I will start this piece by assuring you that it is not an argument for a flat earth. Ok, now that I have that out of the way, let's proceed. As I walked into the Geography-Geology building at the University of Georgia earlier this week, the iconic globe in the lobby caught my attention. I snapped the photo while ascending white Georgia marble staircase. On my commute home later that evening, I thought about that globe and virtually every other one that I have seen in classrooms over the years. In two ways, they are "sort of" lying to us. Here's why.
A very large globe in the Geography-Geology building at the University of Georgia
When you see most globes, they are usually presented on a fixed axis at an angle of 23.5 degrees. It is tempting to assume that Earth is always tilted on its axis and for the most part, it is. This axial tilt, as the Earth moves around the sun, is why we have seasons. It is also why the Northern Hemisphere experiences the opposite season from the Southern Hemisphere (so it can actually snow in Australia during August). However, you may be surprised to learn that unlike that rigidly fixed axis on a globe, the Earth is not always fixed at the same angle. The NASA Earth Observatory website provides good insight:
Today, the Earth's axis is tilted 23.5 degrees from the plane of its orbit around the sun. But this tilt changes. During a cycle that averages about 40,000 years, the tilt of the axis varies between 22.1 and 24.5 degrees. Because this tilt changes, the seasons as we know them can become exaggerated. More tilt means more severe seasons—warmer summers and colder winters; less tilt means less severe seasons—cooler summers and milder winters.
This is a part of a series of well-known orbital changes called Milankovitch Cycles. Milutin Milankovitch was a Serbian astrophysicist who used the early part of the 20th century to argue that the shape of the Earth’s orbit around the Sun changes periodically. I have already described the “axial tilt” change or “obliquity.” Every 90,000 to 100,000 years, the Earth’s orbital plane varies from circular to elliptical. Such changes in eccentricity significantly affect the amount of solar radiation coming into our atmosphere. The third major orbital variation is precession. The Earth wobbles a bit like a child’s toy spin top.
Milankovitch Cycle component
Milankovitch proposed that these three changes were linked to much of the naturally-varying climate system including the Ice Ages. According to the NASA website documenting his contributions:
for about 50 years, Milankovitch's theory was largely ignored. Then, in 1976, a study published in the journal Scienceexamined deep-sea sediment cores and found that Milankovitch's theory did in fact correspond to periods of climate change (Hays et al. 1976). Specifically, the authors were able to extract the record of temperature change going back 450,000 years and found that major variations in climate were closely associated with changes in the geometry (eccentricity, obliquity, and precession) of Earth's orbit. Indeed, ice ages had occurred when the Earth was going through different stages of orbital variation.
The U.S. National Academy of Sciences and other organizations have now generally accepted Milankovitch's model.
Unfortunately, people that think in “either/or” terms mistakenly point to natural cycles like this as the “only” driver of our climate system. It’s actually more accurate to think in terms of “and.” Increased greenhouse gases in the atmosphere are causing the Earth’s surface, on average, to warm, according to the recent State of the Climate report. In recent decades, accelerating warming has caused a climate crisis and disrupted many aspects of the Earth system.
Speaking of Earth, there is another way that globes are "lying." Most people that look at a globe would likely say that it is a perfect sphere. NOAA’s History of Geodesy website says:
Since the days of ancient mythology, scientists and philosophers have debated the shape of Earth. Since about 500 B.C., the idea that the Earth was a perfect sphere has dominated most scientific thinking, even though the concept of a flat Earth may have persisted in some regions for another millenium. Around the end of the 16th century, the idea that the Earth was a perfect sphere evolved into a radical new idea: that the Earth was an imperfect sphere. This new way of thinking was initially divided into two major schools of thought. One believed the Earth was egg-shaped (prolate). The other believed the Earth was flattened at the poles (oblate).
As modern geodesy (the study of the Earth’s shape and area) evolved, the concept of Earth as an oblate spheroid as been proven to be the correct description. It would not be very noticeable on a globe as my NASA GISS colleague Gavin Schmidt tweeted, but a true representation would not be a perfect sphere. I wonder if some readers will think differently now about that “deceitful” classroom globe (smile).
The Earth is an oblate spheroid
Updated at 8:13 pm on August 19th.
