Watch out for perigee
FRIDAY OCTOBER 02, 2009
When any two bodies in space orbit each other they are never equidistant. There is a closer orbit point (perigee) and a furtherest away position (apogee). The closer flypast causes the smaller body to speed up and the larger one to slow. For instance in the case of sun-orbiting-earth the closest earth/sun point is always over the first week in January, which is why southern summers are slightly hotter than summers in the northern hemisphere, and why weather systems typically speed up in the first week of January bringing changeable weather. On the other hand, the sun is furthest from earth in the first week in July, which is partly why we usually get a cold snap then, setting in for a few days.
When the moon comes closer, on its 27-day perigee beat, the increased lunar gravitational pull on the sea causes more currents, greater swells, higher tides and rougher sailing conditions. For instance the Wahine disaster in April 1968 occurred within four days of a full moon in perigee. The tide of the air, which manifests in a daily change in air-height, also increases at perigee time. Weather conditions are exaggerated as a result. Around this time a passing shower may build to a storm event, or a dry patch can produce hotter than usual temperatures. The Earth-Tide, by which the ground surface lifts every day of every year towards the daily transiting moon by up to 50cms in some places, also increases around perigee. This brings land disturbances that manifest as increased eruptions, volcanoes, and underwater emissions. Old mariners' manuals always warned about perigees, and the annual Nautical Almanac produced by the NZ coastguard listed all perigees and apogees for the year because storms and waterspouts were well known to accompany higher and rougher tides, hazardous to sailors entering channels.
Closer perigees are powerful. Perigees bring wind, and closer perigees more wind, and in the last week of October there was mayhem everywhere. Thousands of homes across Canterbury were without power as strong winds felled trees and power poles. A weather bomb also hit coastal Southland and Otago bringing high winds, driving rain and lightning, hampering efforts to reconnect the 1500 Southland and Otago homes who also lost their power. By the end of that closest-perigee month, October had entered the record books as being the third windiest month in New Zealand in 18 years. MetService spokesman Bob McDavitt said 41 severe wind warnings had been issued.
Because the whole earth revolves beneath the same moon in 24-hours, a closer perigee affects all countries to some extent. Vandal-lit fires in California fanned by abnormally big winds ran the fire out of control. Bush fires also raged in Australia, tornadoes raced through Oklahoma and in Florida a shopping mall blew over. There is however, good in everything. The Apollo 11 moon mission left on 21 July 1969, six days before the July perigee, meaning the moon was 50,000kms closer to earth than it normally is. Because July was the month of closest perigee that year, another 10,000kms worth of rocket fuel was not required.
Another example: a closest year’s perigee occurred on the last day of February, 2006. What happened then? Within two days, on 3 March, widespread strong to gale southerly winds arrived in Canterbury with squally showers, some with hail. 82km/h was the highest wind gust recorded at some stations, easing by the 4th. The next annually closest perigee was in mid December 2008. Because it coincided with full moon, and the January perigee also combined with full moon to be 2009’s second closest perigee, we could expect and received a hotter 2008/9 summer than had been experienced in a while.
The closer the perigee, the more freakish the weather, because the moon’s closer proximity exaggerates the height of the daily air tide. The cyclone cycle is plottable as an 8.85 year cycle of changing lunar latitude. Cyclones are more numerous and of greater magnitude when perigee sits astride the equatorial latitudes, as during the Katrina/Monica years of 2004-2006. This is because the curvature of the earth is greatest at the equator and when perigee is equatorial earth-moon distances are less. The closer moon creates more gravitational turbulence, more summer heat and more destructive rains.
There should be more tropical cyclones in 2011 than in 2010. We need only look at our own history to see the trends and the causes-and-effects. From July 1935 to April 1936 the perigees were at or near the equator. The cyclone of February 1936 (they only started naming them in the 1970s) was probably the most destructive storm to hit New Zealand in the 20th century. The depression that crossed the North Island on the 2nd/3rd of February 1936 brought widespread heavy rain, causing every major river in the North Island to flood. Perigees were also over or near-to-over the equator between 1966 and 1968. That was a black year for NZ. On 10 April, 1968 the moon was at 11degN. In April 10, 1968, Cyclone Giselle brought flooding and destructive winds to many parts of New Zealand and the inter-island ferry Wahine sank in Wellington Harbour, with the loss of 51 lives.
Perigees were about the equator from 1974 until the following year. On Xmas Eve 1974, Cyclone Tracy blew away Darwin. The moon was sitting on 12degN that day, and Darwin is sited at 12deg S. Between Feb 1988 and Nov 1989 the moon was averagely closer to the equator than to either northern or southern declination points, sitting at latitude 12deg30S when Cyclone Bola struck NZ on 7 March, 1988. The heat from the equatorial belt picks up ocean moisture and a southward-trekking closer moon can deliver turbulence to points south of the equator. But if the closer moon is going north, cyclonic weather systems become hurricanes, striking targets in the northern hemisphere. An example was the New Orleans Hurricane Katrina, on Aug 29, 2005. The moon was at 28degN that day. New Orleans is 29degN.
After about two years striking around the equator, after March 2007 perigee moved further north and tropical cyclones in the southern ocean became relatively scarce. They still occurred 2008-2010 but in fewer numbers. Perigees in 2008 were midway between the equator and the northern declination. Cyclone Nagris struck Burma on the day of third closest perigee for 2008, the 6th of May. That day and time, 6 May, was also New moon and northern declination, and the moon’s latitude was 22N. Myanmar (Burma) sits at latitude 22N. Late in 2008 the perigeal moon was at its northernmost point, the first time since January 2000. Destructive cyclones diminish at these declinational peak points, making 2007 the last of seriously destructive cyclonic events until 2011. In 2009, perigees started drifting south again to lie between northern declination and lunar equinox, and accordingly tropical cyclones were expected and duly arrived in the first week of February and about the last week in March. Then, March 2010 saw the return of perigee at lunar equinox, the cyclonic breeder, and cyclones began to resume. By mid 2010 with the perigee moon again around the equator NZ will again be at risk, (update: this came in the form of cyclonic activity in the Pacific). A similar storm is expected around the end of November/beginning of December 2010.
Perigees will continue to drift slightly south in years to come, reaching Southern declination in July 2012. By June 2013 the closest perigee for the year will be at Southern declination. Perigee will sit astride the equator again in March 2015, and by the winter of 2016 the perigee will be past the equator heading north. In the first week of January 2018, the closest perigee for that year will coincide with northern declination. Perigee will again be level with the equator by November 2019. It will pass that point and again have entered the southern hemisphere by the following summer, which means 2020 will be a heightened cyclonic season.
The so-called Metonic Cycle that was independently discovered by the Greek astronomer Meton (born about 460 B.C.) is a 19 year cycle, after which time the phases of the moon are repeated on the same days of the year, or approximately so. Take, for instance, 2007's full moon of 24 Nov. Nineteen years hence, in 2026 there'll be another full moon on 24 Nov. Another interesting cycle: is that after 2 years, the preceding lunar phase occurs on or very nearly the same calendar date. Thus, in 2009, it will be the First Quarter moon that occurs on 24 Nov. After 8 years, the same lunar phases repeat, but occurring one or two days later in the year. The Greeks called this 8-year cycle the octaeteris. Indeed, in 2015, a full moon occurs on 25 Nov.
We can mathematically predict where the moon will be in years to come, just as we do for the tides, so we can predict the timing and severity of cyclone-years. Because the moon is such a clockwork phenomenon it was used for thousands of years for calendar measurement. The word measurement comes from me, which is the Indo-European root meaning moon. Other derivative words are monsoon, menstruation, season and metre. In our Gregorian Calendar, 372 years provides an excellent long period cycle for the recurrence of a particular phase on a given date. Thus, we know with absolute certainty that the same full moon that shines down on us on 24 Nov 2007 will also be shining on 24 Nov 24 in the year 2379 and was shining in the exact same part of the evening sky, back in 24 Nov 1635. Moonrise in Canterbury on 24/11/1635 was 8.26pm and on 24/11/2007 was 8.06pm. That means over 372 years the moon gained only 20 minutes, just 2 seconds per year.
If I had a watch that accurate I think I’d be over the moon.