Many older maps still use epoch 1950 coordinates. To convert right ascensions and declinations from 1950 to 2000, find the entry in Table B.1 nearest the object's position in the sky and add the corrections indicated there.

To convert 2000 coordinates to 1900 for AAVSO variable-star designations and the like, double the tabulated correction and apply it in the opposite direction (subtracting instead of adding and vice versa).

To calculate precession, let α and δ stand for the object's initial right ascension and declination respectively, both expressed in degrees. (Recall that 1h = 15°.) Let N stand for the number of years between epochs (negative if you are converting from a later epoch to an earlier one). Then compute:

R.A. correction (in seconds) = (3.073 + 1.336 sin α tan δ) × N

Declination correction (in arc-seconds) = (20.042 cos α) × N

and add the corrections to the original R.A. and declination.

These formulae are for dates within a couple of centuries of 2000 and positions at least 0.1° away from the celestial poles. For more accurate formulae see the Astronomical Almanac published by the U.S. and British governments.

The Moon

Phases of the Moon

We often make jokes about activities that depend on the phases of the Moon, but amateur astronomy really does. When the Moon is high in the sky, especially if it is full or nearly full, you can't see faint stars, nebulae, or galaxies. Conversely, if you want to observe the Moon you need to know when and where it is going to appear.

Accordingly, all astronomical observers need to keep track of the phases of the moon. Figure 3.1 summarizes the whole cycle. The Casio “Forester” wristwatch, marketed to hikers and fishermen, keeps track of the cycle for you.

The Moon stays close to the ecliptic, though not precisely on it, and moves eastward, making a full circle relative to the stars every 27.32 days (one sidereal month). This means that it moves its own apparent width (half a degree) in slightly less than an hour. You can watch this happen when the Moon passes near a bright star.

The cycle of phases, or synodic month, takes 29.53 days. This is longer than the sidereal month because the Sun and Moon are moving in the same direction; the Moon has to move more than a full circle in order to catch up.

Introduction

This chapter describes the original Meade LX200 (Figure 10.1), which was made from 1992 to 2001. The newer LX200 GPS is optically and mechanically similar but has an enhanced version of the Autostar computer described in Chapter 12.

The information here is based on my experiences with an 8-inch LX200 purchased in 2000. I assume that you also have the Meade manual available for reference. This is not a complete guide to all the LX200's features.

This chapter is more detailed than the next two, for several reasons. The original LX200 was on the market for nine years, so there was plenty of time for the amateur community to learn all about it. All LX200s use similar firmware, and the total number of LX200s in use is very large, so this detailed information is useful to a large number of people. Finally, the original LX200 is at the end of its product life cycle, so there will be no further changes.

Even after it is discontinued, the LX200 will remain in wide use for many years. It is to computerized telescopes what the Nikon F is to cameras: an army of loyal users complain about its quirks but continue to trust it for serious work.

Evaluation of the LX200

Two useful features are conspicuously missing from the LX200: the ability to download software updates, and the ability to do a two-star alignment in equatorial mode to keep pointing accuracy from depending on polar alignment.

No light without shadow

A shadow is a volume of space not directly illuminated when light is intercepted by an object. Usually we are aware of these shafts of darkness only when they fall upon an illuminated surface, where they are seen as dim, distorted outlines. But if the medium through which the shaft of the shadow passes is filled with particles able to scatter light, such as dust-laden or hazy air, fog, or turbid water, the shaft itself becomes visible.

We are apt to overlook the effect of shadows on the way the world looks to us. If we notice them at all, it is probably as a diversion.

This book is about things that can be seen in the sky. We all look at the sky from time to time, though usually it is to check the weather. By and large we don't look at it for enjoyment, in part because we don't know what to look for. Very few people who are unfamiliar with the many wonderful sights to be seen in the sky accidentally notice halos or sundogs, two of the most common optical phenomena. To be sure of seeing these and other sights, you must know what to look for and when to look. This is where I hope this book will come in useful. It has been written to help you find your way around the sky, and see for yourself the many wonderful things that it has to offer.

My earliest memory of looking at the sky is of having the three stars that make up Orion's Belt pointed out to me. I can't recall what I made of them; I remember being told that they are distant suns, though that didn't mean much to me at the time. I was, I think, six or seven years old.

It was, nevertheless, a defining moment, the start of a lifelong fascination with the sky.

Comets

There is a long association between comets and disastrous events on Earth. Over the ages, they have been variously blamed for floods, droughts and pestilence. As far as the ancients were able to tell, comets appear unpredictably, seemingly from nowhere, and after a few weeks vanish, never to be seen again. They were seen as a challenge to the harmony and predictability that normally reigns in the skies, and were excluded from the heavens on the authority of Aristotle. He argued that the fact that comets appear and disappear without warning, means that they must be due to events close to the Earth, the only part of the universe where he believed change was possible. According to Aristotle, comets were atmospheric phenomena involving friction between layers of the atmosphere furthest from Earth.

Amateur astronomy for a new generation

This is a handbook for the modern amateur astronomer. As far as possible, I've tried to write the book that I'd like to have in my own hands while at the telescope – along with a star atlas and the Handbook of the B.A.A., of course.

Amateur astronomy isn't what it used to be. A generation ago, most serious amateurs observed from their homes with large Newtonians; one star atlas and two or three reference books were the amateur's complete guide to the sky; the latest news, arriving by magazine, was two months old; and most of the stars visible in the telescope were absent from even the largest catalogues and atlases.

Those days are gone, thank goodness. Telescopes have changed – they are nearly all portable, and compact designs such as the Schmidt–Cassegrain are popular. As often as not, the telescope is computer-controlled.

More importantly, computers have brought high-quality data sources within the amateur's reach. Alongside star atlases, we use software that plots the star positions measured by the Hipparcos satellite. We can compute the positions of comets, asteroids, and artificial satellites at the touch of a button. We can even track clouds by satellite to see if we're going to have clear weather.

Accordingly, a major theme of this book is the effective use of astronomical data, especially the Internet. Web addresses are given throughout, as well as detailed information about classic and modern catalogues of celestial objects.

Magnitude

The magnitude system

The magnitude of a star is its brightness measured in a somewhat peculiar way. In ancient times, Ptolemy and Hipparchos classified stars as “first class” (brightest) to “sixth class” (barely visible). These brightness classes were termed magnitudes, but there was no provision for exact measurement.

In 1856, Norman Pogson proposed the logarithmic magnitude scale that is now standard. The advantage of a logarithmic scale is that it can span a tremendous brightness range without using very large or very small numbers (Figure 8.1). Each difference of five magnitudes corresponds to a factor-of-100 difference in brightness. One magnitude corresponds to a brightness ratio of 2.512.

In this system, most stars still have roughly the magnitude that Ptolemy assigned them, but some of the brightest stars have negative magnitudes. The full moon is magnitude – 12 and the Sun is –27. This 15-magnitude difference means that the Sun is a million times as bright as the Moon.

The star Vega is defined to be magnitude 0.0, but in practice, the average of several stars is used as a standard for measurement.

The human eye's response to light is not actually logarithmic, but it is close enough for practical purposes. If a star appears to be halfway between two other stars in brightness, it will also be halfway between them in magnitude.

The importance of double stars

More than half of all stars belong to double- or multiple-star systems. That is, they are gravitationally bound to other stars and orbit around the system's common center of gravity.

Amateur astronomers today have forgotten the excitement that accompanied the gradual discovery of this fact during the nineteenth century. Double stars provided the first opportunity to measure the mass of stars and to test the laws of physics outside the Solar System.

Many double stars show measurable orbital motion over just a few years. Many others need to be measured now, since determination of an orbit requires observations many years or centuries apart, and double-star work fell out of fashion in the twentieth century. The Hipparcos satellite made accurate measurements of numerous double stars in early 1991; even these observations are now old enough that it is worth while looking for subsequent changes.

It is often unknown whether a particular double is really a gravitationally bound binary star or merely an optical double comprising two unrelated stars in the same line of sight. In between are common proper motion pairs, stars that appear to be the same distance from Earth and have roughly the same proper motion, but in which orbital motion has not been observed.

A visual binary is a binary star whose orbit can be observed with telescopes. Less than a thousand orbits are known in any detail, and the available information about them is often inaccurate.

Atmospheric refraction

When looking at something, we not unnaturally assume that what we see is exactly as we see it and precisely where we see it. In almost all situations in which we find ourselves this assumption is perfectly justified because we are close enough to the things that we see for their light to reach us by travelling in a straight line to our eyes through a homogeneous atmosphere.

Over larger distances, however, the atmosphere is not homogeneous because its density is not uniform. As you might expect, the density of air decreases with height because the lower layers are compressed by the weight of those above.

Observing the Moon

The Moon is probably the only celestial object, apart from the Sun, that we can all recognise without being prompted. It is large and bright enough to catch our eye unexpectedly, and regularly goes though a remarkable sequence of phases that have no parallel in nature.

There was a time when the Moon played an important part in people's lives. Moonlight made activity possible outdoors after sunset. The first calendars were based on a lunation, the time taken for a complete cycle of lunar phases, from one new Moon to the next. In fact, the word ‘Moon’ is derived from an archaic term for measurement.

The Julian date (JD) is the number of days elapsed since noon UT on January 1 of 4713 B.C. It is the standard way of giving the date and time of variable star observations and is also used in astronomy for other purposes.

The Julian date system was introduced in 1582 by Joseph Justus Scaliger, who named it in honor of his father Julius Scaliger; it has nothing to do with the Julian calendar of Julius Caesar. The starting date in 4713 B.C. was chosen for easy conversion between several ancient calendars.

Note that the Julian day begins at noon, not midnight. Astronomers also use the modified Julian date (MJD), which is the JD minus 2 400 000.5. The MJD day begins at midnight.

AAVSO publications also use the JD minus 2 400 000, ignoring the 0.5. With long-period variables, this makes little difference.

Table C.1 gives the MJD for 0:00 UT on the “zeroth” day of every month (i.e., last day of the previous month) from 2001 to 2020, with instructions for computing the JD.

The AAVSO publishes a calendar giving the Julian date for each day (including an online version at http://www.aavso.org), and most if not all computer sky chart programs give the Julian date.