Mid-Autumn Festival moon-gazers are in for a big surprise this year. The full moon will be the fullest one seen in years, the Hong Kong Observatory says.

The distance between the Moon and the Earth that day will be 361,000km, slightly shorter than the average 385,000km, making the celestial body appear larger than usual.

The traditional Chinese festival sees scores of Hong Kong people camp outdoors for the night, lighting lanterns and burning candles while gazing at the Moon and eating mooncakes.

This year the festival falls on September 25 and the full moon will appear at 3.45am after moonrise on September 27. The Mid-Autumn full moon usually occurs on the 15th or 16th day of the 8th lunar month, and occasionally on the 17th day.

The Moon will be at its zenith at 10.24pm on September 24, at 11.13pm on the 25th, and 3am on the 26th.