Dear Dr. Universe: How do scientists know how to predict a solar eclipse? -Beau, 11 ½
 
Dear Beau,
 
Before humans even knew how to predict solar eclipses, they were fascinated with the phenomenon. To figure out how to predict an eclipse, astronomers asked lots of questions and made observations about the motion of our moon, sun and Earth.
 
My friend Jose Vazquez, an astronomer at Washington State University, told me all about it.
 
“It took centuries to figure it out,” he said. “It was a journey.”
 
A total solar eclipse happens when the sun, moon and Earth are lined up just right and for a few minutes, the moon blocks the Earth’s view of the sun. During a total solar eclipse, the moon casts its shadow down to some places on Earth.
 
Vazquez told me about the Greek astronomer Hipparchus who was really curious about the moon. Using knowledge from early astronomers, he studied lunar eclipses—that is, when the moon passes into the Earth’s shadow. He watched the night sky and recorded his observations about the moon.
 
Maybe you can try this, too. Every day for a week, stand in the same spot outside your house. You may notice that the moon seems to move across the sky west to east.  
 
When Hipparchus was studying the moon, his models were based on an idea that the moon orbited Earth in a circle. About 400 years later, Claudius Ptolemy would use mathematics to show many objects in our solar system orbit in a circle. But today, we know that isn’t the case.
 
The astronomer Johannes Keppler created a more accurate model and showed us that the moon orbits Earth in a shape called an ellipse. You can imagine this shape by taking a hula hoop circle and squashing it on one side so it forms more of an oval.
 
Also, the moon doesn’t move in a perfectly straight path in its orbit. It sort of goes up and down as it moves along its path.
 
Through watching the sky and observing the patterns and orbits that the moon, sun and Earth follow, astronomers can help determine when the three objects will line up. They can make an educated guess about when the eclipse will happen. 
 
While many people, including in the ancient Mayan, Babylonian and Chinese civilizations, have observed solar eclipses and recorded their observations, it was the astronomer Edmond Halley who used mathematics to get some of the most accurate predictions in 1715. These days, we also use computers that make calculations to help predict eclipses.
 
Believe it or not, scientists at NASA are now able to predict eclipses for the next thousand years. We know that the last eclipse of 2021 will be on Dec. 4. While we won’t be able to see it in North America, it will be visible in places like South Africa, southern Australia, South America and Antarctica.
 
If you are curious about upcoming solar eclipses, you can also visit NASA’s database. They make it easy for anyone around the world to know when they can catch the next solar eclipse.  
 
Sincerely,
Dr. Universe