An eclipse can only occur during the times of the New Moon and Full Moon, when the Sun, Earth and Moon are essentially lined up. A solar eclipse can only occur at the time of the New Moon, when the Moon is interposed between the Earth and the Sun, and the Moon’s shadow is directed toward the Earth. Similarly, a lunar eclipse can only happen during a Full Moon, when the Moon is opposite the Sun, and generally behind the Earth’s shadow.
However, we don’t have eclipses at every single New or Full Moon! This is because the Sun, Earth and Moon are not perfectly lined up most months. The plane of the Moon’s orbit slightly tilted to the plane of the Earth’s orbit around the Sun, at an angle of 5.2 degrees. Though not a very large angle, this inclination of the Moon’s orbit insures that eclipses occur only in certain months each year.
The plane of the Earth’s orbit is called the plane of the ecliptic. The places where the Moon’s orbit crosses the ecliptic are the nodes. Eclipses can only occur when the Sun and Moon are both located sufficiently close to one of the nodes. The Sun is always found somewhere on the ecliptic, but the nodes are not always near the Sun. It is only when the Moon is near the ecliptic than an eclipse can occur, which is why this plane is called “the ecliptic.”
During most months, the Sun and Moon are not lined up with the nodes, so Moon sails high above or low under the Sun and the Earth’s shadow. Depending on the current location of the nodes, the Moon can be so far away from the Sun at the time of the New Moon that 10 Moons could fill the gap! Thus, the Moon’s shadow passes far away from the Earth most of the time during the New Moon, so that no solar eclipse occurs. Similarly, the Moon usually misses the Earth’s shadow by a wide distance during most Full Moons, so that we don’t have a lunar eclipse.
Both the Earth and the Moon cast two types of shadows, the umbra and the penumbra. The umbra is the full shadow of a body, what we usually think of as a “shadow.” While inside the umbra, the bright rays of the Sun are fully obscured. We experience nighttime when our location is fully within the umbra of the Earth. And the dark portions of the Moon seen in the phases are the areas where it is nighttime on the Moon, where the lunar umbra obscures the Moon’s surface.
In addition to the umbras of the Earth and the Moon, each also has a penumbra, or partial shadow. The term “penumbra” means “almost a shadow.” As seen from within the penumbra, the Sun is only partially obscured. Close to the outside edge of the penumbra, only a small portion of the Sun is blocked. But deeper within the penumbra, closer to the umbra, a greater percentage of the Sun is blocked.
The penumbras of the Earth and Moon diverge from their umbras, and this divergence is greater at greater distances, like the distances between bodies in space. The umbra is a converging cone that tapers down toward a narrow point. But the penumbra is a diverging cone that flares outward, encompassing a large volume of space in which the Sun appears to be partially obscured.
Both solar and lunar eclipses progress in phases. In a partial eclipse, only a portion of the Sun or Moon is obscured. A partial eclipse is observed in the areas covered by the penumbra. During total eclipse, the entirety of the luminary is obscured, resulting in a darkening for a period of time. A total eclipse is only experienced in locations where the tiny dot of the lunar umbra touches the Earth, along the narrow path of totality. Sometimes, an eclipse is only partial, never entering a phase of totality, since the Moon’s umbra never contacts the Earth. But all total eclipses begin and end with stages of partial eclipse.
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Each year, there are two eclipse seasons when the Moon’s nodes line up with the Sun, and each season includes at least one solar eclipse and at least one lunar eclipse. So there are at least two of each type eclipse every year, and sometimes more. It is a fairly common sight to see a lunar eclipse from a particular location, but not as common to see a solar eclipse from any specific place.
A lunar eclipse is something physically happening on the surface of the moon, when the Earth’s shadow passes over the lunar surface. Eclipses of the Moon can only be seen from locations where it is currently nighttime while the eclipse is underway, so everyone on the side of the Earth currently facing the Moon sees the same thing. Entire continents can observe the same lunar eclipse at the same time. But continents where it is still daytime will completely miss seeing that eclipse. Lunar eclipses can last up to an hour and a half in duration.
However, a solar eclipse is very different. For a solar eclipse, the Moon’s shadow is passing over the Earth. Since the Moon is smaller than the Earth, the “footprint” of the Moon’s shadow is smaller than the Earth itself. The Moon’s penumbra passes over a smaller section of the side of the Earth facing the Moon, so that a partial solar eclipse is only seen from only a part of the Earth. Under a partial eclipse, the bright sunshine is only partially obscured, and special eye-protective solar viewing glasses are required in order to see the eclipse.
The converging cone of the Moon’s umbra comes to nearly a point as it passes over the Earth, so a total solar eclipse is only seen in the few places where the umbra passes over. The umbra is only about 150 miles wide at most during a total solar eclipse, usually much smaller, so only a relatively small area on the Earth can see the total eclipse of the Sun. Everywhere else outside the path of totality, everyone is standing in the sunshine!
Under a total eclipse, the Moon appears larger than the Sun in the sky, and completely covers the Sun’s bright disc. It’s totally safe to look at a total solar eclipse, since no portion of the Sun’s blindingly bright surface is visible. But you still need special solar viewing glasses to see the stages of partial eclipse, before and after totality.
Not every location on the globe where the lunar umbra passes sees the same duration of totality. Eclipse durations are longest as seen from locations near the point of greatest eclipse, near the middle of the eclipse path, where the axis of the lunar umbra passes closest to the center of the Earth. In these areas, totality occurs in the middle hours of the day, around noon. This can be thought of as the “prime” portion of the eclipse path, locations more ideal for observing the eclipse. The longest solar eclipses last only about seven and a half minutes, and are still short compared to a lunar eclipse.
Conversely, durations are shortest near the ends of the path of totality, where the umbra grazes the edges of the Earth. In these places, totality is seen in the early morning after sunrise, or in late afternoon before sunset. These areas can be thought of as the “tail ends” of the eclipse path, locations that are not at all ideal for observing maximum duration of totality.
Because of the small areas involved, it’s been calculated that any particular city in the world will only see a total eclipse of the Sun every 375 years, or only three times in a millennium! And because of the way things work out, the 20th century was a “dark age” of total solar eclipses. There were “tail end” eclipses and others that covered small areas of American real estate. Only two “prime” eclipses passed over the USA, in 1918 and 1970, that spanned larger areas. And no total eclipses at all were seen over the Continental USA for the thirty-eight years between 1979 and 2017.
But after this long lean stretch, the cycles have worked out that so that the 21st century will be a “golden age” of American eclipses! There will be six “prime” eclipses to pass over the United States in the current century, with four of them within a thirty-five year period, in 2017, 2024, 2045 and 2052! So the young Americans of the early 21st century have an opportunity to spend their lifetimes chasing eclipses, right here at home!
For more information, read the Future USA Eclipses page!
There is another special type of partial solar eclipse, called an annular eclipse. With this type, the eclipse is “central,” like a total solar eclipse, except the Moon appears smaller than the Sun, so that it does not completely cover the Sun’s disc. An annular eclipse appears in the sky as a bright ring of sunlight, and protective solar viewers are required in order to see the annular shape of the Sun.
Annular eclipses occur because the Moon’s orbit is shaped like ellipse, rather than a circle. The Moon follows an eccentric path around the Earth, sometimes closer and sometimes farther away. The close point in the Moon’s orbit to the Earth is called perigee, and the Moon appears largest in the sky. Perigee has lately come to be known as a “supermoon,” since the Moon looks larger than average. Total solar eclipses tend to occur close to perigee, since the Moon’s disc is larger than the Sun’s.
The farthest point away from the Earth is apogee. At apogee, the Moon appears smallest in the sky. Annular eclipses occur when the Moon is near apogee, so that the Moon’s disc is not large enough to cover the Sun. In between the total eclipses of the early 21st century, there will also be several annular eclipses over the USA, and also several partial eclipses. So there will be a some sort of solar eclipse over the United States every few years for decades to come! So hang onto your Eclipse Shades, they’ll come in handy!
(The content of this page is adapted from the Understanding Eclipses ebook, coming soon!)
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To learn some basic information, including the differences between a lunar and a solar eclipse, visit our What is an Eclipse? page.
To learn more about what you can expect to see before, during and after a total eclipse of the Sun, visit our Total Solar Eclipse Phenomena page.