Adam Hincks is a Jesuit priest and an assistant professor in the Department of Astronomy & Astrophysics and at St. Michael’s College at the University of Toronto, where he holds the Sutton Family Chair in Science, Christianity and Cultures.
What is an eclipse and how does a solar eclipse differ from a lunar one? Partial versus full?
Most generally, an eclipse happens whenever one body in space passes in front of another body and blocks its light. The most well-known eclipses are lunar eclipses and solar eclipses. Lunar eclipses happen around the time of a full moon if the Earth passes between the Sun and the Moon and casts a huge shadow over the Moon. Solar eclipses happen around the time of a new moon if the Moon passes between the Sun and the Earth. Because the Moon is a lot smaller than the Earth, the shadow it casts during a solar eclipse only covers a small area on the Earth’s surface, whereas during a lunar eclipse the whole Moon can be enveloped in Earth’s shadow. For this reason, lunar eclipses can be seen by more people at a time than solar eclipses.
Both solar and lunar eclipses can be partial or total. Total eclipses are when the whole disc of the Sun is covered by the Moon, in the case of solar eclipses, or when the whole disc of the Moon is covered by the Earth’s shadow, in the case of lunar eclipses. Partial eclipses happen when only a portion of the disc is covered. In the case of solar eclipses, there are also annular eclipses. This is when the Moon passes exactly over the Sun, but at a time when it is further away in its orbit around Earth so that its size in the sky is slightly smaller. In that situation, the Moon cannot completely cover the face of the Sun, but leaves a thin ring, or annulus, of the Sun visible.
While many people think of an eclipse as a rare event they happen regularly. What factors are involved in whether or not I am able to see an eclipse?
The orbit of the Moon around the Earth is tilted about 5 degrees relative to the orbit of the Earth around the Sun. This means that during most new and full moons, the Moon is either above or below the Sun and you can’t get an eclipse. However, every five to six months the orbits line up so that the Moon and the Sun are level with each other. These are called eclipse seasons, and you typically get two or three eclipses of some kind during each season.
As mentioned above, the Earth’s shadow is big compared to the size of the Moon, so when lunar eclipses happen, you can see them from pretty much anywhere on Earth where it is nighttime. In Toronto, the next total lunar eclipses will be on March 14, 2025 and March 3, 2026. If you have never seen one before, it is well worth trying to catch a glimpse of the next one!
On the other hand, the Moon’s shadow is a lot smaller than the size of the Earth. The full shadow is typically 100 to 200 km in diameter—similar to the widths of Lakes Erie and Ontario that it will pass over in the upcoming April 8, 2024 eclipse. You need to be inside this shadow, the umbra, to see the total eclipse. So while many years there are one or two total or annular eclipses, they are only visible from a tiny fraction of the Earth’s surface. As a solar eclipse proceeds, the umbra moves along the face of the earth, tracing out a narrow strip. What is special about the April 2024 eclipse is that this strip passes over Mexico, the United States and Canada and will be viewable by a huge population. The path of totality will just miss Toronto, unfortunately—see below for notes on safe viewing—since Toronto is only in the much wider penumbra from which you can see a partial eclipse. However, the umbra will cross over other locations in Southern Ontario, like Niagara Falls, Hamilton and Kingston. You can use this interactive map to see where the umbra passes; clicking on the map will tell you when the eclipse will happen.
Unlike the upcoming eclipse when lots of people will have the chance to witness it, in many cases the strip of the Moon’s shadow passes over less populated regions—and remember, most the Earth is covered with oceans. For instance, the annular eclipse later this year, on 2 October 2024, will pass mainly over the Pacific ocean and only cross land at Easter Island and the very south of Chile and Argentina.
You can use this website to explore other upcoming eclipses
In the past eclipses were more mysterious events and sometimes seen as omens. We now know when one is going to happen and we know it’s not the end of the world. Why do we remain fascinated?
Just because you understand how something works does not make it less fascinating! The Sun and the Moon are such familiar objects to us, and we literally could not live without the Sun, so when something unusual like a solar eclipse occurs, it disrupts our normal relationship to the sky in a way that naturally evokes great curiosity. It is not surprising that they have been seen as significant portents by some cultures. Today, they can be a great reminder of how awesome our Solar System is and a wonderful opportunity to learn more about Creation. When people disagree about so many things down here on Earth, it is nice when there are events in the heavens that everyone can appreciate and enjoy together.
What sorts of things have we learned from studying eclipses?
The curved shape of the shadow during a lunar eclipse was one piece of evidence—but not the only one—that ancient peoples had that the Earth is round. Aristotle, for instance, lists this as a reason for the Earth being spherical.
Precise measurements of the timing of lunar eclipses were also used to determine geographic longitude before clocks were good enough to keep time at sea. Here in Canada, the Jesuit missionaries used this method to figure out the geographical location of the Ste.-Marie mission near the Georgian Bay. The most precise measurements were made by Fr. Joseph Bressani, but Sts. Jean de Brébeuf and Gabriel Lalement also recorded observations of lunar eclipses. Bressani’s observation of the lunar eclipse on January 30, 1646 is also the first record we have of a telescope being used in Canada for astronomy.
What are some fun facts about eclipses that lay people might not know?
It is actually a striking coincidence that the Moon and the Sun are almost the same size in the sky. In reality, the Sun is 400 times bigger than the Moon, but it is also a little less than 400 times further away from Earth than the Moon is. These two factors cancel out so that the two appear similar in size. There is no deep reason for this, but it means that we can observe total solar eclipses, which we couldn’t if the Moon had a slightly larger orbit around Earth than it does, and it also means that we can also sometimes see annular eclipses, which wouldn’t happen if the Moon had a slightly smaller orbit.
This coincidence of sizes and distances was propitious for a famous crucial experiment last century. Einstein’s new theory of gravity, called general relativity, predicted that massive bodies should bend the path of light. A team of astronomers, including the great Arthur Eddington, put this prediction to the test during the total solar eclipse on 29 May 1919. During the period of totality, the Sun’s light was blocked enough that stars near the Sun could be seen, and it was confirmed that their apparent positions had shifted due to their light getting bent by the Sun’s gravity along its path to us. If in some twist of cosmic history the Moon’s orbit had been slightly bigger, so that the Moon were further away from Earth and not big enough to create a total solar eclipse, that experiment wouldn’t have been possible! There are many other ways that general relativity has been tested, so it would not have been a fatal blow to Einstein’s work. As it stands, though, the observations of 1919 made headlines and propelled him to international fame.
How can we safely watch an eclipse?
You should never look directly at the Sun, even briefly, because it can cause serious and permanent eye damage. We do not have pain receptors in our retinas, but normally we instinctively avert our eyes from the Sun due to its brightness. During a partial or annular eclipse when some of the light is blocked, though, that instinct might not kick in, which means that you need to be extra vigilant because there is still more than enough light to do damage.
You should only look at the Sun through properly certified eclipse glasses. Regular sunglasses are not safe for solar viewing. You can find a list of vendors who comply with the certification here. As that site infers, some unscrupulous manufacturers may try to sell fakes, so do check any vendor against their list to be safe. (Here on campus Gerstein and Robarts Libraries were giving away certified glasses, but they have now run out.) If you do not have access to eclipse-viewing glasses, you can still view it indirectly with a pinhole camera, which still provides an exciting and worthwhile experience. See this video for more information on eclipse safety.
You can look up during the brief period of totality, when all of the sunlight is blocked by the moon. Note, however, that this will not be possible in Toronto. Even though more than 99% of the Sun will be blocked in Toronto, leaving only a sliver of sunlight, anything less than 100% is not safe to look at directly.
When is the next time the St. Mike’s campus will experience an eclipse?
As mentioned above, there will be total lunar eclipses visible from Toronto on March 14, 2025 and 3 March 3, 2026. The next partial solar eclipse will be on August 12, 2026, but only a tiny part of the Sun will be covered and you won’t notice it unless you look at the Sun with certified solar glasses (see above). The next annular eclipse in Toronto will be on July 2, 2057, and the next total solar eclipse will be on October 26, 2144, more than 120 years away!
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