Astronomy Principles and Practice Fourth Edition.pdf

The role of the observer 17
Earth and on micrometeoritic material which is scooped up by rocket probes in the upper atmosphere.
Some thirty years ago the Apollo and Lunakhod missions brought back our first samples of lunar
material for laboratory study. Interplanetary space probes have sent and still are sending back new data
from the experiments which they carry. They are able to transmit information about the planets that
could not have been gained in any other way. Astronomers have also gleaned information about the
planets by using radar beams. However, all these active experiments and observations are limited to
the inner parts of the Solar System, to distances from the Earth which are extremely small in relation
to distances between the stars.
When it comes to stellar work, the experiments, whether on board space vehicles or Earth
satellites, or at the bottom of the Earth’s atmosphere, are more passive. They involve the measurement
and analysis of radiation which happens to come from a particular direction at a particular time. It is
very true to say that practically the whole of the information and knowledge which has been built up of
the outside Universe has been obtained in this way, by the patient analysis of the energy which arrives
constantly from space.
As yet, the greater part of this knowledge has been built up by the observer using ground-based
telescopes though in recent years a wide variety of artificial satellite-based telescopes such as the
Hubble Space Telescope and Hipparcos have added greatly to our knowledge. The incoming radiation
is measured in terms of its direction of arrival, its intensity, its polarization and their changes with time
by appending analysing equipment to the radiation collector and recording the information by using
suitable devices. The eye no longer plays a primary role here. If the radiation has passed through the
Earth’s atmosphere, the measurements are likely to have reduced quality, in that they are subject to
distortions and may be more uncertain or exhibit an increase of noise. In most cases, however, these
effects can be allowed for, or compensated for, at least to some degree.
The task of the observer might be summarized as being one where the aim is to collect data
with maximum efficiency, over the widest spectral range, so that the greatest amount of information
is collected accurately in the shortest possible time, all performed with the highest possible signal-tonoise
ratio. Before the data can be assessed, allowances must be made for the effects of the radiation’s
passage through the Earth’s atmosphere and corrections must be applied because of the particular
position of the observer’s site and the individual properties of the observing equipment.
It may be noted here also that with the advent of computers, more and more observational work is
automated, taking the astronomer away from the ‘hands-on’ control of the telescope and the interface
of the data collection. This certainly takes away some of the physical demands made of the observer
who formally operated in the open air environment of the telescope dome sometimes in sub-zero
temperatures. Accruing data can also be assessed in real time so providing instant estimates as to
its quality and allowing informed decisions to be made as to how the measurements should proceed.
In several regards, the application of computers to the overall observational schemes have made the
data more objective—but some subtleties associated with operational subjectivity do remain, as every
computer technologist knows.
We cannot end this chapter without mentioning the role of the theoretical astronomers. Part of
their tasks is to take the data gathered by the observers and use them to enlarge and clarify our picture
of the Universe. Their deductions may lead to new observational programmes which will then support
their theories or cast doubt upon their validity.
Several comments may be made here.
It goes without saying that an astronomer may be both theoretician and observer, though many
workers tend to specialize in one field or the other. Again, it has been estimated that for each hour of
data collecting, many hours are spent reducing the observations, gleaning the last iota of information
from them and pondering their relevance in our efforts to understand the Universe. The development of
astronomical theories often involves long and complicated mathematics, in areas such as celestial mechanics
(the theory of orbits), stellar atmospheres and interiors and cosmology. Happily in recent years,
the use of the ubiquitous computer has aided tremendously the theoretician working in these fields.