A BIG MOON
WHEN: 3 December, all night long
The Moon is full on 3 December at 15:48 UT. It also reaches perigee, its closest orbital position to Earth, 17 hours later, making this the perigee full Moon for 2017. For those directly under the Moon at 15:48 UT, it will appear slightly larger and brighter than the other full Moons of the year.
Compared to the apparent size of the apogee full Moon, the one which occurs when the Moon is farthest from Earth, the difference can be as much as 14%. In reality, these extremes never occur next to one another in the lunar calendar, being separated by approximately six months. This month’s perigee full Moon will also be beaten by the one due to occur next month on 2 January, the perigee full Moon for 2018.
A popular unofficial term encompassing perigee full Moons is ‘Supermoon’. This original astrological definition describes the situation when a full or new Moon occurs within 90% of the perigee or apogee distance; closer than 361,430km or farther than 401,670km. The correct astronomical definitions for these situations are apogee-syzygy or perigee- syzygy Moons. The term ‘syzygy’ describes when three bodies in a gravitational system appear in a line. This is the case with the Sun, Earth and Moon when the Moon is either full or new.