43
atmosphere (airglow), the zodiacal light (from the interplanetary medium),
integrated starlight, light from our Galaxy, and extragalactic light (light from
objects beyond the Galaxy). The greatest difficulty in measuring each of these
components is separating them from one another.
According to the IAC archives, a total of 260 hours of observations were
carried out in 1973 with the Bordeaux Telescope and 720 hours with the
double telephotometer. There are thousands of entries of nocturnal sky brightness data. The section was active until 1982, and the researchers in the team
formed the embryo of various new IAC research groups. I must confess that I
am responsible for taking the decision to disband the team in order to continue to push for the formation of the Institute, a task to which I was now
dedicating all my waking hours. It was only in this way that I could avoid a
conflict of interests between heading my own research team and directing the
entire Institute. I had in effect ‘left research’. From that point on it became
clear to everybody that their Director now minded only collective interests.
Things were going swimmingly between Dumont and myself. We were
among the leading groups working on the interplanetary medium and we
completed our theses using data from the Bordeaux Telescope.
We made the fullest use of our advantages; namely, the quality of the Teide
sky, an innovative instrument, and a method of observing and data reduction
developed by Dumont himself. For that reason, we were able, among other
things, to carry out reliable measurements of the zodiacal light over the whole
sky, including the gegenschein (a small, extremely feeble luminous patch on
the sky diametrically opposite the sun, which moves in synchrony with it).
Our empirical model of the brightness and degree of polarization of the zodiacal light are still used today to separate the zodiacal light from other sky glows.
By inverting the integral of the measured brightness in different directions,
we were able to determine the scattering functions per unit volume in the
zodiacal cloud, and thence values for the density of dust particles, together
with the physical and chemical properties of the grains. Scattering functions
depend on the size and refractive index of the particles. We thus concluded
that the density of the dust cloud decreases in inverse proportion to its distance from the sun, and that the particles which are the main cause of the
zodiacal light are approximately micron-sized.
The permanence of the zodiacal light might lead one to think of an immobile and stable cloud of particles: nothing could be further from the truth. To
begin with, the measured Doppler shifts make it clear that the dust grains
describe orbits around the sun like all other bodies in the solar system. To
understand the renewal process, we need to take into account the forces to
which these micrometeoroids are subjected to bring about changes in their
5 The Zodiacal Light (1962–1982)
atmosphere (airglow), the zodiacal light (from the interplanetary medium),
integrated starlight, light from our Galaxy, and extragalactic light (light from
objects beyond the Galaxy). The greatest difficulty in measuring each of these
components is separating them from one another.
According to the IAC archives, a total of 260 hours of observations were
carried out in 1973 with the Bordeaux Telescope and 720 hours with the
double telephotometer. There are thousands of entries of nocturnal sky brightness data. The section was active until 1982, and the researchers in the team
formed the embryo of various new IAC research groups. I must confess that I
am responsible for taking the decision to disband the team in order to continue to push for the formation of the Institute, a task to which I was now
dedicating all my waking hours. It was only in this way that I could avoid a
conflict of interests between heading my own research team and directing the
entire Institute. I had in effect ‘left research’. From that point on it became
clear to everybody that their Director now minded only collective interests.
Things were going swimmingly between Dumont and myself. We were
among the leading groups working on the interplanetary medium and we
completed our theses using data from the Bordeaux Telescope.
We made the fullest use of our advantages; namely, the quality of the Teide
sky, an innovative instrument, and a method of observing and data reduction
developed by Dumont himself. For that reason, we were able, among other
things, to carry out reliable measurements of the zodiacal light over the whole
sky, including the gegenschein (a small, extremely feeble luminous patch on
the sky diametrically opposite the sun, which moves in synchrony with it).
Our empirical model of the brightness and degree of polarization of the zodiacal light are still used today to separate the zodiacal light from other sky glows.
By inverting the integral of the measured brightness in different directions,
we were able to determine the scattering functions per unit volume in the
zodiacal cloud, and thence values for the density of dust particles, together
with the physical and chemical properties of the grains. Scattering functions
depend on the size and refractive index of the particles. We thus concluded
that the density of the dust cloud decreases in inverse proportion to its distance from the sun, and that the particles which are the main cause of the
zodiacal light are approximately micron-sized.
The permanence of the zodiacal light might lead one to think of an immobile and stable cloud of particles: nothing could be further from the truth. To
begin with, the measured Doppler shifts make it clear that the dust grains
describe orbits around the sun like all other bodies in the solar system. To
understand the renewal process, we need to take into account the forces to
which these micrometeoroids are subjected to bring about changes in their
5 The Zodiacal Light (1962–1982)