15
If the light source is in motion toward the observer, the distance between waves
will be shortened compared to those emitted when the source was at rest; on the
other hand, if the source is moving away from the observer, the wavelength will be
greater (Fig. 1.8).
Secchi was well aware of this possibility. However, the tools available at the time
were not precise enough to measure the effect. Only toward the end of the nineteenth century would the first reliable measurements of radial velocities of celestial
bodies be obtained, thanks to photography becoming sensitive enough to be applied
to spectroscopy.
Precisely this kind of observations during the 1920s would bring Edwin Hubble
(1889–1953) and Georges Lemaître (1894–1966) to formulate the well-known law
of expansion of the universe (Hubble-Lemaître law). This was the first observational
indication of the need to move from the idea of a static universe to an evolving universe model.
An expanding universe fits very well with a model for the universe based on
general relativity. In 1915, Albert Einstein (1879–1955) introduced a new way of
describing gravitation by changing the common way of thinking about the relationship between matter, space, and time. The universe was no longer described as
immersed in a three-dimensional space, but in a continuous space–time of which
time was the fourth coordinate. This new theory, providing a link between the
geometry of space–time and energy, paved the way for the study of the universe as
a whole.
The transition from a static to a dynamic universe can be considered as a decisive
turning point in twentieth-century cosmology. Until the middle of the century, there
was still strong support for a stationary cosmological model. It was the discovery in
the early 1960s that the electromagnetic radiation in the region of microwaves is
spatially isotropic and of cosmic origin (the Cosmic Microwave Background) that
provided unequivocal evidence of the existence of an era in which the whole universe had gone through a phase where its density and temperature were very high.
These are the experimental data, together with the observation of the receding of
distant galaxies (Hubble- Lemaître law) and of the abundance of light elements in
the universe (primordial nucleosynthesis), that form the pillars of the Big Bang
theory (Fig. 1.9, for a schematic view of the main stages of the evolution of the
universe).
1.5 Solar Physics and the Sun–Earth Connection
In the second half of the nineteenth century, the study of the physical and chemical
principles of the Sun represented one of the main efforts in the nascent field of astrophysics (Chinnici 2000). Secchi made important contributions toward understanding the phenomena that occur in the solar photosphere and chromosphere, such as
spots, flares, and prominences. His observations were obtained both through traditional visual methodology and through photography, a technique that was just
1 An Introduction to Angelo Secchi and his Collegio Romano Observatory
If the light source is in motion toward the observer, the distance between waves
will be shortened compared to those emitted when the source was at rest; on the
other hand, if the source is moving away from the observer, the wavelength will be
greater (Fig. 1.8).
Secchi was well aware of this possibility. However, the tools available at the time
were not precise enough to measure the effect. Only toward the end of the nineteenth century would the first reliable measurements of radial velocities of celestial
bodies be obtained, thanks to photography becoming sensitive enough to be applied
to spectroscopy.
Precisely this kind of observations during the 1920s would bring Edwin Hubble
(1889–1953) and Georges Lemaître (1894–1966) to formulate the well-known law
of expansion of the universe (Hubble-Lemaître law). This was the first observational
indication of the need to move from the idea of a static universe to an evolving universe model.
An expanding universe fits very well with a model for the universe based on
general relativity. In 1915, Albert Einstein (1879–1955) introduced a new way of
describing gravitation by changing the common way of thinking about the relationship between matter, space, and time. The universe was no longer described as
immersed in a three-dimensional space, but in a continuous space–time of which
time was the fourth coordinate. This new theory, providing a link between the
geometry of space–time and energy, paved the way for the study of the universe as
a whole.
The transition from a static to a dynamic universe can be considered as a decisive
turning point in twentieth-century cosmology. Until the middle of the century, there
was still strong support for a stationary cosmological model. It was the discovery in
the early 1960s that the electromagnetic radiation in the region of microwaves is
spatially isotropic and of cosmic origin (the Cosmic Microwave Background) that
provided unequivocal evidence of the existence of an era in which the whole universe had gone through a phase where its density and temperature were very high.
These are the experimental data, together with the observation of the receding of
distant galaxies (Hubble- Lemaître law) and of the abundance of light elements in
the universe (primordial nucleosynthesis), that form the pillars of the Big Bang
theory (Fig. 1.9, for a schematic view of the main stages of the evolution of the
universe).
1.5 Solar Physics and the Sun–Earth Connection
In the second half of the nineteenth century, the study of the physical and chemical
principles of the Sun represented one of the main efforts in the nascent field of astrophysics (Chinnici 2000). Secchi made important contributions toward understanding the phenomena that occur in the solar photosphere and chromosphere, such as
spots, flares, and prominences. His observations were obtained both through traditional visual methodology and through photography, a technique that was just
1 An Introduction to Angelo Secchi and his Collegio Romano Observatory