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Light Through the Year
The Light Seasons are easiest to understand by watching daylight over several weeks rather than by looking at a single date. From one day to the next the difference is often small, but over a week sunrise and sunset move, the Sun reaches a different height at midday, and the amount of usable daylight can change noticeably.
The Light Seasons give names and fixed positions to these broader changes, especially to the transitions where the behaviour of the day-length curve changes from one mode to another.
Day length does not increase or decrease at the same rate throughout the year. Near a solstice the change from one day to the next is small. Moving towards an equinox, the daily difference becomes progressively larger; around the equinoxes, day length changes particularly quickly, then the rate slows again towards the next solstice.
This is also why the conventional astronomical season boundaries sit in unusual places if a season is understood as a phase of changing light. A solstice is an extremum, the centre of the long-light or dark repose, while an equinox lies near the inflection point and strongest slope of Waxing or Waning light. These are characteristic points within the phases, not the transitions between them.
Around the winter solstice the day-length curve is almost flat. Later, during Departure from darkness, the increase becomes more perceptible. Waxing light starts well after the winter solstice, when the gain in daylight is gathering pace towards the March equinox. The opposite happens after the summer solstice: the days initially shorten only slowly, the loss becomes more apparent during Departure from light, and through Waning light it accelerates towards the September equinox.
Changes in day length are not always divided evenly between morning and evening. Sunrise and sunset follow the annual motion of the Sun, but solar noon also shifts during the year. The earliest sunset therefore does not occur on the winter solstice, and the latest sunrise does not occur there either; comparable offsets occur around the summer solstice.
The total day length gives the overall pattern, while sunrise and sunset show how that change appears in everyday life.
A Light Season boundary marks a change in the behaviour of the curve, while the visible change develops across the surrounding days and weeks. A few minutes of additional daylight are easy to ignore, but over a week or two morning light returns at familiar times, evenings remain bright longer, shadows change and the midday Sun climbs or falls.
The Light Seasons add this information to ordinary calendar orientation: not only that it is February or August, but whether daylight is gathering pace, approaching the long-light repose, declining rapidly, or passing through the quiet part of the year around the winter solstice.
Light is one of the environmental signals involved in plant development and biological timing. Many plants respond to day length as well as temperature and other conditions, and human circadian rhythms are strongly synchronised by light. These processes do not follow Light Season boundaries as fixed dates; they have their own mechanisms, delays and local conditions.
What can be compared is their position within the same annual light cycle. Leaves may appear during rapidly increasing light, growth continues through the long-light part of the year, and ripening, leaf fall and dormancy occur while light is declining. The exact timing varies by species, place and year, while the astronomical reference remains fixed.
The descriptions above use the Northern Hemisphere. In the Southern Hemisphere the same annual pattern is shifted by half a year: increasing light occurs while the Northern Hemisphere is losing light, and the December solstice is the long-light maximum rather than the minimum. Gordon's Sun Clock therefore reverses the Light Season assignment between the hemispheres.
Towards the equator, day length remains close to twelve hours and the strong annual rise and fall seen at temperate latitudes largely disappears. Gordon's Sun Clock therefore does not display the Light Seasons between 18° south and 18° north latitude. This is a practical boundary used by the app, not an astronomical division of the Earth.
The Light Seasons can be used without adopting a new calendar. Their simplest use is observational: knowing where the Sun is in its annual cycle and comparing that with what is happening outside. The Gregorian dates repeat only approximately, weather and vegetation vary from year to year, while the astronomical cycle provides the fixed reference.