Close up of the Moon near the Sea of Tranquillity. The approximate location of the landing of Apollo 11 in July 1969 is marked with an X. Image Nick Lomb
On Friday and Saturday evenings 12 & 13 December 2008 the full Moon appeared unusually large. This was because the time when the Moon was closest to Earth (perigee) and hence at its largest almost coincided with the time of the full Moon. Perigee was at 9:00 am (eastern summer time) while the instant of full Moon was a few hours earlier at 3:37 am on 13 December 2008.
Where to look for the Moon was discussed by your blogger on Channel 7’s Sunrise with Melissa Barr and David Koch Friday 12 December 2008.
Perigee and full Moon do not normally coincide as they follow different periods. The time from one full Moon to the next is on average 29 days 12 hours 44 minutes while the time from one perigee to the next is on average 27 days 13 hours 19 minutes. The two days difference in periods means that they will quickly get out out of step and may take a year or so to coincide again. The former period is technically known as the synodic month while the latter is the anomalistic month.
Tides are mainly due to the Moon but there is also some assistance from the Sun. Drawing Nick Lomb
There are always higher tides than normal at full Moon as the Sun and the Moon are in the one line, albeit they are on opposite sides of the Earth. The Moon being much closer to the Earth than the Sun has a greater effect in creating tides than the Sun, but the Sun also has a effect and when they combine at full Moon or new Moon we have king tides. On this full Moon, as discussed above, the Moon was at perigee, at its closest to Earth, so it pulled on the oceans more than usual. Plus it was near the summer solstice when the Sun is at its highest in the sky and the full Moon at its lowest. This also added to the tides. All in all it is not surprising that in Sydney there have been tides of over 2 metres over the last few days and the sea walls of a number of unit blocks have been seriously damaged by the high water.