THE HARVEST MOON
WHEN: EARLY SEPTEMBER
The full Moon on 10 September is the Harvest Moon for ➤0➤➤, defined as the closest full Moon to the September equinox, which in ➤0➤➤ occurs on ➤3 September at ➤:04am BST. The tilt and orientation of Earth’s axis relative to the Sun makes the Sun appear to rise and fall annually against the background stars. An equinox occurs when the Sun’s centre crosses the celestial equator, the projection of Earth’s equator into the sky. The September equinox, also known as the northern hemisphere’s autumn equinox, is the second of two annual equinoxes.
The nearest full Moons to the September equinox have the property that their fuller phases rise with the minimum difference in time from one night to the next. For example, the 90 per cent-lit waxing gibbous Moon on 7 September rises at 7:1›pm BST. On 8 September, now at 96 per cent illumination, the Moon rises at 7:40pm BST, ➤› minutes later than it did the previous day. On 9 September, the virtually full Moon rises at 7:›7pm BST, 17 minutes later than the day before.
As a result, the fuller phases appear at similar times of the evening, lighting the fields for harvest collection. This is why the nearest full Moon to the September equinox is known as the Harvest Moon. The graphic shows just how close the rise times are either side of the full Moon on 10 September.
The earliest Harvest Moon occurs on 8 September, and the latest on 7 October. In reality, the full Moons that occur a month each side of the Harvest Moon exhibit only marginally greater rise time differences. The average fuller phase moonrise difference is around ›0 minutes, the greatest occurring near the March equinox when it’s around 80 to 90 minutes.