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.

Why equatorial?

Table 4.1 sums up the advantages and disadvantages of equatorial and altazimuth mounts.

There are two main reasons for using an equatorial mount (such as the one in Figure 4.1): to eliminate field rotation (Figure 4.2) in long-exposure photography, and to establish which way is north in the sky so that you can use charts and measure double-star position angles. Apart from that, altazimuth mode is almost always preferable. Setup is simpler and quicker, and you don't need an equatorial wedge to tilt the base.

There is one situation in which an equatorial mount is easier to set up than an altazimuth one. That is when you have a permanent telescope stand that is accurately aligned with the Earth's axis. In that case, all you have to do is attach the telescope and sync on one star. That's all – the telescope is aligned, calibrated, and ready for both visual observing and photography.

Must field rotation be eliminated?

Equatorial mounts get rid of the field rotation illustrated in Figure 4.2. Celestial objects tilt as they rise, travel across the sky, and set. An equatorially mounted telescope tilts with them, so that everything remains stationary in the field of view, but altazimuth-mounted telescopes suffer field rotation. With an altazimuth mount, the object that you're tracking remains centered, but everything else rotates around it.

What eyepieces do you need?

The ease and comfort of using a telescope depend to a surprising degree on the eyepiece, which contains more optical elements than the rest of the instrument. Most importantly, the eyepiece determines the magnifying power.

The magnifications that work well with any telescope are proportional to its aperture. For example, a 4-inch (10-cm) telescope at 25× and an 8-inch (20-cm) at 50× are both at the low end of their power ranges. They are operating at 2.5 power per centimeter of aperture, or 2.5×/cm for short, equivalent to about 6 power per inch.

What this implies is that the eyepiece focal lengths that work well with any telescope depend on its f-ratio. For example, a 20-mm eyepiece gives low power with an f/5 telescope but medium power with an f/10 telescope.

With that in mind, Table 6.1 makes specific recommendations. You do not have to get the exact focal lengths in the table, of course, but the table will show you where any particular eyepiece stands. If you have an f/10 telescope and choose to get a 32-mm eyepiece, which is not in the table, you'll know that it's in the low- to very-low-power range.

You do not need a lot of eyepieces. Three eyepieces are enough for almost anybody; two (low and high power) are almost enough; and one eyepiece (low or medium power) will get you started. Go for quality, not quantity.

Sunset

Are there significant differences between sunrise and sunset? It is a question I often ask myself, and to which I have yet to find a definitive answer, if indeed there is one. Other than that events in one occur in the reverse order to events in the other, it seems to me that any physical differences are those of detail only. By and large, it seems the conditions that give rise to a splendid sunset are exactly the same as those that produce an equally memorable sunrise, although it has been said that these conditions occur more often at the end of the day. Most of us have seen the Sun set many more times than we have seen it rise, and so it's difficult to judge the truth of this.

Introduction

Although it came on the market slightly later than the Meade Autostar, the original model Celestron NexStar is a simpler design, so it is logical to describe it first.

This chapter is based on my experiences with a NexStar 5 purchased in 2001. This is a 5-inch (12.5-cm) Schmidt–Cassegrain and was initially the flagship of the NexStar fleet. It is practically identical to the original (one-armed) NexStar 8.

Related products

Celestron has subsequently introduced a wide range of NexStar telescopes, ranging from small refractors to an 11-inch Schmidt–Cassegrain. Some of the smaller NexStars have been sold under the Tasco Starguide label.

No two NexStar models have entirely the same firmware, and the product line is still evolving rapidly. Accordingly, this description of the NexStar 5 will serve only as a general guide to the rest of the product line; changes are ongoing. Many different versions of the NexStar 5 firmware are already extant.

The top-of-the-line NexStar GPS telescopes are radically different and are not covered here. They include GPS receivers to determine the latitude, longitude, and time; periodic-error correction; and other advanced features.

Evaluation of the NexStar 5

Compared with Meade LX200 and Autostar telescopes, the NexStar 5 is appreciably easier to use because it has fewer features; the overall design of the firmware is simpler. The optics are from the classic (and, in my opinion, underrated) Celestron 5.

General concepts

The first thing a telescopic observer of the planets notices is that they all look rather small and blurry. Some training of the eye is required in order to see planetary detail. Not only are planetary features faint, but they require constant attention in order to take advantage of brief moments of steady air.

Drawing the planets is one of the best ways to learn to observe them (Figure 4.1). You can always see, and therefore draw, more detail than you can photograph with the same telescope. A good scale is about 5 cm (2 inches) for the diameter of the planet. Always draw what you see, not what you think you ought to see.

Every planetary drawing should be labeled with the date and time, particulars of the telescope, and eyepiece, and quality of seeing (atmospheric steadiness, Table 2.2, p. 12). Colored filters are often helpful. Such drawings have considerable scientific value and are collected for research purposes by the B.A.A. and A.L.P.O. (see p. 30).

Good handbooks for planet observers include, among others, Thomas A. Dobbins, Donald C. Parker, and Charles F. Capen, Introduction to Observing and Photographing the Solar System (Willmann–Bell, 1992), and Fred W. Price, The Planet Observer's Handbook (Cambridge, 1994). For current planetary research, see J. Kelly Beatty et al., The New Solar System (Cambridge, fourth edition, 1999; revised regularly).

Ice halos

Take the advice of the engaging Edmund Halley (of comet fame), and ‘… always look out …’ for ice halos in the sky. Sooner or later you're bound to develop a nose for them, just as he did. Despite being an eminent astronomer and mathematician, and a leading member of the English scientific establishment of his day, Halley was never so busy that he hadn't time to glance up at the sky in search of halos and rainbows, and share his observations with others in his characteristically informal, easy-going manner.