Astronomy Principles and Practice Fourth Edition.pdf

Simple observations 35
Figure 6.1. The hand as a means of estimating angles.
the IAU convention, the names of constellations are usually referred to by a standardized three-letter
abbreviation.
6.2 Simple observations
There are many aids to help provide information about what is available for view in the night sky at any
particular time. Many PC software packages W6.1,W 6.2 offer active demonstrations of the behaviour of
the night sky according to the observer’s location and the local time. These can be very informative
as motions which, in reality, may take some months to execute can be simulated on the screen and
speeded up to take just a few seconds. Thus, for example, the apparent motions of the Moon and the
complex planetary paths may be readily appreciated. It is a relatively easy matter to learn which stars
are in the sky at a particular time and where the planets are relative to the stellar background. If a
planetarium is available, constellation identification can be learned very quickly, especially if a pattern
projector is attached for highlighting each constellation.
Familiarity with the night sky, however, is best gained by spending a few hours on different nights
observing the ‘real’ panorama. The true feeling of being under a hemispherical rotating dome with
stars attached can only be obtained by outdoor activity. Appreciation of the angular scale associated
with the well-known asterisms and identification of the constellation patterns is best gained by direct
experience. On the early occasions, it is useful to be armed with a star atlas (such as Norton’s Star
Atlas) or a simple planisphere. This latter device is a hand-held rotatable star map with a masking
visor, allowing the correct part of the sky to be seen according to the season and the local time.
In the previous chapters, reference has already been made to angular measure and, in the first
place, it is useful to have an appreciation of angular scales as projected on the night sky. For example,
the angular sizes of the Sun and Moon are approximately the same being ∼ 1 2 ◦ . If either the Sun or full
Moon is seen close to the horizon it is readily appreciated that their apparent diameters would need to
be extended 720 times to ‘fill’ the 360 ◦ of the full sweep around the horizon. Rough estimates of larger
angles between stars, so providing the impression of just how large an area a particular constellation
covers, can be made by using the ‘rule of thumb’ technique as practised by artists.
If the arm is fully extended, different parts of the hand can be used to provide some simple
angular values. Typical values are indicated in figure 6.1 but a system and scale should be developed
individually by comparing observations with a star map. In the first place it will be noted that the
angular extent of the thumb at arm’s length is about 1 ◦ . This means that if the Moon is in the sky it
should be easily blocked from view by the use of the thumb. Figure 6.1 indicates that the knuckle-span