Astronomy Principles and Practice Fourth Edition.pdf

Chapter 4
The nature of the observables
4.1 Introduction
Energy is arriving from space in the form of microscopic bodies, atomic particles and electromagnetic
radiation. A great part of this energy is, however, absorbed by the Earth’s atmosphere and cannot
be observed directly by ground-based observers. In some cases, the absorption processes give rise
to re-emission of the energy in a different form. Macroscopic bodies have kinetic energy which is
converted into heat; atomic particles interact with the gases in the higher atmosphere and liberate their
energy in the form of light, giving rise to such phenomena as the aurorae. Electromagnetic energy
of particular frequencies, say in the ultraviolet or x-ray region, is absorbed and re-emitted at other
frequencies in the visible region. Thus, besides gaining knowledge about the sources which give rise to
the original energy, observation of the re-emitted energy leads to a better understanding of the nature of
our atmosphere. However, by far the greater part of our knowledge of astronomical objects is based on
the observation of electromagnetic energy which is collected by satellite instrumentation or transmitted
directly through the atmosphere and collected by telescope.
4.2 Macroscopic bodies
As the macroscopic bodies penetrate the Earth’s atmosphere, the air resists their motion and part of their
energy is lost in the form of heat. The heat generated causes the ablated material and the atmospheric
path to become ionized and, when the atoms recombine, light is emitted and the rapid progress of the
body through the upper atmosphere is seen as a flash of light along a line in the sky. The flash might
last for a few seconds. The event is known as a meteor (popularly known as a shooting star). The rate
of burning of the meteor is not constant and fluctuations in brightness may be seen on its trail, usually
with a brightening towards the end of the path. Positional measurements can be made of the meteor and
the event can be timed. Simultaneous observations of a meteor at different sites allow determination of
its trajectory within the Earth’s atmosphere.
On occasions, many meteors can be observed during a relatively short period of time and, by
observing their apparent paths across the sky, it is noted that there is a point from which the shower
of meteors seems to originate. This point in the sky is known as the radiant of the shower. Meteor
showers are often annual events and can be seen in the same part of the sky at the same time of the year,
although the numbers counted vary widely from year to year. The regular appearances of showers result
from the crossing of the Earth’s orbit of a fairly tight band of orbits followed by a swarm of meteoritic
material.
Meteors can also be detected during the day by radar. As a meteor passes through the upper
atmosphere, as has already been mentioned, some of the gases there are ionized. The ionized trail
which persists for a short time acts as a good reflector for a radar beam and the effect of any daytime
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