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Eclipses

We generally talk of eclipses of the Sun and Moon but other bodies inside and outside the Solar System exhibit eclipses and are very important in astronomy. Eclipses of the moons of Jupiter were used in one of the first measures of the speed of light and eclipsing binary stars give us fundamental data on the masses of stars.

An eclipse occurs when a body cuts off the light from a light source so that we can no longer see it shining. An eclipse can be due either to a dark body coming between us and a light emitter, so that we can no longer see the source, or it can be a body coming between a light source and the body that the light is illuminating, so that we no longer see the illuminated body.

An eclipse of the Sun occurs when the Moon comes directly between the Sun and the Earth so that the Earth lies in the shadow of the Moon. An eclipse of the Moon occurs when the Earth lies directly between the Sun and the Moon and the Moon lies in the shadow of the Earth.

If the orbit of the Moon about the Earth lay in the same plane as the orbit of the Earth about the Sun then there would be eclipses of the Sun and Moon at every New and Full Moon respectively. The orbits are inclined, however, and eclipses can only occur when the Moon is close to the nodes of its orbit (when it is near to the places where the orbital planes cross).


Lunar eclipses

The amount of the Moon's disk that is eclipsed depends on how close the Moon is to the node of its orbit at Full Moon. Like all shadows of light from an extended source the shadow produced by the Earth has an umbra, where all the light from the Sun is shadowed, and a penumbra, where only some of it is. Penumbral eclipses of the Moon occur when the Moon passes only through the Earth's penumbral shadow. Although these are catalogued they are inconspicuous events and are not noteworthy.

When the Moon passes through the Earth's umbral shadow we can either see a Partial Eclipse, when only part of the Moon is obscured, or a Total Eclipse. The Earth's shadow is much larger than the Moon and so eclipses can last up to 3 hrs 40 mins, with totality lasting up to 1 hr 40 mins. They can be seen from anywhere on the side of the Earth which faces the Moon. During a Total Eclipse, the Moon does not, as might be expected, disappear entirely but turns a deep, dark red. The brightness and colour depend on the state of the Earth's atmosphere for the Moon, during eclipse, is illuminated by light that has passed through the Earth's atmosphere and has been bent towards the Moon by refraction.


Solar eclipses

These, like Lunar Eclipses, can only occur when the Moon is near the nodes of its orbit but in this case at New Moon. The shadow of the Moon can then pass over the surface of the Earth. Because the Moon is much smaller than the Earth its shadow only covers a small part of the Earth's surface and a solar eclipse can only be seen from a restricted area. Like the Earth's shadow, the Moon's has an umbra and a penumbra. Viewed from the Earth a person in the umbra sees the whole of the Sun eclipsed while someone in the penumbra sees only part of the Sun obscured. These are called a Total and a Partial Eclipse respectively.

Quite by chance the apparent sizes if the Sun and Moon are very nearly the same. The apparent angular size of the Sun does not change very much due to the Earth's non-circular orbit but the Moon's apparent size varies quite a lot. For most solar eclipses the Moon's apparent diameter is less than the Sun's and so the whole solar disk is nowhere totally obscured. It is only when the Moon is close to the Earth that, at some places, the whole disk is obscured and a Total Eclipse is seen. The track of the small area on the Earth's surface where a total eclipse can be seen is several thousand miles long but only up to 160 miles wide. Outside this track and outside the short time of totality, maximum about 7 minutes, a Partial Eclipse is seen.

When the Moon is not at its closest to the Earth its apparent diameter is less than that of the Sun and even where the Moon's disk obscures the Sun centrally the outer ring of the Sun's disk is still visible. This is called an Annular Eclipse.

Total eclipses of the Sun are much more spectacular than Partial eclipses as virtually all the light from the Sun is blocked out by the Moon and it becomes as dark as night and stars can be seen. The solar chromosphere and corona can be seen. The former as a reddish rim around the eclipsing Moon and the latter as a whitish glow surrounding the eclipsed Sun.

The length of totality depends on how close the Moon is to the Earth. The 1991 total eclipse was the longest for 140 years. The last total solar eclipse to be visible in Britain was in August 1999. It was only visible from parts of Cornwall and Devon.

Warning - do not look directly at the Sun


The Metonic Cycle and the Saros

The Greek astronomer Metos, in the fifth century BC, discovered that the dates of the phases of the Moon repeated exactly after a period of 19 years. Mathematically, it uses the fact that 19 tropical years contain 6939.60 days while 235 synodic months contain 6939.69 days.

Since it is almost equal to 20 eclipse years, 6932.4 days, it is possible for a series of four or five eclipses to occur on the same dates 19 years apart. The metonic cycle was used to determine how intercalary months could be inserted into a lunar calendar so that the calendar year and the tropical (seasonal) year were kept in step.


The Saros cycle
Edmond Halley, whose name is associated by most people with the comet carrying his name, was interested in classical writings, especially those concerning astronomy. He mistakenly connected the naming of a cycle of 223 synodic months by the tenth century Greek lexicographer Suidas with the eclipse cycle of the same period. The name given to the cycle by Suidas was the Saros.

This cycle was almost certainly known to the ancient Babylonians and was possibly used by Thales around 585 BC.

Eclipses of the Sun and Moon can only occur at New or Full Moon respectively and these have to occur close to the nodes of the Moon's orbit. The nodes are the places in the orbit where the plane of the Moon's orbit and the ecliptic cross. The time between successive passages by the Moon through one of its nodes is called the Draconic month and equals 27.212220 days. The time between successive New or Full Moons is called the Synodic month and equals 29.530589 days.

If we take 223 synodic months (6585.321 days) and compare them with 242 draconic months (6585.357 days) we can see that they are almost the same. This period is the Saros and it amounts to 18 years, 10 and a third days.

This means that eclipses can be expected in families whose members are separated by the length of the Saros. Thus knowing the date of one eclipse allows the prediction of others.

It also happens that the Saros is also nearly equal to 239 anomalistic months (the time between successive closest approaches of the Moon to the Earth) and so the length of the eclipses in each cycle will be approximately the same.


Glossary

Synodic month: the interval between two successive New Moons. 
Draconic month: the interval between two successive passages of the Moon through the same node of its orbit. 
Anomalistic month: the time between successive perigee passages of the Moon. 
Eclipse year: The period between two successive passages of the Sun through the same node of the Moon's orbit. 346.620 days. There are very close to 19 eclipse years in one Saros.


Other eclipses

The eclipse of an apparently small object by one which appears much larger is generally called an occultation. Thus the Moon occults many stars as it moves across the sky. Observations of occultations by the Moon were used for a long time to get the most accurate positions for the Moon and have been used to determine the position and size of such strange objects as radio stars.

Eclipses of the satellites of Jupiter by the planet, and also by one another, were used in one of the first determinations of the speed of light. And occultations of stars by the planets have allowed analyses of the planetary atmospheres.

Eclipsing binary stars, in which two stars are in orbit about each other and each passes in front of the other as seen from the Earth, have given us most of our knowledge of the masses of different kinds of stars.


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