If this is your first time viewing a total solar eclipse, you are in for a treat. Imagine the astronomical coincidence that places the sun about 400 times more distant from the Earth than the moon, and at the same time, the sun is about 400 times the moon’s diameter. This set of circumstances makes the sun and moon appear almost exactly the same size from our perspective. Solar eclipses are not rare since they happen every year or two, being visible in a very narrow band somewhere on the planet. But for any single location, they only happen on average every few hundred years.
Because the Earth’s orbit around the sun and the moon’s orbit around the Earth are not perfectly circular, sometimes the sun appears a bit larger than the moon, resulting in an annular eclipse (so named because a ring of the sun is still uncovered). Sometimes the moon appears larger than the sun, resulting in a total solar eclipse. They are each completely different in terms of the visual experience.
During an annular eclipse, the sky doesn’t get dark, because the sun is not completely covered. There is a noticeable dimming, however. And when viewed safely (it is not safe to look directly at an annular eclipse without protection), the sun appears as a bright ring, also known as the “ring of fire”. During a total solar eclipse, the moon completely covers the sun. And only then is it safe to look at the sun/moon safely without protection.
What are the phases of an eclipse?
Total solar eclipses are divided into phases. The first phase, also known as first contact, happens when the edge of the moon just touches the edge of the sun. This phase progresses until second contact, when the moon finally reaches the opposite edge of the sun, resulting in totality. At the beginning of second contact, just as the last of the sun is being covered, there is a diamond ring effect, resulting from the last tiny bit of the sun still being uncovered. And then, just before totality, the uneven surface of the moon still allows some points of sunlight to peak through. These points are known as Bailey’s Beads. This process is repeated as the sun begins to peek out from behind the moon, known as third contact. The portion of the sun that is covered continues to grow until the moon’s edge leaves the sun’s edge, also known as 4th contact.
Why are all eclipses not the same duration?
You might wonder why some eclipses last less than a minute and others last over 6 minutes. The answer is simple. The orbit of the Earth around the sun is an ellipse (not perfectly circular). The same can be said about the moon’s orbit around the Earth. If the Earth is slightly farther away from the sun than average at the same time the moon is slightly closer to the Earth than average, the moon will appear noticeably larger than the sun and take longer to cross completely past the sun, as long as 7.5 minutes. On the other hand, if the moon is slightly farther than average from the Earth and the Earth is slightly closer to the sun than average, the moon will appear only slightly larger than the sun and only cover it for a short period of time, sometimes even less than a minute. The extreme is when the moon cannot completely cover the sun resulting in an annular eclipse. There is also something known as a hybrid eclipse when at the beginning of the eclipse, the moon is not large enough to cover the sun, resulting in an annular eclipse, but as the day progresses, the orbital geometry shifts enough for the moon to completely cover the sun, resulting in a very short total eclipse.
Why aren’t there solar eclipses every month during a new moon?
This is a very interesting question. If the Earth was not tilted on its axis and the moon orbited precisely over the equator without tilt, we would have a solar eclipse every 28 days or so during a new moon. But the reality is that the Earth is tilted compared to its orbit around the sun, and the moon doesn’t revolve exactly around the equator. This results in nodes during which the moon does pass exactly between Earth and the sun. It is when the moon passes through these nodes that total eclipses can occur every year or two. The cycle of alignment of the nodes to the sun repeats in a cycle of nearly identical eclipses every 18 years and 11.3 days (the Saros Cycle). The astronomical geometry of each cycle is the same, but Saros cycles are not a whole number of days. And because of the rotation of the Earth, these eclipses don’t happen in exactly the same location each time.