Observations

The largest Moon for 2008 causes high tides

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

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.

Drawing of tides due to the Moon and also assistance from the Sun. Drawing Nick Lomb

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.

6 responses to “The largest Moon for 2008 causes high tides

  • Hi Nick
    Surely the high tides are higher on new moon when the moon and the sun are on the same “side “of the earth and therefore combine their gravitational pull?

    • Hello John. Remember there are two tidal bulges due to the Moon, one directly below the Moon and one on the opposite side of the Earth to the Moon. Similarly there are two (smaller) tidal bulges due to the Sun. Whenever the Sun and the Moon are in the one line, either at new or full Moon, the two sets of tidal bulges overlap and reinforce each other giving us have higher than normal tides.

  • Hi Nick,

    So there is a third variable, that of the sun’s height in our sky? So the solstice is important for latitudes greater than 22 degrees since at those latitudes, the highest the sun gets in our sky is at the solstice.

    So full moon+ perigee + solstice = really high tides! (and low?)

    Thanks for this great info.

    • Hello Greg. There are a number of factors involved in determining the tides, but these are amongst the most important. Note that as far as the tides are concerned full Moon and new Moon are equivalent.

  • Hello Phil. The Sun being further away it has less of an effect than the Moon, but it still acts the same way. So it will have more of an effect on the tides when it is closer, that is when the Earth is near perigee, than when it is further, that is the Earth is near apogee. When the Sun and Moon are in the one line as at full Moon or new Moon, their effects add. When they are at right angles to each other as at first or last quarter phase, than the Sun will act to reduce the effect of the Moon on the tides.

  • Nick

    “…but the Sun also has a effect and when they combine at full Moon or new Moon we have king tides.”

    The earth is in an elliptical orbit around the sun so I was wondering when the highest possible tide would occur?

    Earth at sun apogee – moon at perigee and full moon
    Earth at sun perigee – moon at perigee and full moon

    my instinct tells me that the sun should be furtherest away (earth at sun apogee) so it dilutes the moon’s gravitational pull the least.

    Cheers
    Phil

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