A BIG MOON

Sky at Night Magazine - - DECEMBER -

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.

This month's full Moon will be the big­gest and brightest of the year

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