The relative distances of the sun and the moon from the Earth and their relative sizes are responsible for one of the most fortuitous coincidences in astronomy.
It just so happens that the apparent disks of the sun and moon, as seen from Earth, are almost exactly the same size. That makes it possible for the moon to just cover the sun as it passes between the sun and the Earth, and because the size match is so exact, people on Earth can see the sun's corona. The odds of this occurring are, well, astronomical.
When the moon passes in front of the sun, people on Earth experience an eclipse, but not all eclipses are total. Sometimes the moon doesn't line up exactly with the sun, and instead of a total blackout, people only see the sunlight grow dim.
And sometimes, the moon is too far from the Earth in its orbit to completely cover the sun, even when it passes directly in front of it. This is an annular eclipse. It would be a total solar eclipse if the moon was closer.
'Tis the Season ... for an Eclipse
Solar eclipses occur during new moons. Conversely, a lunar eclipse occurs when the moon is full and the Earth moves between it and the sun.
If the moon's orbit was on the same plane as Earth's orbit around the sun, we would see a solar and lunar eclipse every month, but that isn't the case. The plane of the moon's orbit is inclined at 5.1 degrees to the plane of the Earth's orbit. That adds an extra condition for an eclipse to occur. Not only must it be new or full moon, but the moon must also be close enough to the plane of Earth's orbit to block part of the sun.
Each month, the moon crosses the plane of the Earth's orbit twice, once on its southward path and another time two weeks later when it's on its northward path. These crossings are called nodes, and for an eclipse to occur, the sun must be within 17 degrees of one of the nodes. This occurs twice a year. The sun travels 0.99 degrees per day, so it stays in the vicinity of a node for about 34 days. This 34-day period is called an eclipse season.
During a given eclipse season, there is one solar eclipse and one lunar eclipse. An eclipse season is longer than a month, though, so it's possible for two solar or two lunar eclipses to occur during a single season.
Four Types of Solar Eclipses
Total solar eclipses are visible along a fairly narrow path on the Earth's surface, but partial eclipses are visible over a much wider area. The type of eclipse that people see depends on three factors:
- The separation of the sun from the moon's node.
- The distance of Earth from the sun.
- The distance of the moon from Earth.
The four types of eclipses that can occur are as follows:
Total: This is the classic solar eclipse during which the moon completely covers the sun, and viewers in the moon's umbra are able to see the sun's corona. It can occur only if the sun is within a few degrees of the moon's node. At the same time, the sun must be far enough away from the Earth for its disk to be small enough to be covered by the moon. The moon, for its part, must be close enough to the Earth to have a disk big enough to cover the sun.
Partial: When an eclipse season occurs, but the sun is far from a node at full moon, some people on Earth may see the moon blocking only part of the the sun. This is a partial eclipse. The sky darkens slightly as a portion of the sun's disk is obscured.
Annular: An annular eclipse occurs when the sun is close enough to a node for a total eclipse to occur, but it's either too close to the Earth or the moon is too far from the Earth for the moon's disk to completely block the sun. Viewers in the umbra see the complete disk of the moon in front of the sun with a bright ring of sunlight surrounding it.
Hybrid: A hybrid eclipse is rare. It occurs when the sun and moon are positioned to create an annular eclipse, but as the umbra moves across the face of the Earth, the Earth's curvature reduces the distance to moon just enough to make the moon's disk large enough to completely block the sun and create a total eclipse for a short time.
What's an Annular Eclipse?
Both the Earth and moon have elliptical orbits. There is a distance of almost 5 million kilometers between the Earth's aphelion, or maximum distance from the sun, and its perihelion, or minimum distance from the sun. That makes a difference of about 1 arc-minute in apparent size.
The difference in the moon's distance from the Earth at apogee (maximum distance) and perigee (minimum distance) is about 50,000 kilometers, creating a difference in apparent size of 4 arc minutes, or about 13 percent of its average size. The moon changes more in relative size than the sun, so it has more of an effect on the type of eclipse people see.
For an eclipse to be annular, the moon must appear smaller than the sun. This definitely occurs when Earth is at its closest approach to the sun, which happens in January, and the moon is at its farthest distance.
However, the orbit of the Earth is very close to being circular, so the apparent size of the sun doesn't change that much. Consequently, an annular eclipse could also occur in July if the moon is at its apogee. If an eclipse occurs when the moon is at perigee and appears as a "supermoon" when it's full, you definitely won't see an annular eclipse, no matter what time of year it is.
When an annular eclipse occurs, the moon passes completely in front of the sun, but the sun doesn't darken completely. Instead, a ring of fire is visible around the edges of the moon's shadow, and this sunlight partially illuminates the sky, creating a kind of ghostly twilight. Because the sun is still visible during an annular eclipse, looking directly at the eclipse is even more dangerous than looking at a total eclipse.
Total vs. Annular Eclipse
When you see a diagram of a total solar eclipse, you see the moon's shadow, or umbra, depicted as a cone that tapers to a point on the Earth's surface. The area inside the umbra is about 100 miles in diameter, and anyone within it sees a total eclipse. The combined movement of the moon and the rotation of the Earth causes the umbra to move in a characteristic path along the Earth's surface at a speed between 1,000 and 3,000 mph, depending on latitude.
If you examine an annular eclipse diagram, you'll see the umbra come to a focus at some distance above the Earth's surface. Earth-bound viewers, who are beyond this focal point, are not thrust into complete shadow as they are during a total eclipse. Light from the outer ring of the sun – from where the name "annular" originates – extends beyond the focal point of the umbra and illuminates the region beyond. Sunlight is reduced, but not extinguished, creating an effect similar to a heavy cloud cover.
People are able to witness totality for no more than 7 1/2 minutes before the umbra moves eastward. Once outside the umbra, viewers remain in the penumbra, or partial shadow, for a longer period. What they see while in the penumbra is the shadow of the moon blocking only a portion of the sun's disk. In contrast, an annular eclipse can last for up to 12 1/2 minutes. The extra time is due to the smaller apparent size of the moon's disk. By virtue of its smaller size, it has more distance to cover in its path across the face of the sun.
Types of Lunar Eclipses
In any given eclipse season, at least one lunar eclipse will occur either two weeks before or after a solar eclipse. Remember, lunar eclipses happen when the moon is full – that is, it's at the opposite end of its orbit – and the Earth passes between it and the sun. Lunar eclipses can be partial or total, but never annular. The Earth is too large relative to the moon to fit inside the disk of the sun, as seen from the moon.
The Earth's umbra is 1.4 million km long, which is more than three times the distance between the Earth and the moon. If you were on the moon, you would see the Earth blocking the sun, but instead of being in total darkness, you would witness something very strange. You would see the Earth bathed in a ring of red light. This is sunlight being deflected by Earth's atmosphere. Higher-energy sunlight is completely deflected, but red light is able to penetrate the atmosphere and is refracted, much like light passing through a prism.
This refraction is the reason why people refer to a lunar eclipse as a blood moon. The refracted light illuminating the lunar surface turns the moon a ghostly red color. Because the Earth's disk is so much larger than that of the moon, the period of totality during a lunar eclipse can last for as long as 1 hour and 40 minutes. On either side of totality, the sun is partially occluded by the Earth for another hour or so. A lunar eclipse can last as long as six hours from the moment the Earth's disk begins to hide the moon to the moment it completely moves away.
Predicting Eclipses and the Saros Cycle
Conditions on Earth's surface may be unpredictable, but the movements of Earth and all the other planets are not. Scientists catalogue these movements, and if your area is due for a spectacular solar eclipse, you'll know about it years before the actual event.
Since Mesopotamian times, astronomers have recognized that eclipses occur in 18-year cycles (actually, 18 years, 11 days, 8 hours) called Saros cycles. At the end of one Saros, the sun assumes the same position with respect to the moon's nodes that it had at the beginning of the cycle, and a new Saros cycle begins. Eclipses in each Saros cycles follow the same pattern as in the one before, with small changes due to orbital perturbations and other factors.
The fact that solar eclipses don't occur on the same part of the Earth's surface at 18-year intervals is due to the Earth's rotation. When factoring that in, NASA astronomers have created a calendar of eclipses good until the year 3000.