ALIENS last came to visit Hong Kong on Tuesday - a follow-up to their inspection the night before. But that is nothing out of the ordinary: beings from other planets have come to call on our little patch of Earth hundreds of times before. A sharp-eyed resident of Ta Kwu Ling, near the Chinese border, saw them come last Monday in a ''fluorescent oblong with a line of white dots, shiny and golden, brighter than a star'', heading northwest for about 20 minutes before disappearing from sight. And at the same time the following night, about 8.15pm, he saw the visitors again, though this time descending - presumably to take a closer look at mainlanders below. In June they came on three nights, in a flat purple saucer, in a bright round object, even in a team of 30 in circling round objects 20 times the size of the moon. Although the trips are unbeknown to most of us, luckily there are a few quick-witted sky-watchers who not only spot the visitors, but report them to the Royal Observatory. ''A round, flat shape'', ''bright and shiny'', ''green with a long tail'', ''hovered for half an hour before disappearing'', ''gone in a few seconds'' - piles of questionnaires completed in careful or untidy script by Observatory duty officers give a glimpse of the craft used by visitors from worlds beyond ours. The reports are stored in a folder labelled ''Reports of Sighting Mysterious Objects in the Sky by Residents of Hong Kong''. Senior scientific officer Tam Cheuk-ming is remarkably cool for someone whose job it is to look after such potentially explosive information. ''We get one sighting about every few weeks or so,'' he says. He hesitates to call them UFOs - unidentified flying objects - because, he says, they aren't. ''Most of them can be explained,'' he claims. But many can't or haven't been. The Observatory only starts checking what's happening if it gets a clutch of reports in one night. April 9, 1993, holds the record. That night 110 people rang the duty phone off the hook with sightings of ''pillars of light from heaven'' and fears that Judgment Day had come. The Observatory investigated and found . . . ''Spotlights from Parkview,'' says Mr Tam. A spokesman for the development said light from two laser beams used during festivals must have been reflected off low clouds. The previous record of 56 callers from Kowloon and the New Territories who saw a light shooting over Lion Rock on October 31 - Hallowe'en - in 1990 is less convincingly explained. The Observatory claimed the object was a meteor. But the Observatory has no telescopes. So how could it prove it? Then there was the occasion on November 16 last year, when four people - again in the New Territories - reported similar sightings. The green-headed object with a white or light-green tail, very bright and travelling very fast, was seen by people at Chinese University, Tuen Mun, Tin Shui Wai and Tsuen Wan. Dr Tam shrugs. ''It only lasted a few seconds. How can we check?'' But Dr Tam has checked on other occasions - personally. From the leafy Observatory grounds in Tsim Sha Tsui, he watched for - and saw - a UFO on July 2 after two callers on June 30 and July 1 each reported a rocket-shaped object, with a very bright head with silver sparkles and a long tail. They saw it from Tolo harbour highway and the Cultural Centre at Tsim Sha Tsui, at about 7.15pm on both days. ''It looks like a rocket ascending,'' said one caller. ''I went out at 7.15 on July 2,'' said Dr Tam. ''There was a clear sky and I saw it. ''It was a plane,'' he claims. ''A plane flying so high that people will only see a cigar with a long tail from the engines.'' The Civil Aviation Department confirmed there would have been such a flight at such a time. Dr Tam agrees that for every caller, there must be many who see the aliens but don't bother to ring. ''This'll be the tip of the iceberg,'' he says. He is quite surprised that pragmatic Hong Kongers take the trouble to ring at all. Sometimes they are excited by what they've seen, but mostly, he suspects, they guess it'll turn out to be nothing.