Seasons change as the earth revolves around the sun. Why? That's a question I began to wonder about as I picked up a leaf from the ground.
The tree 
in this picture came from Pixabay. Everything else in the picture came from a single leaf, 
and my imagination.
Did
you know that the earth wobbles? Did you know that the rotation of
earth on its axis is slowing? Did you know that in the future, days on
earth will be longer? And that, when our distant descendants look into
the night sky, their North Star will not be Polaris?
All of this I discovered after I picked up a small leaf from the ground and began to wonder about the mystery of seasons.
Image credit: Mauler CC4.0 license
A complete rotation of the earth takes twenty-four hours. However, that number is not fixed: the earth's rotation is slowing down. Scientists attribute the slowing to the moon's gravitational pull.
As a matter of fact, when astronomers look back 2,000 years, they note
that an eclipse recorded then occurred four hours earlier than would be
expected, based on current movements of the earth.
As a result of the earth's slowing rotation, it is estimated that in a hundred years, a day will be about 2 milliseconds longer.
In addition to this rather intriguing fact, I learned that the earth wobbles,
and wobbling may have a significant influence on the planet's seasons.
Wobbling has gone on, and will go on, without any provocation from
humans. However, researchers at NASA suggest the phenomenon may be
exaggerated by human-precipitated climate change. As reported in Forbes, scientists believe that as polar ice melts, the balance of the earth is disturbed and its tendency to wobble increases.
We know that the rotation of earth affects the rising and setting of the sun--however, this rotation does not govern the seasons. Seasons change because the earth is tilted on its axis. As this tilted orb makes its way around the sun, different areas of earth are exposed to different intensity of sunlight. With varying exposure comes a shifting of seasons.
The
diagram above shows that the earth's orbit is not perfectly circular,
but is slightly elliptical. Because of this elliptical path, the
distance between earth and sun is not constant. The point at which the
earth is closest to the sun is called the perihelion. The point which
it is farthest is called the aphelion. However, the difference in
distance between the perihelion and aphelion is not significant.
The perihelion (closest) occurs in early January. At this time earth is 147.5 million km away from the sun. The aphelion (farthest) occurs in early July. The distance between earth and sun at that time is 152.6 million km.
I've already asserted that the earth's slightly elliptical orbit does not significantly affect the planet's seasons. However, if the orbit were to become dramatically more elliptical, then earth's orbit would influence climate noticeably. In the past, this has been the case. Scientists estimate that such orbital changes are cyclical and that the cycle covers a span of about 100,000 years. With a more elliptical orbit the earth-sun distance would change, and that would affect climate.
Image Credit: Silver Spoon CC 1.0, 2.0,2.5,3.0 licenses
When the earth wobbles, the angle at which it tilts may be affected. With an increase in tilt comes more extreme seasons: winters are colder and summers are longer.
Currently, the earth is tilted at 23.5 degrees. The angle of tilt is called the earth's obliquity. However, the tilt has varied over thousands of years. According to Live Science (different sources give slightly different ranges), the obliquity of the earth in the last 41,000 years has varied from between 21.5 to 24.5 degrees.
Gyroscope: LucasVB public
Globe: NASApublic
So far, I've established (based on my sources) that the shape of earth's orbit is not fixed, and the tilt of its axis may vary. There is yet another uncertainty in earth's passage through the heavens. The orientation of its rotation axis will change, and has changed in the past.
Just as the gyroscope spins in the picture above, so does the earth
spin on its axis. It is pulled always by the moon and the sun. Today,
the earth's rotation axis points north, toward Polaris. In 13,000
years, the axis will point toward another star, Vega.
The Three Uncertainties
Perhaps no one besides me may call them that, but here they are: the eccentricity of earth's orbit, the obliquity of its tilt, and the precession of its axis. Alterations in these three, researchers surmise, may have been responsible for glacial ages in the past, and may bring about dramatic climate changes in the future.
Because of the earth's tilt, the only places on the planet that ever receive direct overhead sunlight (called the subsolar points) are located between the Tropic of Cancer and the Tropic of Capricorn.
As the earth turns its face away from, or toward the sun, along with a
change in season comes a change in the length of day and night. This
is true everywhere but at the equator. At the equator, day and night
are approximately equal in length year round, and there is little variability in season, beyond a wet and dry season.
Summer and winter solstice indicate the times when sun shines directly on either the Tropic of Cancer or the Tropic of Capricorn. June 21 is the day of summer solstice in the Northern Hemisphere and winter solstice in the Southern Hemisphere. Likewise, December 21(22?) marks the winter solstice in the Northern Hemisphere and summer solstice in the Southern Hemisphere.
In the United States, Hawaii has the distinction of being the only place that experiences direct, overhead sunlight. This happens twice a year, at the beginning and end of summer, and is called Lahaina Noon. During Lahaina Noon (and you have to catch it just at noon) no perpendicular object, such as a lamp post, will throw a shadow.
Image credit Caelus5, CC 4.0 license
A subsolar point occurs when sunlight hits earth at a 90 degree angle.
And so, after visiting the heavens and exploring celestial events, I come back to my humble leaf that fell to the ground. How did the leaf know to do that? What was the trigger that signaled to a tree it was time to sleep--to go into dormancy? It turns out, finding the answer to this question is as challenging as my inquiry into celestial realms had been.
Different kinds of plants have different triggers. It may be one trigger, or a combination of triggers that signals plants to sleep and to reawaken from dormancy. For some the trigger is moisture. For others, the trigger is temperature. For some plants, amazingly, smoke is the signal to wake up and start growing.
The Dry Sclerophyll is not unique in requiring smoke to germinate. Many species located in areas where fire is part of the natural life cycle also require smoke.
For my little leaf, I will make an assumption (founded in pure conjecture) that the phenomenon of photoperiodism triggered the leaf to fall, and its parental tree to go into dormancy. Plants that are affected by photoperiod respond to the change in proportion of night to day. It is likewise this trigger which suggests to birds in the fall that it's time to migrate. And it is this trigger which prompts humans to release melatonin as winter approaches.
There's a wealth of information, some of it still evolving, about how plants are triggered to go to sleep and reawaken. It seems the secrets of the earth are as abundant as the riddles of the skies. As I finish my blog I'm left with wonder and amazement at the mystery of life and the unity that ties it all magnificently together.
Thank you for reading. I hope you like my many GIFs. It was impossible for me to write this blog and not be inspired to create.
Images my husband captured on his cell phone, the same afternoon I found my leaf: two shrubs, in brilliant fall display.
Thank you for spending time with me
Illustrations/GIFS
- All leaves used in this picture were derived from the one small leaf I picked up from the ground. (Imagine if I'd picked up a rock đŸ˜‡)
- Globes used in GIFs: Pixabay
- Sun used in GIFs: Pixabay
- Seascape (Day and Night GIF): Pixabay
- Tree used in the GIFs: adapted from Pixabay
Selected Sources
NASA: What Is Orbit
Astronoo: Eccentricity of the Earth
Science Magazine.org: Ancient Eclipses Show Earth's Rotation Slowing
Earth's Changing Face: Has the Day Always Lasted 24 Hours?
NASA, Soace Place:Dr. Marc Earth Rotation
Astrobio.net: How Earth's ORbital Shift Shaped the Sahara
NASA, Earth Observatory: Milutin Milankovitch, Orbital Variations
NASA: Sun Seasons
Earthsky.org: Can You Explain Why the Earth Has Four Seasons
Live Science: Milanklovitch Cycles
Stardate.org: Astro-Guide Precession
EOS.org/Research Spotlights: How Variations in Earth's Orbit Triggered the Ice Ages
Skymarvels.com/info pages: Subsolar Point
Timeanddate.com: September Equinox
Worldatlas.com: Do Places Near the Equator Jave Seasons
Beatofhawaii.com: Lahaina Noon 10 tips to Avoid Sunburn on Hawaii Vacation
sfsu.ed: Earth-Sun Relations
Washington.edu: Smoke Infusion for Seed Germination in Fire-adapted Species
Frontiers in Neuroendocrinology: Influence of Photoperiod on Hormones, Behavior, and Immune Function
New South Wales Office of Environment and Heritage: Clarence Dry Sclerophyll Forests
Australian Academy of Science: Weird Way Plants Germinate
Forbes: Earth's Rotation Is Mysteriously Slowing Down: Experts Predict Uptick In 2018 Earthquakes
Britannica: Perihelion
U. Michigan: Precession
Emporia State U.: The Milankovitch Theory
YouTube: Earth's motion around the Sun, not as simple as I thought
Scientific American: Earth’s Orbital Shifts May Have Triggered Ancient Global Warming