|Above: Locations around 40°N get only about a third of the solar energy at the winter solstice than they do at the summer solstice—but the sun is actually closest to Earth in early January. Image credit: Bob Henson.|
Unless you’re an astronomy or meteorology nerd, you might scoff if someone told you that the sun is going to be closer to Earth this week than at any time until 2020. Better hold that scoffing, though. Early January is indeed the time of the year when our home planet and its life-giving star draw nearest to each other.
Perihelion—the closest juxtaposition of the Earth and Sun—will occur at 12:19 am EDT Thursday, January 3, 2019. Aphelion—the most distant juxtaposition—will take place at 6:10 pm on July 4. Earth and Sun are about 3% closer to each other during perihelion (about 91.4 million miles apart) than during aphelion (about 94.5 million miles apart). The reason we get perihelion and aphelion is because of Earth’s elliptical orbit around the sun, which puts Earth slightly closer to the sun at one side of the ellipse than the other. Also in the mix is the fact that the sun and Earth both rotate around a common point—their barycenter.
Just as the opening bells of summer, autumn, winter, and spring typically shift back and forth by a day or two from year to year, so do the dates of perihelion (ranging from January 2 to 5) and aphelion (July 3 to 6), with the year-to-year variations caused mainly by the monthly rotation of the moon and Earth around their own barycenter. In addition, these date ranges of perihelion and aphelion are also sliding forward on the calendar ever so gradually—less than two days per century—as Earth’s tilt itself undergoes a slow-motion wobble. In 13,000 years, perihelion will be arriving in July and aphelion in January.
|Figure 1. Earth is about 3% closer to the Sun during early January (right), even though the Northern Hemisphere is colder at that time than during early July (left). Image credit: Colivine/Wikimedia Commons.|
Why do we have seasons?
On a basic level, it would be easy to think that the “reason for the seasons” is because Earth is closer to the sun in summer than in winter. But this is true only in the Southern Hemisphere, which is home to only about 12% of our planet’s population. Instead, our seasonality is driven by the tilt of Earth’s rotational axis relative to the orbital loop (about 23.4°), which brings the Northern Hemisphere its longest days in June and its shortest days in December. Many if not most folks aren’t aware of this core science concept (see bottom of this post).
Because of Earth’s elliptical orbit, the amount of solar radiation reaching Earth is about 6.5% larger in January than in July. However, roughly three times more solar radiation falls across midlatitude areas at the summer solstice versus the winter solstice, so it’s easy to see why Earth’s axial tilt predominates over eccentricity in shaping the seasons.
You might expect modern-day summers to be hotter in the Southern Hemisphere than in the Northern Hemisphere, given that the sun is closer to Earth during the southern summer than during the northern summer. Earth’s continents get in the way of this idealized picture, though. Most of Earth’s land area (about two thirds) is in the Northern Hemisphere, and land masses heat up and cool down more quickly than oceans. Geography thus works to blunt seasonality in the Southern Hemisphere and amplify it in the Northern Hemisphere.
|Figure 2. NASA satellite imagery from September 2004, during North Africa’s wet season, shows lush vegetation extending northward across the Sahel region, in stark contrast to the parched Sahara further north. Image credit: NASA Earth Observatory.|
Greening and de-greening the Sahara
Turn back the clock a few thousand years—to the last time that perihelion occurred during the northern summer—and you’ll find something remarkable: lush grasslands and lakes extending across large parts of what we now know as the Sahara Desert.
One factor believed to be responsible for the greener Sahara that prevailed from about 14,000 to 6000 years ago is, ironically, the more intense summer sunlight at that time. The stronger solar input was not only because of perihelion occurring in the warm season but also because Earth’s tilt was slightly larger back then. (The tilt varies from about 21.8° to 24.4° and back over a period of about 41,000 years, and it’s been on the decrease for the last few thousand years.)
Rather than parching the landscape, the more intense summer sunlight that prevailed in Africa at the time is believed to have triggered a more powerful monsoonal process. The huge thunderstorm complexes that churn westward across the Sahel (and that sometimes serve as the seedlings for Atlantic hurricanes) extended further north, sweeping across more of the present-day Sahara, and they may have pulled in more moisture from the tropical Atlantic.
As perihelion moved farther away from the summer solstice and Earth’s tilt decreased, the monsoons gradually weakened, and the grasslands slowly reverted to the sandy, arid landscape that we now associate with the Sahara.
The legacy of “A Private Universe”
A majority of people surveyed in several countries know that the Earth rotates around the Sun, but it seems likely that those who understand the role of Earth’s tilt in producing the seasons (including you, dear reader) are a more elite group.
Confusion about seasonality became a focus of science educators thanks to a powerful documentary called “A Private Universe”, produced by the Harvard-Smithsonian Center for Astrophysics in 1987. In the opening clip of the documentary, several freshly minted Harvard graduates were quizzed on commencement day about what caused the seasons—and the results were eye-opening. Only 2 of the 23 randomly interviewed Harvard grads gave a complete and correct answer.
“A Private Universe” not only prompted a rethinking of how seasonality was taught, but it also led to a more general look at how misunderstandings can persist throughout one’s education, as students fit new knowledge into flawed mental models. Science educators looked back on the film and shared its impact on them in a 2007 conference session, part of a set of videos available online from Annenberg Learner on the project’s legacy—including the original “A Private Universe”. You can’t help but feel for the (happily anonymous) students whose misconceptions have been preserved on video for decades, but at least their struggles helped to improve science education for millions of students who followed them.