The Earth and the Moon throw long conical shadows into space and when one of these bodies moves into the shadow cast by the other there is an ECLIPSE. The shadow has two parts: the total shadow, called the UMBRA, and the partial shadow, termed the PENUMBRA. The Sun is totally obscured to an observer in the umbra but in the penumbra it is partially visible.

The umbra of the Earth’s shadow extends into space for 1382000km on average, but because of the ellipticity of the Earth’s orbit this distance varies by 22500km between its greatest and least values. Since the length of the umbra is more than three times the distance of the Moon from the Earth it must strike the Moon if it is pointing in the proper direction: i.e. if the Moon, Earth and Sun are in line .There is then a total eclipse of the Moon. Such an eclipse starts when the Moon enters the penumbra. The darkening that results is not very-conspicuous and may not be noticed by someone who is not fore¬warned. The more conspicuous partial phase starts when the Moon enters the umbra and the eclipse is total when all of the Moon is in the umbra. When the total phase ends, there are second partial and penumbral phases before the eclipse is finally over. Although those parts of the Moon in the umbra are not illuminated directly by the Sun they are not completely dark. The Sun’s rays which pass through the Earth’s atmosphere without striking the Earth’s surface are refracted and scattered slightly so that some light enters the umbra and illuminates the Moon during the eclipse. Since red light is refracted less than blue, the Moon is given a coppery-red hue . The exact colour and intensity of this light depends on the conditions in those parts of the atmosphere where the light is refracted and astronomers speak of dark and bright eclipses. Dust in the atmosphere can also have an effect. The phenomenal volcanic eruption at Krakatoa in August 1883 sent a great amount of dust into the atmosphere and caused some subsequent eclipses of the Moon to be very dark indeed.

There can also be partial eclipses when the Moon enters the umbra but is not completely immersed, and penumbral eclipses when the Moon only enters the penumbra. As is the case with the penumbral phases of partial and total eclipses, a penumbral eclipse is often not noticed by people who do not know that it is taking place.

An eclipse of the Moon can only occur at Full Moon. However, there is not an eclipse every month for the following reason: because the Moon’s orbit is inclined to the ecliptic, the Moon often passes to the north or south of the Earth’s shadow. Whether there is an eclipse depends on how far the Sun and Moon are from a node of the Moon’s orbit. If the difference is greater than 12° 15′ a total eclipse of the Moon cannot occur; if it is less than 9° 30′ an eclipse must occur. For positions between these limits, eclipses may or may not occur according to other circumstances. When an eclipse does occur, its duration depends upon the exact circumstances, but the partial and total phases can last at most 3hr 40min of which totality is at most 1 hr 40 min. The eclipse will be visible (weather conditions permitting) from the hemisphere of the Earth for which the Moon is above the horizon.

Since the Moon is smaller than the Earth, the umbra of its shadow extends into space for only 375000km on average. This distance is less than 384390km, its average distance from the Earth. Under these average conditions, the Moon’s umbra cannot reach the surface of the Earth when the Moon is between the Earth and Sun. However, the umbra is longer than average when the Earth and Moon are further than average from the Sun; the longest possible shadow occurs when the Earth is at aphelion. Similarly the Moon can be nearer to the Earth than average. Under the most favourable circumstances the Moon’s umbra can extend 29300km beyond the Earth’s surface. A TOTAL ECLIPSE occurs in this case and the Sun is totally obscured . The greatest possible diameter of the umbra at the Earth’s surface is 269km but if the shadow falls obliquely onto the Earth the projected shadow is approximately an ellipse with a minor axis of 269km and a larger major axis. As the Moon moves round its orbit its shadow moves across the Earth to give a zone of totality.

More common than total eclipses are ANNULAR ECLIPSES which occur when the umbra fails to reach the Earth’s surface. At a total eclipse the Moon appears slightly larger than the Sun and is able to cover it completely; at an annular eclipse the Moon appears slightly smaller than the Sun and the rim of the Sun is visible all round the Moon. The fact that the Sun and the Moon have almost the same angular diameter is a pure coincidence. Sometimes an eclipse can be total at some places on the Earth and annular in other places. At both annular and total eclipses the penumbra of the Moon’s shadow falls on part of the Earth’s surface. In these regions the Sun is only partially obscured and a partial eclipse is visible about 3000 km on each side of the eclipse path.

The Moon’s shadow moves rapidly across the surface of the Earth . The slowest possible speed is 1680km/hr and this can only occur at the equator, where a total solar eclipse can last at most 7min 40 sec. Elsewhere the maximum duration is shorter; for example at latitude 45° it is 6min 30sec. Annular eclipses can be longer; at the equator the maximum is 12 mm 24sec.

A total eclipse of the Sun starts with the partial phase when the Moon covers part of the Sun. As totality approaches, the visible part of the Sun becomes a narrow crescent. Immediately before totality this crescent is broken up by the irregularities of the mountains at the edge of the Moon’s visible disc to form what are known as BAILY’S BEADS. The last visible part of the Sun can give the appearance of a DIAMOND RING. During totality the Sun’s atmosphere, the corona becomes visible (figure 8.26). Since the corona is much less bright than the Sun’s disc, it is not normally visible against the glare of the Sun, but at totality this glare is removed and the corona emerges in splendour. The resultant spectacular view of the corona is only possible because of the co¬incidence that the apparent diameters of the Sun and Moon are very nearly equal. After totality the partial phases are repeated in reverse order.

If you want to view an eclipse please take great care. The Sun is very bright object and the dangers of looking at it directly cannot be overstressed. For naked-eye observations, a very dark filter is required to prevent eye damage. To view the Sun directly through a telescope can result in blindness. The only satisfactorily safe method for the amateur to view the Sun with a telescope is to project the image onto a screen.

Eclipses of the Sun can only occur at New Moon but, as with eclipses of the Moon, there is not an eclipse every month. The Moon’s shadow may miss the Earth completely and whether there is an eclipse or not depends on how far the Sun and Moon are from a node of the Moon’s orbit. If the difference is more than 18° 31′, a solar eclipse cannot occur and if it is less than 15° 31′, an eclipse must occur.

Because of the restrictions on the Moon’s position in its orbit at an eclipse there can be at most seven eclipses during a year, of which either four or five are eclipses of the Sun. There must always be at least two eclipses (both of the Sun). Solar eclipses are more common than lunar eclipses but, at a given location, lunar eclipses are more frequent because such an eclipse can be seen from a much larger part of the Earth’s surface. On the average there is a total eclipse of the Sun visible somewhere on the Earth once every 18 months but because of the narrowness of the band on the Earth s surface inside which an eclipse is total, a total eclipse occurs at any one place only once every 360 years on average.

It has been known for several thousand years that eclipses recur at an interval of 18 years 11 days 8 hours – the so-called Saros cycle. This period is almost equal to 223 synodic months, or 242 nodal months or 19 eclipse years. An ECLIPSE YEAR is the interval at which the Sun passes the ascending node of the Moon’s orbit; it is 346.6200 days long. At an eclipse the Moon is near one of the nodes of its orbit and is at either the full or new phase; consequently the Sun must also be in the direction of one of the nodes. After the passage of a Saros cycle this situation is repeated and there is another eclipse of the same type. Because the periods are not exactly equal, a sequence of eclipses ends after a long period. Similarly, new sequences start from time to time. Since the Saros cycle is not an exact number of days in length, the eclipses in a sequence are not all at the same place on the Earth’s surface. The odd eight hours in the length of the cycle causes each eclipse to be 120° to the west of the previous eclipse. Three such displacements add up to 360° so that the eclipse position repeats itself almost exactly after three Saros cycles, that is after 54 years and 34 days.

Lists of lunar and solar eclipses from AD 1977 to 1985

Comments are closed.