Perigees on the move
SUNDAY JANUARY 24, 2010
In 2008 over one month there were three major extreme weather events wreaking havoc in the northern hemisphere. What was going on? The answer lies in the moon's declinations and perigees.
Declination is the name given to the moon's cycle of the changing latitudes. This is a 27.3 day cycle which from space would look as if the moon is slowly bouncing north then dropping south of the equatorial line. It is a cycle independent of the daily rotation of the earth. Over a typical declination cycle's duration, taking the equator as starting point, after about 7 daily rotations of Earth beneath the moon, the moon will have slid upwards to about Japan. Over the next 7 days, over an ever-rotating Earth, the moon works its way back to the level of the equator. During the following 7 days it gets down to be over about Brisbane and over the next 7 it makes it back to being over the equator. The movement is about 4deg of latitude per day, or 450kms, which is about 19kms/hr. In just one day, the moon can move a distance equivalent to Oamaru to Blenheim. It has been performing this north-south zigzag for many trillions of years.
Our gray neighbour is also continually coming closer and further away. The mean earth-moon distance is 384401 km. The closest (perigee) in the years 1750 through 2125 was 356375 km on 4th January 1912; and the most distant (apogee) in the same period will be 406720 km on 3rd February 2125. This monthly zooming in and out occurs at the rate of 100,000kms over 27 days, or 150kms per hour. During perigee, the moon moves faster past the earth at about 14.8deg per day, whilst at apogee it slows to about 11.6deg/day. Perigee is not always over the same part of Earth but itself changes hemispheres according to an 8.85 year cycle. That means it spends 2-3 years over the southern hemisphere, a couple of years about the equatorial line, and another 2-3 years over the northern hemisphere latitudes. The perigee-moon was over the equator between 2006-7 and by 2008 was further north.
As for the moon moving east to west over the course of a day, as every school child knows, this is an illusion. The moon only moves 13 deg to the east every day as it goes through space around earth once a month, but the earth rotates beneath the moon every day at around 3500kph. The moon's apparent transiting speed relative to the ground depends on latitude and closeness, but the average is 3675.7kph.
To recap, the moon is cruising through the sky at over 3000kph, towards us or away from us at 150kph, eastwards through space at 53- 68kph and moving between latitudes at about 19kph. It has rightly been called the inconstant moon. Determining its exact position at any one moment demands consideration of these four main cycles, over a day and over three different versions of months.
Combinations bring memorable events. January 4, 1912, the day of closest earth-moon distance, was also the day of full moon. On that day the full moon all around the world was 25% brighter than average.
When the perigee moon is equatorial, we get blessed with visits from tropical cyclones. Between Feb 1988 and Nov 1989 the moon was averagely closer to the equator than to either northern or southern declination points, and Cyclone Bola struck NZ on 7 March, 1988. The moon was sitting at 12deg30S. 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 WellingtonHarbour, with the loss of 51 lives. Perigees were over near-to-over the equator between 1966 and 1968. (On 10 April, 1968 the moon was at 11degN). On Xmas Eve 1974, Cyclone Tracy blew away Darwin. The moon was sitting on 12degN that day, and Darwin is 12deg S. Perigees were about the equator until the following year. 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 NorthIsland to flood. The MangakahiaRiver in Northland rose by 19 metres. And from July 1935 to April 1936 the perigees were at or near the equator.
When perigees are around the equator or heading south, we get the possibility of tropical cyclones hitting NZ, and this is roughly an every 4th year occurrence. We do not seem to get them here at other times in this declination-perigee cycle. As the perigeal moon creeps northward from either southern declination or equator, the cyclonic weather systems get to be called hurricanes, and strike targets in the northern hemisphere. An example is the New Orleans Hurricane Katrina, which occurred on Aug 29, 2005. The moon was at 28degN that day. New Orleans is 29degN. Closer to the present we have had Cyclone Nagris, which struck Burma on the day of third closest perigee for this year, 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. Over the next year the perigeal moon is beyond the equator and heading towards northern declination, which perigee-declination point will be reached in April and May of 2009. By mid 2010 the moon will be once again around the equator and NZ will again begin to be at risk, at least from the remnants of cyclonic activity happening around Samoa and Tonga. A coincidence? It does not seem so. In 2008 the first weeks of May, June and July had really close perigees. P#2 was on the 4th of June and I had been expecting some event along the 26deg latitude line, which takes in China (again) Pakistan or elsewhere , even possibly 26 SOUTH. Then we had P#5 on the 2nd of July which lined Japan or Mexico up in its sights. Result?
TC "Arthur" arrived in the Carribbean.
© Ken Ring 2008