During the total solar
eclipse on Aug. 21, skywatchers will direct most of their attention to
the sun, but don't forget about the moon: Its slow progress away from
Earth means these celestial events won't keep happening forever.
Next month’s total solar eclipse will sweep across the continental
U.S. from Oregon to South Carolina along a stretch of land about 70
miles (113 kilometers) wide. A total solar eclipse occurs only when the
disk of the moon passes between the Earth and the sun, briefly blocking
the sun’s bright light and casting a long shadow on the planet.
“A total eclipse is a dance with three partners: the moon, the
sun and Earth,” Richard Vondrak, a lunar scientist at NASA’s Goddard
Space Flight Center in Maryland, said in a statement. “It can only
happen when there is an exquisite alignment of the moon and the sun in
our sky.”
Total solar eclipses occur because the moon and the sun have the
same apparent size in Earth’s sky — the sun is about 400 times wider
than the moon, but the moon is about 400 times closer.
But the moon is slowly moving away from Earth by about 1-1/2
inches (4 centimeters) per year, according to the NASA statement. As a
result, total solar eclipses will cease to exist in the very distant
future, because the apparent size of the moon in Earth’s sky will be too
small to cover the sun completely.
“Over time, the number and frequency of total solar eclipses will
decrease,” Vondrak said in the statement. “About 600 million years from
now, Earth will experience the beauty and drama of a total solar
eclipse for the last time.”
A total solar eclipse occurs during an alignment of the Earth, the moon and the sun.
Credit: NASA
For now, a total solar eclipse is visible from somewhere on the
Earth’s surface about once every 18 months, on average. However, seeing a
total solar eclipse from a specific location is rare, because the
moon’s inner shadow is relatively small, which limits the total area
from which the total eclipse is visible, according to a video from NASA
about the moon’s role in a solar eclipse.
“You have to be on the sunny side of the planet, and you have to
be in the path of the moon’s shadow,” NASA officials said in the video.
“So, if you find your area in the path of totality one year, you’ve hit
the jackpot, because on average, that same spot on Earth will only get
to see a solar eclipse every 375 years.”
But note that partial solar eclipses, in which the moon obscures
only part of the sun, are visible across a much larger area. The two
parts of the moon’s shadow, the umbra and penumbra, determine which kind
of eclipse an observer sees on Earth. The moon’s umbra, or the dark
inner shadow, is the part of the moon’s shadow where the entire sun is
blocked by the moon. The penumbra is where only part of the sun’s disk
is obscured.
Most eclipse maps highlighting the path of totality show a dark
circle that represents the umbra. However, the “true shape of the umbra
is more like an irregular polygon with slightly curved edges,” according
to the video. Features on the surface of the moon determine the shape
of the umbra.
Using data from NASA’s Lunar Reconnaissance Orbiter (LRO), NASA
scientists were able to map the lunar surface in unprecedented detail,
showing the mountains and valleys that affect the passing sunlight and
subsequent shape of the moon’s shadow during a total solar eclipse,
according to the video.
These topographic maps, along with Earth elevation data, allow
scientists to determine the exact areas on Earth that fall in the path
of totality for the Aug. 21 solar eclipse.
“With this new visualization, we can represent the umbral shadow
with more accuracy by accounting for the influence of elevation at
different points on Earth, as well as the way light rays stream through
lunar valleys along the moon’s ragged edge,” Ernie Wright, a NASA
visualizer at Goddard, said in the statement.
The LRO data also helps scientists better predict when and where
skywatchers will see “Baily’s beads,” the irregular dots of light
visible around the edge of the moon during a total solar eclipse. This
phenomenon is caused by the last rays of sunlight streaming through the
moon’s rugged mountain valleys, and it occurs for just a few seconds
before and after totality.
So when you look up for the solar eclipse on Aug. 21 — whether
you are seeing a total or partial eclipse — be sure to admire the moon
as well as the sun.
