Achi news desk-
NEWS RELEASE
BROCK UNIVERSITY
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The rarity of a total solar eclipse, which will occur over Niagara and select areas across North America on April 8, can be better understood by analyzing the math and science involved, says a Brock University expert.
The celestial phenomenon, where the moon completely blocks the sun and turns the daytime sky dark enough to see stars for several minutes, will be a once in a lifetime experience for many.
Jan Vrbik, Professor of Mathematics who studies the movement of the sun, planets and moons, says that there are four factors that make it quite rare to see a total solar eclipse.
Distances of the sun and the moon from the Earth
“Although the sun is 400 times larger than the moon, when we look at them in the sky, they appear to be almost the same size,” said Vrbik. “This is because their distances from Earth are roughly in the same ratio of 400 to one.”
That is why the moon can cover the sun, either partially or completely, to create a solar eclipse.
The inclination of the moon’s orbit
The moon’s orbit around the Earth is not a perfect circle – it is an ellipse – so the moon’s distance from the Earth varies depending on where it is along its orbit.
“It varies by 11 percent every month, which suggests that the moon’s disk also varies in size relative to the sun,” Vrbik said.
About 35 percent of the time, the moon’s disk is larger than the sun’s disk, and the other 65 percent of the time, it’s the other way around, he said.
The intersection of the orbits of the sun and the moon
The Earth orbits the sun once a year, while the moon orbits the Earth once a month. If the orbits of the sun and the moon were on the same plane, an eclipse would occur every month, but the moon’s orbit is tilted by about five degrees from the sun’s orbit.
“This means that on most new moons, the moon is either noticeably higher or lower in the sky than the sun,” Vrbik said. “Nevertheless, the moon needs to cross the plane of the sun’s orbit twice during its monthly cycle, at a crossroads called a ‘node.’ Only when the direction of the sun closely coincides with one of these nodes, which happens regularly twice a year, during a new moon.”
It says 23 percent of solar eclipses are ‘total,’ when the moon completely covers the sun, and 42 percent are ‘annular,’ when the moon’s disk is smaller than the sun’s disk and is seen ‘ring of fire’.
“Thirty-five percent of solar eclipses run so far off the center of the Earth that only a partial eclipse is observed on one side of the globe – with an actual path missing the Earth entirely, ” he said.
The shadow of the moon along the path of totality
For a total solar eclipse, the moon’s shadow needs to cross the Earth’s surface.
The size of the moon’s shadow varies depending on how close in time the event is to when the moon is closest to the Earth. Typically, the shadow is 150 kilometers (km) in circular area. A much larger part of the Earth’s surface, about 7,000 km across, will simultaneously experience a partial eclipse, where only part of the sun’s disk is covered by the moon.
Vrbik says the path of the shadows across the globe is a combination of the moon’s straight-line motion of about one km per second and the Earth’s rotation of almost 0.5 km per second, further complicated by the fact that the axis of the Earth tilts.
If people are in the right place at the right time, they can witness a total solar eclipse for three to four minutes.
Add it all up
“When one considers the motion of the sun, Earth and moon, as well as their orbits, speed, distance and alignment with each other, it is easier to understand why total solar eclipses are so rare ,” said Vrbik.
While a total solar eclipse occurs on Earth every 18 months, it usually occurs over water. The last time a total solar eclipse happened over Ontario was in 1979. The next one won’t happen again over the province until 2099.
“If you know the cycle length of each celestial body, you can easily predict when an eclipse will occur again,” he said. “Experiencing a total solar eclipse from a specific location is a rare privilege, usually separated from the next such event by several centuries.”
Vrbik will be among a group of Brock experts leading discussions and educational exhibits around the solar eclipse during ‘Eclipse on the Escarpment,’ a free community event being held on Brock’s main campus on Monday, April 8.
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