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.