Imma Perfetto
Cosmos science journalist
Scientists and eclipse enthusiasts alike will no longer need to worry about accidentally missing out on their view of upcoming solar eclipses.
NASA has developed eclipse maps that plot in fine detail the pathway of the Moon’s central shadow (umbra) as it crosses Earth.
Their new study reveals not only the true shape of the umbra, but also the reason it occurs.
They found that the valleys along the edge of the Moon act like pinhole cameras, projecting images of the Sun onto the surface of the Earth. The umbra is the small hole in the middle where none of these projected Sun images reach.
“Beginning with the 2017 total solar eclipse, we’ve been publishing maps and movies of eclipses that show the true shape of the Moon’s central shadow — the umbra,” says Ernie Wright, from the Scientific Visualization Studio at NASA’s Goddard Space Flight Center.
“And people ask, why does it look like a potato instead of a smooth oval? The short answer is that the Moon isn’t a perfectly smooth sphere.”
Traditional 19th century approaches to calculating the path of the umbra relied on the significant assumptions that everyone observing the eclipse is doing so at sea level and that the Moon is a smooth sphere perfectly centred around its centre of mass.
Both are incorrect, of course. People live well below and above sea level, and the Moon has a cratered, uneven surface from its long history of asteroid and meteorite impacts.
Eclipse map predictions that ignore the Earth’s and Moon’s terrains can be kilometres off – in terms of where the umbra will occur – and seconds off in the duration and contact times of totality (when an observers see the Moon eclipse the Sun completely).
This may not seem significant, but the researchers say such errors are becoming “increasingly intolerable” for both photographers and scientists recording observations.
The paper in The Astronomical Journal describes their new approach. It involves incorporating data on the mountains and valleys around the edge of the Moon, gathered by NASA’s LRO (Lunar Reconnaissance Orbiter), as well as an elevation map of Earth so that eclipse observer locations are depicted at their true altitude.
And, unlike the hand calculation method invented 200 years ago, the new way renders eclipse maps a pixel at a time, the same way 3D animation software creates images.
