Why are they so infrequent, and why do solar eclipses always come in pairs with lunar eclipses? It is all down to a mix of orbital alignments and cosmic coincidences. With all that sky above our heads though, what causes the Sun and the Moon to be in the same position in the sky at the same time?
Earth orbits the Sun along a plane that we call the ecliptic — all of the planets orbit along this plane, more or less. The Moon, however, orbits Earth at an angle that is tilted to the ecliptic, by about five degrees. This means that on each orbit around Earth a lunar orbit lasts 27 days , the Moon only crosses the ecliptic at two locations and these are the only two opportunities for an eclipse, be it a solar or a lunar eclipse. The Moon's variation is more pronounced.
Our planet is closest to the Sun perihelion in early January and farthest aphelion in early July. When near perigee, the Moon can easily cover the entire solar disk and create a total solar eclipse. But at apogee the Moon is too small to cover all of the Sun's brilliant face.
At mideclipse an annulus ring of sunlight surrounds the lunar silhouette, resulting in an annular eclipse. This path is called the ecliptic, for reasons that will become obvious in a moment. More often than not, the new Moon passes above or below the Sun, and the lunar shadow misses Earth completely. But every On the other side of this swarm of hundreds of thousands of minor planets we encounter the four gas giants.
Jupiter has three moons that are larger than our own, namely Io, Ganymede and Callisto; Europa is only slightly smaller, but they are dwarfed by the , km diameter parent planet. An image of Saturn itself, eclipsing the Sun with ease, is shown in Fig. It is really astonishing that the Earth has such a relatively large satellite Table 1. The sun's ,mile diameter is fully times greater than that of our puny moon, which measures just about 2, miles.
But the moon also happens to be about times closer to Earth than the sun the ratio varies as both orbits are elliptical , and as a result, when the orbital planes intersect and the distances align favorably, the new moon can appear to completely blot out the disk of the sun. On the average a total eclipse occurs somewhere on Earth about every 18 months. There are actually two types of shadows: the umbra is that part of the shadow where all sunlight is blocked out.
The umbra takes the shape of a dark, slender cone. It is surrounded by the penumbra , a lighter, funnel-shaped shadow from which sunlight is partially obscured.
During a total solar eclipse, the moon casts its umbra upon Earth's surface; that shadow can sweep a third of the way around the planet in just a few hours.
Those who are fortunate enough to be positioned in the direct path of the umbra will see the sun's disk diminish into a crescent as the moon's dark shadow rushes toward them across the landscape. During the brief period of totality, when the sun is completely covered, the beautiful corona — the tenuous outer atmosphere of the sun — is revealed.
Totality may last as long as 7 minutes 31 seconds, though most total eclipses are usually much shorter. A partial solar eclipse occurs when only the penumbra the partial shadow passes over you. In these cases, a part of the sun always remains in view during the eclipse. How much of the sun remains in view depends on the specific circumstances. Usually the penumbra gives just a glancing blow to our planet over the polar regions; in such cases, places far away from the poles but still within the zone of the penumbra might not see much more than a small scallop of the sun hidden by the moon.
In a different scenario, those who are positioned within a couple of thousand miles of the path of a total eclipse will see a partial eclipse. The closer you are to the path of totality, the greater the solar obscuration. If, for instance, you are positioned just outside of the path of the total eclipse, you will see the sun wane to a narrow crescent, then thicken up again as the shadow passes by. An annular eclipse, though a rare and amazing sight, is far different from a total one.
The sky will darken The annular eclipse is a subspecies of a partial eclipse, not total. The maximum duration for an annular eclipse is 12 minutes 30 seconds. However, an annular solar eclipse is similar to a total eclipse in that the moon appears to pass centrally across the sun. The difference is, the moon is too small to cover the disk of the sun completely.
Because the moon circles Earth in an elliptical orbit, its distance from Earth can vary from , miles to , miles. But the dark shadow cone of the moon's umbra can extend out for no longer than , miles; that's less than the moon's average distance from Earth. So if the moon is at some greater distance, the tip of the umbra does not reach Earth. During such an eclipse, the antumbra , a theoretical continuation of the umbra, reaches the ground, and anyone situated within it can look up past either side of the umbra and see an annulus, or "ring of fire" around the moon.
A good analogy is putting a penny atop a nickel, the penny being the moon, the nickel being the sun. These are also called annular-total "A-T" eclipses. This special type of eclipse occurs when the moon's distance is near its limit for the umbra to reach Earth. In most cases, an A-T eclipse starts as an annular eclipse because the tip of the umbra falls just short of making contact with Earth; then it becomes total, because the roundness of the planet reaches up and intercepts the shadow tip near the middle of the path, then finally it returns to annular toward the end of the path.
Because the moon appears to pass directly in front of the sun, total, annular and hybrid eclipses are also called "central" eclipses to distinguish them from eclipses that are merely partial.
Eclipses do not happen at every new moon, of course. This is because the moon's orbit is tilted just over 5 degrees relative to Earth's orbit around the sun.
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