Full moon to rise on day following Mid-Autumn Festival: Largest 'Supermoon' of the year to appear on Sept 18

Next Tuesday (Sept. 17) marks the Mid-Autumn Festival. This year, the largest and fullest moon will appear the day after the Mid-Autumn Festival, on Sept. 18.

Even more exciting is that in this Mid-Autumn Festival, HK residents will witness the largest "supermoon" of the year.

"Supermoon" is not an official astronomical term but originates from astrology. A supermoon occurs when the full moon coincides with the moon's closest approach to Earth in its orbit.

While textbooks state that the average distance between the Earth and the Moon is 380,000 kilometers, the Moon's orbit around the Earth is not a perfect circle but an ellipse. The farthest distance from the Earth to the Moon is about 400,000 kilometers, while the closest is approximately 360,000 kilometers. This closest point is known as the Moon's "perigee" relative to Earth.

When combined with the familiar concept of lunar phases such as new moon and full moon, a "supermoon" can be simply understood as the largest and brightest moon visible to the naked eye. It appears about 14% larger and 30% brighter than the smallest full moon.

The Hong Kong Observatory said that the full moon will appear on the morning of Sept. 18, but residents can still enjoy the bright and perfectly round moon in the early hours and evening of the 18th. 

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