Night and Day

“You go to the Caves of Winter. That is the Plan.”

“Winter, yes. The cold. Mother told us. And after the cold winter comes the warm. I remember. The winter will pass, won’t it? Why did she say, the winters grow? Teach me, Old One. . . .

~ “Love is the Plan, The Plan is Death” by James Tiptree, Jr.

Here in the Northern Hemisphere we are moving through autumn towards winter. That means the days are becoming shorter and the weather is getting cooler. The seasonal changes become more dramatic the further North you go; for people living near the Arctic Circle, the shortest day of the year – December 21st – will have nearly 24 continuous hours of darkness.

The change in seasons is due to the axial tilt of the Earth relative to its orbital plane (see image). In the summer, the sun is more directly overhead, which not only makes the days longer but the intensity of the sun’s rays are greater. It also means that the Southern Hemisphere experiences summer while the Northern Hemisphere is experiencing winger.

Seasonal changes on other planets in our solar vary considerably. Mars , with a similar axial tilt to Earth, has seasons that last roughly 6 months. Uranus has a much greater tilt and longer orbit has 20-year-long seasons with extremely variable weather. On Venus, on the other hand, the seasonal differences are pretty insignificant, since the planet has little axial tilt.

Not surprisingly, the change in the amount of daylight over the course of the year has had evolutionary consequences.   There are many plants and animals on Earth that have evolved to use day length as a cue for physiological changes that affect growth, reproduction and migration.

Plant reproduction, for example, is often dependent on having the right amount of darkness in a 24-hour period.  “Long day” plants – like soybeans and ryegrass – require fewer than a certain number of hours of darkness to flower, while “short day” plants – like spinach and coffee – won’t flower if there are too few hours are darkness. There are even “long-short-day” plants – like aloe – that require long days followed by short days to reproduce, and so only flower in the fall and “short-long-day” plants – like white clover – that flower in late spring. Disruption of the light-dark cycle can prevent such picky plants from reproducing.

Four Seasons - Fenner Nature CenterThe shortening of days in the fall also triggers changes in deciduous trees that helps them prepare for the cold of winter. The green chlorophyll in their leaves breaks down – unmasking already present yellow pigments – and is used to provide nutrients to the trunk and roots. Some trees also start making red pigments in their leaves, likely to help maximize the nutrients it can store for the cold winter months. Not only do these changes produce lovely fall color, but they also help trees survive until spring.

Animals are also affected by changes in day length. An extreme example is the Siberian hamster, which in the summer grows quickly and reproduce. As days grow short in the winter, the male hamsters’ testosterone levels drop, their testes shrink, and they store fat. Female Siberian hamsters go through similar changes.  While it’s not quite so dramatic in humans, we also respond to seasonal changes in daylight,  particularly with regard to our moods.

So how can these changes be used in science fiction? I think seasonal changes in day length are an often-overlooked aspect of realistic world-building.

While some science fiction stories include planets with extreme cold and hot “seasons”, these are often due to the unusual orbits of the planets. Examples of this are Hal Clement’s Abyormen and Brian Aldiss’s Helliconia*. Imagine a fictional planet that has an axial tilt similar or greater than Earth’s. The length of day should change with the seasons, and there should be indigenous life forms that have adapted to those changes, such as the creatures in Tiptree’s  short story “Love is the Plan, The Plan is Death”.

Day length is also plays a role when a planet is terraformed for human settlement. The Earthly plants and animals brought by the colonists would have to be specially selected (or engineered) so they are not adversely affected if the nights are significantly longer or shorter than Earth’s, or if the day length doesn’t change over the course of a year.

And imagine what might happen to life if the tilt of Earth’s axis were altered. Assuming any life survived such a dramatic event, it would have to adapt to changes in the seasons.

The cold grows, it grows, and your Mother-eyes are growing, glowing. Soon you will be alone with our children and the warm will come again.
Will you remember, my heartmate? Will you remember and tell them?
Tell them of the cold Leelyloo. Tell them of our love.

~ “Love is the Plan, The Plan is Death” by James Tiptree, Jr.

More reading:

Top Image: Diagram Earth and Sun from Wikipedia
Bottom Image: Four Seasons – Fenner Nature Center by Aunt Owwee, on Flickr

* Aldiss’s Helliconia may have has Earth-like seasons in on top of the centuries-long “seasons” due to its unusual orbit. It’s the latter changes that are the focus of the Helliconia novels.

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