The Distribution Of Eclipses

There are about 66 total solar eclipses per century, but many have ground tracks unfavorable for potential viewers. These may be located either at very high latitudes (over the Arctic or Antarctic), or over regions in which the weather is likely to be poor, such as the tropics during the monsoon, or completely over the ocean. In practice, a total solar eclipse track traversing accessible places with a good chance of clear weather occurs about once every three years. Nevertheless there is many a keen eclipse watcher who has spent an enormous amount of time and money getting to a well-considered prime spot, only to be stymied by an unseasonably cloudy day.

The number of eclipses per century given above is an average, which would result if they occurred randomly in time. The reality, though, as we have seen, is eclipses are not random at all; they repeat on regular cycles. The tracks of total solar eclipses within specific saros sequences advance consistently by steps across the Earth, as in Figure 2-2, and there are other systematic trends.

Eclipses do not occur randomly in terms of geography either. If they happened entirely by chance then any particular location would get a total solar eclipse about once every 410 years. In fact more of these events occur during the summer than the winter in the Northern Hemisphere, because the Earth passes aphelion in July, when the Sun has its smallest apparent size. This makes it more likely that the Moon will be large enough to cover it completely. The Northern Hemisphere is tilted towards the Sun in the summer, meaning that there is a greater probability that it will get an eclipse, and so places north of the equator are visited about once per 330 years. Being the summer there's also a greater chance of clear skies. In contrast Southern Hemisphere locations receive total solar eclipses about once per 540 years. As the bulk of the population lives in the north, this quirk of nature increases the likelihood that a person picked at random from the whole of humankind will experience a total solar eclipse without needing to chase after one.

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