Why the Northern Hemisphere Sees More Total Solar Eclipses

Spain recently experienced its first total solar eclipse in over 100 years. NASA collaborated with a team to film this rare event from space using balloons. The team initially hesitated to cover the eclipse due to existing content on YouTube but found several unanswered questions that piqued their interest. These included:

• Why does the Northern Hemisphere experience more total eclipses than the Southern Hemisphere?
• Why does the sun appear white only during pure totality?
• What causes the elusive shadow bands seen on the ground during an eclipse?

The team traveled to Burgos, Spain, a prime location for observing totality, to meet with NASA and other science groups launching balloons to the edge of space to film the eclipse and potentially answer these questions.

Eclipse Frequency and Patterns

Angela, an experienced eclipse observer with three total eclipses under her belt, explained that while a random point on Earth experiences a total eclipse approximately every 300 years, certain patterns lead to more frequent occurrences in specific regions.

Mapping total solar eclipse paths from 2000 BCE to 3000 CE reveals that the Northern Hemisphere consistently experiences about 15% more total eclipses than the Southern Hemisphere. This phenomenon is primarily due to two factors:

1. Moon's Elliptical Orbit: The Moon's distance from Earth varies, causing its apparent size in the sky to change by up to 30% over a month. For a total solar eclipse, the Moon must appear larger than the Sun.
2. Earth's Elliptical Orbit: The Earth's orbit around the Sun is also elliptical. The Sun's apparent size is about 7% smaller in July than in January. This increases the likelihood of a total solar eclipse around July, which coincides with the Northern Hemisphere's summer when it is tilted towards the Sun.

Conversely, the Southern Hemisphere experiences more annular eclipses, where the Moon appears slightly smaller than the Sun, creating a "ring of fire." This is because the Sun is closer during the Southern Hemisphere's summer.

This pattern will eventually reverse due to Earth's axial precession and the slow shift of its elliptical orbit. In approximately 9,500 years, the Southern Hemisphere will experience more total solar eclipses.

The Rarity of Eclipses

Despite the perception of eclipses as rare, there is never a year without some form of solar eclipse (total, annular, or partial). At least two solar eclipses occur annually, and sometimes up to five.

The Moon's orbit is tilted about five degrees relative to the Earth-Sun plane. This tilt usually prevents the Moon from casting a shadow on Earth during its monthly pass in front of the Sun. However, there are two points in the Moon's orbit, called nodes, where it crosses the Earth-Sun plane. These nodes align with the Earth and Sun approximately every six months, creating "eclipse seasons" – 34-day windows during which solar eclipses are possible.

Since a new moon occurs every 29.5 days, at least one new moon must fall within each 34-day eclipse season, guaranteeing at least two solar eclipses per year. If a new moon occurs near the middle of an eclipse season, it typically results in a total or annular eclipse.

Partial eclipses, which occur when the new moon is closer to the edges of an eclipse season, can happen twice within a single season, leading to four eclipses in a year. Five eclipses in a year are possible if the eclipse seasons drift early enough in the year, allowing for multiple partial eclipses and a final one in December.

An interesting correlation is that every solar eclipse is accompanied by a lunar eclipse either two weeks before or after, as the Earth is positioned between the Sun and Moon during those times.

The Sun's Color During an Eclipse

During the partial phases of an eclipse, the Sun often appears yellowish in time-lapse videos, but during totality, it turns white. This is not due to a change in the Sun's actual color. The yellowish hue in partial phases is an artifact of the filters used on cameras to safely photograph the bright Sun. These filters commonly impart a reddish-orange tint. During totality, the Sun's direct light is blocked, making it safe to view without filters, revealing its true white color.

NASA's Eclipse Ballooning Project

The team joined a NASA-funded lab from Bozeman, Montana, participating in the Nationwide Eclipse Ballooning Project. This project involved launching weather balloons (radiosondes) in Reykjavik, Iceland, and larger engineering balloons in Spain. The engineering balloons carried cameras to film the eclipse from the edge of space. The team even used water bottles filled with rocks as makeshift weights for their balloon payload.

Chasing Totality: Aircraft Missions

While ground observers experience totality for a maximum of two minutes and 18 seconds, NASA also deployed a mission on a 70-year-old Cold War-era bomber plane. This aircraft, flying at 460 mph, aimed to extend the duration of totality to around three minutes. This isn't a new endeavor; in 1973, the Concorde flew at Mach 2 and remained in totality for 74 minutes.

The aircraft mission's primary goal isn't just extended viewing time but also stability. Unlike balloons, a jet provides a much more stable platform for sensitive equipment, allowing for detailed study of specific parts of the Sun.

Scientific Discoveries from Eclipses

Eclipses have historically led to significant scientific discoveries. During the total eclipse of August 18, 1868, French astronomer Jules Janssen observed the Sun's prominences (arcs of glowing gas at the edge). By using a slit and prism to break down their light, he identified five bright bands, one of which did not correspond to any known element on Earth. Two months later, English astronomer Norman Lockyer observed the same line and named the element "helium" after Helios, the Greek god of the Sun.

In the following year, scientists studying the hotter corona found another unexplained band, which was believed to be a new element called "coronium" for 70 years. However, in 1939, it was discovered to be iron, heated to such an extreme that 13 of its electrons had been stripped away. Thus, eclipses have contributed to the discovery of both real and "fake" elements.

Pinhole Camera Effect and Shadow Phenomena

During an eclipse, everyday objects can act as pinhole cameras, projecting crescent-shaped images of the partially eclipsed Sun. For example, a triangular cutout, when moved away from the ground, will project a crescent shape instead of a triangle. This occurs because light travels in straight lines, and when the object is far enough, the projection is of the light source itself, not the hole. The image is also inverted. This effect is naturally observed through the gaps between leaves in trees, which all project crescent shapes during an eclipse.

Another peculiar phenomenon is the change in shadow sharpness. As totality approaches, shadows cast by objects exhibit both a sharp and a soft direction, 90 degrees apart. This is because the light source (the partially eclipsed Sun) becomes a narrow sliver. In the direction parallel to this sliver, shadows remain blurry. However, perpendicular to the sliver, the light source acts more like a point source, resulting in perfectly sharp shadows.

Elusive Shadow Bands

Shadow bands are long, moving, snake-like shadows that appear on the ground just seconds before and after totality. While their exact cause remains a mystery, the leading theory attributes them to narrow beams of light from the Sun's sliver passing through varying layers of air with different temperatures and densities. These atmospheric layers refract and bend the light, similar to how starlight twinkles.

However, this theory doesn't fully explain why shadow bands are observed on the ground but not necessarily at higher altitudes. An experiment in Burgos in 1905 involved an army engineer ascending in a hydrogen balloon to look for shadow bands. While he didn't see them on a white sheet placed below the balloon, he observed them everywhere else, including on his hands and the balloon's basket.

Another contributing factor to shadow bands might be the Moon's topography. The irregular surface of the Moon creates multiple point sources of light (Baily's Beads) just before totality. The interference and overlapping of shadows from these multiple sources could amplify the shadow band effect, similar to how multiple spotlights in a theater create complex overlapping shadows.

The Experience of Totality

The experience of totality is described as "insane," "unbelievable," and "cataclysmic." Observers noted the stunning view of the Sun's corona and bright spots, possibly solar flares. A unique aspect is the "360-degree sunset" effect, where the horizon appears to be in sunset all around. The emotional impact of witnessing a total solar eclipse is profound, leaving observers "unreasonably fired up."

Balloon Recovery and Sponsorship

After the eclipse, the team tracked and recovered their balloon, which had traveled a significant distance. The article also included a sponsored message for "80,000 Hours," a nonprofit organization that provides evidence-based career advice for making a positive impact on the world. They offer career guides, deep dives into different paths, and a job board for high-impact roles, all available for free.

Finally, the team thanked Astrum for their video on the Sun's corona, Exploratorium for live stream access, and the NASA Borealis team for their assistance. They also offered signed stickers that had flown to the edge of space to their Patreon members.