Equinox and Equilux

Bob McDavitt's ideas for sailing around the South Pacific. Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world. Compiled 20 September 2026 THE EQUINOX Our vernal (spring) equinox (Latin for equal night) is at 00:05UTC on Wed 23 September. That's 12:05pm in NZ . That's a special moment in the earth's orbit of our sun, and it fittingly picked by many to mark the start of the Southern hemisphere SPRING. Many in NZ, for practical reasons, such as climatologists who measures averages, etc. on a monthly cycle, take the MONTHS of September, October and November to be Spring, but astronomers, for logical reasons, use the four 'corners' of the earth's orbit as markers (equinoxes and solstices). The best definition of the equinox is when the sun, as seen from earth, is directly overhead the equator. As viewed from earth the sun is directly overhead a different latitude every day of the year. The difference between this latitude and the equator is called the sun's declination, as all navigators know already. Although equinox comes from Latin for equal night, it is only a 12 hour day if you measure sunrise and sunset using the MID-POINT of the sun. Sunrise and sunset tables actually use the top limb of the sun so that, plus atmospheric refraction, adds an extra 5 to 8 minutes of (rather dull) sunlight. The name given to the day with 12 hours exactly sunlight/darkness is the equilux - in New Zealand that's between 19 and 20 Sept. From now on the days will lengthen. But the rate is not linear-Auckland's dawn is getting earlier at the rate of 10 minutes per week at present, but dusk is only getting later at the rate of 6 minutes per week. At the equinox the delimiter (line on the earth dividing day and night) is shaped like a square wave and during the next few months or so it slowly reshapes into a sine wave. You can watch this by downloading the program Home Planet from http://www.fourmilab.ch/homeplanet/ and watching this in the animate mode. The Delimiter is a square wave at the equinox At this time of the year the days in NZ are getting longer at the rate of around an hour per month. It is thus a good time to do a switch to Daylight saving, and indeed in NZ we SPRING our clocks ahead an hour at 2am next Sunday 27 Sep. This rate starts easing from now onwards, but the extra sunlight is now affecting the weather patterns. Our spin axis is NOT at a right angle to our motion around the sun: the 23 deg tilt influences our heating cycle. OK it nods the southern hemisphere closer to the sun than the northern hemisphere for the next six months - and we are now approaching the flatter side of our elliptical orbit for the next six months, but the MAIN FACTOR is to do with the concentration of sun beams over a smaller area (and, in the northern hemisphere, the spreading out over a wider area. December Solstice, with the southern hemisphere hogging the sunlight. There is a lag between the change in increasing sunlight and the spot with the warmest temperatures on earth. The lag in the ocean is a few weeks more than in the land. Currently the warmest SST band in the open part of the Pacific is at around 8 deg N. Sea surface temperatures. The warmest isotherms are coming south. Note that the 28C isotherm in the South Pacific already extends as far as between Samoa and Fiji. It is extending southwards at a steady rate. This is the time of the year that the subtropical ridge-that zone which divides the trade winds from the roaring 40s- starts to drift southwards. As it drifts south, the isobars on its southern side may bunch closer together in places, and that increases the disturbed westerly winds in the parts of the roaring 40s. This period, known as equinoctial gales, is likely to last until mid-November. A strong El Nino encourages these gales. TROPICS The Atlantic basin so far has not produced a hurricane this year. This is mainly due to stronger than normal upper winds ripping developing tropical systems apart. . Tropical Storm Dujuan formed near the Northern Mariana Islands and travelled north. . Tropical Storm Norbert passed over the open waters of the Pacific, followed by depression FIFTEEN and storm ODALYS. . Depression FAY travelled north in North Atlantic. WEATHER ZONES SPCZ is weak and mainly over Tuvalu and Tokelau . A new convergence zone is expected to develop this week between Vanuatu and Fiji There is a HIGH 1023hPa tonight in north Tasman Sea and a Low 985 L1 near Chatham Islands. Between these systems are strong southwest winds and rough seas. The High is expected to fade away and the low to travel off to the east allowing another low L2 to come in from the southwest and follow it. There is another HIGH H1 in the Aussie Bight that is expected to cross Tasmania on Tuesday and then travel NE across the Tasman Sea by Saturday and then fade away. Low L3 is a broad trough that is expected to come in from the southern Ocean and onto NZ on Sat 26 and Sun 27 Sept. There is expected to be strong to gale winds with the frontal zones in the system as isobars get squashed between L3 and H1. Resulting rough SW swells may reach Minerva om Mon /Tues 28/29 Sept. This seems to be a repeating pattern, with a burst of strong winds affecting northern NZ around the weekend. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> If you would like more details about your voyage, check metbob.com Or Facebook at /www.facebook.com/metbobnz/ Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom). Weathergram archive (with translator) is at weathergram.blogspot.co.nz. Contact is bob@metbob.com or text 64277762212. >>>>>>>>>>>>>>>>