There is a time each year when we get this feeling and its suddenness not purely psychological, it is because we witness the most dramatic changes in the Earth-Sun relationship at two times in the year: spring and autumn.

Some of the things that we tend to assume change gradually, actually don’t at all. On this blog I have mentioned that the bearing of sunrise and sunset change most dramatically at the equinoxes, in March and September, and briefly stand still at the solstices.

There are other things that change with varying speed over the course of the year and they follow a similar pattern. The length of day hovers at the same length at each solstice, but changes rapidly in spring and autumn. For example, in London (latitude is important), the length of the day changes by only 2 minutes over the course of 10 days in late June. However, in late September the day shortens by 40 minutes in just 10 days.

The plants are very sensitive to this rapid change. The leaves on the trees begin to fall as the days shorten, or to be more precise, when the length of night increases. Since these are not steady, gradual changes, it is that much easier for nature to react at the right time. Autumn arrives with a loud rustle, if not a bang.

Happy New Year!

At times like this, I sometimes wonder what the Earth and Sun would say to each other if they could talk. They would watch us celebrating this annual moment at such an arbitrary time…

Sun: I could understand a party at either solstice…

Earth: Yes, or one at either equinox. Would make good sense…

Sun. Quite. But to pick a day about a week after one solstice…

Earth: Very strange.

Sun. Yes. They are a very strange lot.

In this picture of a beech tree in Wiltshire, we can see both moss and lichens thriving in the moist air close to the ground. Water evaporates constantly from the ground and moisture-loving organisms, including mosses, lichens and algae, will be found close to the ground regardless of aspect.

Direction can be found by looking for clues above this area or sometimes by getting to know the lichens in this low band itself. It is much more complex, and fun, detective work than simply thinking ‘green is north’.

On a different note, my book has just come out in the US!

My plans this morning, as announced in the Telegraph, were to head to the top of a hill and catch the sun rising due east. Sadly, the air is cooler than its dewpoint… the humidity is greater than 100%… there is a low level of nimbostratus… however you want to put it: the weather is not very good and the visibility is terrible.

Had I been able to see the sun it would have risen due east. The vernal and autumnal equinoxes being the only two days of the year when the sun rises due east.

Something that you cannot notice on any individual day, but only by studying the sun’s rising position over the course of a year from the same location, is that its rising and setting positions are changing by more at this time of year than at any other time. Near the solstices the sunrise position slows its change to close to a ‘standstill’  (the name ‘solstice’ being derived from the Latin for ‘sun standing still’). At the equinoxes, in the UK, sunrise direction changes by nearly 5 degrees in a week.

The other thing that is changing very rapidly at this time of year are sunrise and sunset times, and therefore the length of the day. At the equinox everywhere on Earth shares a roughly equal day and night (the word comes from the Latin for ‘equal night’). Here in the UK, the day will be half an hour longer one week from now than it is today. This may be why we can sometimes feel ‘stuck’ in winter, but that when we feel the season turning and spring coming it is not a gradual shift, but more like a revolution. That said, it doesn’t feel very spring-like out there at the moment.

A good website for looking at sunrise and sunset times in the UK can be found here. The best one for checking the bearing (direction) of sunrise or sunset anywhere in the world is the US Naval Observatory.

The morning shadow from the dormer window is retreating and the sun is reaching more of the roof with each minute. The frost from the night before is thawing in the warm light. It is also forming a very simple shadow compass. At this time of year the sun rises very close to east and the direction that the frost is retreating will be west to east. The protruding ‘nose’ of frost near the top of the roof can be joined to the jutting part of the shadow to form a near perfect west-east line.

This effect can be seen in so many places at this time of year, as we get closer to the spring equinox (20 March). It will also work close to the autumnal equinox (23 September this year), but not in summer or winter when the sun rises and sets a long way from east and west. At these times there are other, better things to look for.

This balance between the seasons, between light and dark and east and west is something the world shares at this time of year. It comes as the sun finishes its long stay over the southern hemisphere and gets ready to cross the equator, the equinox itself, and begin a six months tour over the northern hemisphere.  This time last year I happened to be checking that the sun was rising where it ought to be in Libya.

This chalk line in this picture shows the shadow tips joined over a period of a few hours on the morning of the 21st September. The line comes very close to a straight line, but even on the equinox it is never a perfectly straight line – unless you happen to be standing on the equator when the sun would rise due east, pass directly overhead and then set due west.

Only a few days of this solar season left, the autumnal equinox is on 22 September. This means that in the UK there are only four more days when the sun will have any north in it at all for another six months. At times like this, close to the equinox, the point on the horizon that the sun rises changes by more each day than at any other time in the year.

‘Equinox is the moment when the Sun crosses a planet’s equator, making day and night the same length.’

I forgot to mention that I delivered the manuscript of my book to my publishers, Virgin Books, three weeks ago. It is an exciting moment, a good line in the sand, but far from a terminal one. Work will continue on it until about November probably.

This is something that I am both more sensitive to and wary of since my trip to the Sahara in March. The lee effect there puzzled me for several days until a sandstorm blew in and blotted out the sky. Ironically it clarified things mentally. There is a difference between a prevailing wind and wind that has a huge short-term impact. The sand that was deposited in some areas that I crossed in Libya indicated a wind direction that clashed with the direction that both the trees and the dunes were suggesting. It became clear that this was caused by a sandstorm blowing in from a different direction to the prevailing wind and leaving its own traces.

There is nothing too mysterious in this picture however. It is late afternoon, near the equinox and we are looking east. A northerly wind has combed the sand and left undulations, but note that the undulations vary subtly in angle showing how locally the wind direction can vary due to terrain. There is a small build of sand in the southern lee of the stunted clumps of grass. The rocks also show signs of weathering, but not equally on all sides.

The small patch of moisture that is in the sun reveals the direction that the shadow is shortening, a crude north-south line as we near the middle of the day.