Hoax”. Starting with some fairly innocuous stories about a powerful new telescope built by a real-life English astronomer, the paper published stories about how life could be observed on the Moon. There were people there with bat wings.
In the 18th Century Voltaire wrote a story called Micromegas. In it two giants from space visit Earth. They are so large that when one of them picks up a whale, it is in his hand like a tadpole is in the hand of a man. This story, incidentally, discusses (and spoofs) an attempt by some astronomers to use “geometric logic” to demonstrate that Mars “should” have two moons.
Jonathan Swift aludes to this idea in Gulliver’s Travels, saying that the people of the scientifically advanced country of Laputa have found that Mars has two moons. Erich Von Danniken cites this in Chariots of the Gods but does so to know apparent purpose except to get readers to say “ooooh…spooky”.
One can trace the belief in extraterrestrial life much farther back than this. Comedian George Carlin used to observe that the idea of reincarnation doesn’t make much sense, as the world’s population keeps growing, so that something must be happening beyond the mere recycling of former inhabitants of this planet. In fact, Hindus have since ancient times speculated that there is life on other planets, and that spirits from such planets can be reincarnated here.
Nevertheless, sightings of exraterrestrial spacecraft has largely been a phenomenon of the late 19th Century and after. Writers on UFOs, especialy in the 1950s and 60s, attempted to demonstrate that it was a longstanding phenomenon. Two stories related by Frank Edwards in his book Flying Saucers: Serious Business, are prime illustrations. One story concerned some monks in England during the Middle Ages who saw a ship hovering over their monastery. An alien creature wearing a space helmet descended to the ground via a ladder. The other story was the one about a craft crashing in Aurora, Texas.
The Aurora story was published during the great Airship scare of the mid-1890s. Numerous writers have demonstrated that details in the story do not match up with facts about the town as it was at that time, and have shown that the town was anxious to draw attention to itself. IIRC, Aurora had been passed over in favor of a neighboring community for a railroad station, and the economy was declining.
The story of the monks was first published in England in the 1950s. It was a pure hoax; some schoolboys simply made it up and sent a letter about it to a paper.
Many other efforts to prove that UFOs were sighted in earlier appear to be similarly bogus; often they consist of interpreting an artifact without providing any historical or cultural context. One I have found particularly interesting is the claim that flying saucers can be seen in the Bayeaux Tapestry. They look remarkably like clouds.
Although belief in life on other planets is of longstanding, the OP is correct: the vast majority of UFO sightings have occured since the time of Kenneth Arnold.
There once was a vogue for suggesting that this was because we had only recently become interesting to the aliens; possibly they were drawn here by television and radio broadcasts, or by the invention of the atomic bomb. This latter idea featured in the classic film The Day the Earth Stood Still. As it became apparent that nearby planets do not support intelligent life, and as people became more sophisiticated about the tremendous distances between objects in space, this idea has lost popularity.
A more likely explanation is that it is only in relatively recent times that a great many people have been actively disposed to believe in the possibility of spaceships.
Frank Edwards noted that descriptions of craft in early UFO sightings were greatly different than in more recent sightings, and suggested this was proof that the aliens were advancing technically. In fact, the early airships resembled the blimps and dirigibles people were soon able to perfect. And flying saucers? They had already been depicted in pulp magazines for years before a wire service editor misintrepreted what Kenneth Arnold said.
Even if I wanted to believe in “god” there are too damn many to choose from.