September 2014 The Autumnal Equinox and the Changing Seasons


Hi everyone, I’m Darrell Heath with the UALR College of Arts, Letters, and Sciences, welcome once again to The Night Sky.

Do you closely monitor the motions of the Sun, Moon, planets, and stars across the sky throughout the year? Chances are you don’t but if you lived in ancient times the very lives of you, your family and your community would have depended on such knowledge. You would have needed to know when the seasons were about to change in order to plant and harvest crops, when to prepare for prolonged cold or heat, wet and dry spells, or when large grazing mammals were about to migrate. Your culture would have devoted much time and energy into obtaining such information. The most important reference source for establishing seasons, months, and years would have come from closely watching the apparent motions of celestial bodies across the day and nighttime sky over long periods of time.

For some cultures this pursuit would become elaborately institutionalized in the form of religion, customs, and even architecture and city planning. Take for example the Mayan city of Chitchen Itza in Mexico’s Yucatan Peninsula. There you will find the Temple of Kukulkan or El Castillo. The temple is a 79 foot high step pyramid devoted to the plumed serpent god Kukulkan. The ancient engineers who built this structure had two things in mind: to honor their serpent deity and also to create a structure that served as an astronomical calendar for the entire populace of the city. The temple is famous for an event that occurs twice a year on the days of the spring and fall equinoxes. On these two days the setting Sun casts a writhing snake-like shadow that connects a statue of Kukulkan’s head at the base of the stairs with his tail end at the top of the pyramid. The design is pure genius and I hope that the city’s ruler well rewarded whoever came up with the idea.

But the temple also serves to underscore a remarkable fact: not only can we determine the change in seasons simply by watching the sky but also that the change is an astronomical event itself.

I remember reading several years ago that a poll was conducted among Ivy League college graduates to determine their knowledge of rudimentary facts. The vast majority of them got completely wrong the question as to why we have seasons. Most of them thought it had to do with Earth’s distance from the Sun but on this planet that has nothing to do with it. The real reason for the seasons has to do with the way the planet Earth is tilted upon its axis by 23 ½ degrees relative to its orbital plane around the Sun. Take for example our Northern Hemisphere winter. Every December, at the time of the winter solstice, our planet is in a position within its orbit where the 23 ½ degree tilt results in the north pole being angled away from the Sun. At this time the Sun, in our particular hemisphere, appears to take a more shallow path across the sky and remains up for shorter periods: shorter days and longer nights. The result is that a lower angled Sun means fewer rays per square meter striking the Earth and warming it and, thus, we experience winter. Meanwhile our neighbors in the Southern Hemisphere are on the part of the Earth that is tilted towards the Sun and they experience summer. Keep in mind that during the winter months the Earth is actually closer to the Sun in its orbit than it is during the Northern Hemisphere summer proving that the Earth-Sun distance has little to do with the change in seasons. Later, in June, the Earth is in a point in its orbit where the Northern Hemisphere is tilted towards the Sun and we see a higher Sun in our sky. The result is longer days and shorter nights along with an increase in the amount of solar heating during the day: summer for us and winter for folks in the Southern Hemisphere who are experiencing the exact opposite conditions.

As our planet continues upon its yearly trip around the Sun, all the while maintaining its 23-½ degree tilt, there are two points along the journey where the Earth is neither tilted away or towards the Sun. On these two days of the year the length of day and night are equal: 12 hours of day and 12 hours of night all over the planet. These days are called ‘equinoxes’ and the one in the March is often called the “Spring” or “Vernal” Equinox while the one in September is called the “Fall” or “Autumnal” Equinox. The word “equinox” comes from the Latin and means “equal night”.

Another consequence of the equinoxes is where the Sun both rises and sets. Most people assume that the Sun always rises and sets due east and west, but this isn’t quite true. During the winter the Sun rises a little bit south of due east and sets a bit south of due west. During the summer it rises north of east and sets north of west. Only on the equinoxes does the Sun actually rise and set more or less due east and due west. It is during the equinoxes that the Sun is directly on the celestial equator, which is an imaginary projection of our own planet’s equator out into space. If you were standing upon the Earth’s equator during an equinox you would see the Sun directly overhead at noon. It was exact knowledge as to the Sun’s rising and setting locations during an equinox that allowed those Mayan engineers to design the Temple of Kukulkan with such precision. No secret knowledge obtained from ancient aliens was needed, only an almost paranoid monitoring of the sky, as if their lives depended upon it, was required.

With our own increase in knowledge of astronomy we have added our own level of precision to marking the time of the seasons. For example, during the month of September of 2014 our knowledge of the Earth’s position in space relative to the Sun allows us to know that the Fall Equinox will occur exactly at 9:29pm CDT on September 23rd.

What else do we get out of an equinox? Well, spring and fall are both transitional seasons and we do notice a change in such things as temperature and rainfall. With changes in the length of night and day we get a corresponding change in the amount of sunlight per square meter upon the Earth and this in turn brings about changes in the weather. So yes, that 23-½ degree tilt of the Earth’s axis has a direct impact on our world and us in many different ways.

These changes in the length of night and day and the weather have a profound affect on plants and animals. One of the most dramatic examples of this is the change in leaf color that occurs among deciduous, leaf shedding trees throughout much of the eastern United States. Leaves are remarkable bits of evolutionary engineering and are Nature’s solar panels and food factories. Plants use the energy contained within sunlight to convert water and carbon dioxide into glucose, a simple sugar the plant can use as its own energy. We call this process “photosynthesis”. It is the change in the amount of light that trees receive around the time of the autumnal equinox that signals them to get ready for the winter months ahead. Remember, in winter there is not only the cold weather to contend with but also a shortage in both sunlight and free water. The best evolutionary strategy the trees have to cope with this change is to become dormant during the winter months and to close down their leafy energy factories. Chlorophyll is the chemical that makes plants green and allows them to carry out photosynthesis, the shorter days and cooler weather triggers a process that makes the chlorophyll disappear. As the green color disappears the leaves of deciduous trees reveal colors such as yellow and orange that have been hidden away all year long. In some trees it is the change in both the amount of light and cooler temperatures that trigger chemical processes that make the leaves turn red or purple. In maple trees for example there are small amounts of glucose left inside the leaf and the decrease in sunlight combined with cooler weather makes the glucose turn red. The next time you are outside in the fall enjoying all of this color or maybe raking up the tree’s shed solar panels remember that these incredible structures have allowed plants to collect the radiant energy being produced from within the core of a star, our Sun, located some 93 million miles away and converting it to energy the plant uses to survive. And be a bit grateful too because the energy that plants harvest from sunlight also gets passed along through the food chain and eventually reaches you in order to keep you alive as well.

In the animal world the change in daylight and temperature triggers behaviors such as migration and physiological changes that allow some animals to enter into hibernation in order to survive the winter months ahead. But even we human animals are not immune to these changes around us. Many people suffer from Seasonal Affective Disorders during the fall and winter and symptoms range from feelings of depression and hopelessness to anxiety and social withdrawal. We still don’t fully understand the causes of this disorder but it is likely that the reduced light levels in fall and winter can disrupt our circadian rhythms and lead to drops in the level of brain chemicals that control mood such as serotonin and hormones, like melatonin, that play a role in governing our sleep patterns. In truth, to say that depression is the result of a chemical imbalance in the brain may be an oversimplification of things, but certainly it is a biological condition whose causes may be far more complicated than we currently realize.

In a modern world we have come to think that astronomy doesn’t have much relevance to our day-to-day lives but the truth is much different. Just look around you at what all comes from living on a planet with a 23 ½ degree tilt upon its axis combined with its orbital motion around a distant and minor star.

Until next time, I encourage you to get outside, look up, and wonder.

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