This week watch for the beautiful harvest moon to rise above the horizon on Tuesday. The harvest moon is defined as the full moon closest to the autumnal equinox. This year, the equinox falls on the 22 September, and the moment of full moon takes place on 21 September at 00.54 BST. The moon will rise from London that evening at 19.37 BST, with 99.8% of its surface illuminated. The nights either side will also present full discs, with almost 99% of the moon’s surface lit. The harvest moon is the best known of the names given to the various full moons of the year. They are derived from Native American culture. Other societies also named the full moons and used them as calendrical markers before it became common practice to use a calendar based on the movement of the sun and the modern months of the year. The equinox is the day on which the lengths of day and night are equal. It signals the true arrival of autumn. From now until the corresponding equinox in March, the nights will be longer than the days, and until the winter solstice in December, the nights will become progressively longer.