Astronomy Principles and Practice Fourth Edition.pdf

Chapter 24
Practical projects
24.1 Introduction
The type of practical work that can be attempted obviously depends on the available equipment
(telescopes, laboratory apparatus, planetarium, etc) or on the access to an astronomical library from
which data can be obtained for exercises. It is, therefore, impossible to generalize and provide a set of
experiments which will be completely suitable for everybody.
This chapter collects ideas for projects of a practical or exercises nature which can be used directly
or adapted according to the resources of the student. Some of them have been or are used as parts of the
course at the Glasgow University Observatory and are described here in relation to particular designs
of equipment. Allowance for this must be made according to the equipment that might be to hand.
As the pieces of knowledge required for particular experiments may be scattered in several
chapters of the book, it is not possible to present a series of exercises following the textual layout
exactly. The scheme outlined here broadly follows the progress through the book.
The number of possible experiments and exercises that can be thought up is practically boundless.
Those described here represent only a small selection but provide sufficient material for any course
and are built from the chapters of this book. They have also been selected with the purpose of
offering material which is more of the ‘hands on’ nature rather than on numerical projects related
to astrophysics.
24.2 The Sun as a timekeeper
By using suitable geometries, it is fairly easy to convert the changing directions of shadows of fixed
objects to the apparent movement of the Sun across the sky and, hence, to the passage of time. Such a
specially designed device is a sundial.
Sundials abound in a variety of forms. Two of the most common types are described here together
with the ideas which are necessary to make and use them properly: these are the horizontal and
the vertical types. In addition, the use of a simple noon-marker is described. Before doing this,
it is important to have an appreciation of two terms which effect that position of the shadow and its
relationship to time. These are longitude and the equation of time.
(a) Effect of longitude. If two sundials are separated some miles apart in longitude, then the times
at which the shadows fall on the noon-markers will be different: the further west the sundial, the later is
the time for this event. Thus, if a time indicated by a sundial is to be converted to a civil time, allowance
must be made for the sundial’s longitude in relation to the reference longitude used to define the local
time zone. In Britain, particularly in the winter period, the situation is very simple. For sundials west
of the Greenwich meridian, a certain period of time according to the value of longitude must be added
403