66
5 Nearly Half a Million Stars
5.1 Tree Rings and Mathematical Series
In his early years, Kapteyn researched systematic long-term patterns in astronomy and in the weather. Thanks to his teacher Buys Ballot, he was probably
quite familiar with meteorological issues. Long-term weather was barely studied and long series of observations were not or hardly available. The weather
forecast was in its infancy. Kapteyn’s interest focused on whether there were
any changes to be found that would recur periodically on timescales of a few
years. That would bring the understanding of long-term weather patterns, and
the facts that played a role in it, within reach. The underlying question was
whether, as with solar and lunar eclipses, there was a regularity or pattern in
the occurrence of weather.
Eclipses are based on the systematics of the orbits of the Earth and the
Moon. The orbit of the Moon in the sky is not the same as the ecliptic (that
of the Sun as seen from Earth in the course of a year). This is because the
orbit of the Moon makes an angle (of over five degrees) with the orbit of the
Earth around the Sun. On the sky, the corresponding orbits of the Sun and
Moon cross each other in two points called nodes. The Moon returns to the
same node with a period of 27.21 days (this is called a draconitic month).
Because of the changing relative position of the Moon with respect to the Sun,
it assumes phases from New Moon, via First Quarter to Full Moon—when
they are opposite on the sky—and Last Quarter. In this too there is a welldefined period, which is 29.53 days (the so-called synodical month). A solar
eclipse occurs when the Moon passes a node during New Moon, and a lunar
eclipse when it does so during Full Moon. The Earth, Moon and Sun then
are on one line in space. Now 223 synodical months is almost exactly equal
to 242 draconitic months, and therefore solar and lunar eclipses occur with
a regular cyclical pattern, which will be maintained for centuries. It fails in
the end because of the ‘almost exactly’ in the last sentence. The corresponding
period is 6585 days and 8 h. 1 This is called the Saros Period, which was already
known in ancient times. Because of that 8 h (one third of a 24 h day) the eclipse
will occur at the same place on Earth after three Saros periods.
The idea of Kapteyn now was that there might be such patterns to be found in
the weather as well, which could then be discovered for example in the thickness
of year rings in tree trunks. To investigate this, he collected slices of trees from
various regions, such as the Trier area in Germany. However, he found the
results too uncertain to publish. It was not until much later, in December 1908,
that he gave a lecture on the subject in Pasadena, California, and published
an article in the local newspaper, the Pasadena Star. Some time later, a slightly
1 A period of 6585 days is equal to either 14 ordinary and 4 leap years plus 11 days, or 13 ordinary and 5
leap years plus 10 days.
5 Nearly Half a Million Stars
5.1 Tree Rings and Mathematical Series
In his early years, Kapteyn researched systematic long-term patterns in astronomy and in the weather. Thanks to his teacher Buys Ballot, he was probably
quite familiar with meteorological issues. Long-term weather was barely studied and long series of observations were not or hardly available. The weather
forecast was in its infancy. Kapteyn’s interest focused on whether there were
any changes to be found that would recur periodically on timescales of a few
years. That would bring the understanding of long-term weather patterns, and
the facts that played a role in it, within reach. The underlying question was
whether, as with solar and lunar eclipses, there was a regularity or pattern in
the occurrence of weather.
Eclipses are based on the systematics of the orbits of the Earth and the
Moon. The orbit of the Moon in the sky is not the same as the ecliptic (that
of the Sun as seen from Earth in the course of a year). This is because the
orbit of the Moon makes an angle (of over five degrees) with the orbit of the
Earth around the Sun. On the sky, the corresponding orbits of the Sun and
Moon cross each other in two points called nodes. The Moon returns to the
same node with a period of 27.21 days (this is called a draconitic month).
Because of the changing relative position of the Moon with respect to the Sun,
it assumes phases from New Moon, via First Quarter to Full Moon—when
they are opposite on the sky—and Last Quarter. In this too there is a welldefined period, which is 29.53 days (the so-called synodical month). A solar
eclipse occurs when the Moon passes a node during New Moon, and a lunar
eclipse when it does so during Full Moon. The Earth, Moon and Sun then
are on one line in space. Now 223 synodical months is almost exactly equal
to 242 draconitic months, and therefore solar and lunar eclipses occur with
a regular cyclical pattern, which will be maintained for centuries. It fails in
the end because of the ‘almost exactly’ in the last sentence. The corresponding
period is 6585 days and 8 h. 1 This is called the Saros Period, which was already
known in ancient times. Because of that 8 h (one third of a 24 h day) the eclipse
will occur at the same place on Earth after three Saros periods.
The idea of Kapteyn now was that there might be such patterns to be found in
the weather as well, which could then be discovered for example in the thickness
of year rings in tree trunks. To investigate this, he collected slices of trees from
various regions, such as the Trier area in Germany. However, he found the
results too uncertain to publish. It was not until much later, in December 1908,
that he gave a lecture on the subject in Pasadena, California, and published
an article in the local newspaper, the Pasadena Star. Some time later, a slightly
1 A period of 6585 days is equal to either 14 ordinary and 4 leap years plus 11 days, or 13 ordinary and 5
leap years plus 10 days.