2019 Guide to the Night Sky Southern Hemisphere: A month-by-month guide to exploring the skies above Australia, New Zealand and South Africa. Wil Tirion

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      New Zealand

      New Zealand uses a single time zone for the North and South Islands: New Zealand Standard Time (NZST) which is 12 hours ahead of UT. Daylight saving time (NZDT) is used in the summer, and begins on the last Sunday in September and ends on the first Sunday of April. (Note that it begins one week before the Australian start of Daylight Saving Time.) As for Australia, the hours shown above should be added to Universal Time (UT). If the result is greater than 24, subtract 24 hours. If the initial or subsequent result is less than 12:00, the zone time is a.m.; if greater, subtract 12 hours to get the time p.m. The dates when Daylight Saving Time begins and ends are shown on the individual calendars inside the book.

      South Africa

      South Africa uses a single time zone: South Africa Standard Time (SAST). This is two hours in advance of UT. Daylight Saving Time / Summer Time is not used. To obtain local (zone) time, add two hours to UT. If the result is greater than 24, subtract 24 hours. If the initial or subsequent result is less than 12:00, the zone time is a.m.; if greater, subtract 12 hours to get the time p.m.

Introduction

      The aim of this Guide is to help people find their way around the night sky, by showing how the stars that are visible change from month to month and by including details of various events that occur throughout the year. The objects and events described may be observed with the naked eye, or nothing more complicated than a pair of binoculars.

      The conditions for observing naturally vary over the course of the year. During the summer, twilight may persist throughout the night and make it difficult to see the faintest stars. There are three recognized stages of twilight: civil twilight, when the Sun is less than 6° below the horizon; nautical twilight, when the Sun is between 6° and 12° below the horizon; and astronomical twilight, when the Sun is between 12° and 18° below the horizon. Full darkness occurs only when the Sun is more than 18° below the horizon. During nautical twilight, only the very brightest (navigation) stars are visible. During astronomical twilight, the faintest stars visible to the naked eye may be seen directly overhead, but are lost at lower altitudes. At Sydney, full darkness persists for about six hours at mid-summer. Even at Christchurch, NZ (not shown), full darkness lasts about four hours. By contrast, as far south as Cape Horn, at mid-summer nautical twilight persists, so only the very brightest stars are visible.

      Another factor that affects the visibility of objects is the amount of moonlight in the sky. At Full Moon, it may be very difficult to see some of the fainter stars and objects, and even when the Moon is at a smaller phase it may seriously interfere with visibility if it is near the stars or planets in which you are interested. A full lunar calendar is given for each month and may be used to see when nights are likely to be darkest and best for observation.

      The celestial sphere

      All the objects in the sky (including the Sun, Moon and stars) appear to lie at some indeterminate distance on a large sphere, centred on the Earth. This celestial sphere has various reference points and features that are related to those of the Earth. If the Earth’s rotational axis is extended, for example, it points to the North and South Celestial Poles, which are thus in line with the North and South Poles on Earth. Similarly, the celestial equator lies in the same plane as the Earth’s equator, and divides the sky into northern and southern hemispheres. Because this Guide is written for use in the southern hemisphere, the area of the sky that it describes includes the whole of the southern celestial hemisphere and those portions of the northern that become visible at different times of the year. Stars in the far north, however, remain invisible throughout the year, and are not included.

       The duration of twilight throughout the year at Sydney and Cape Horn.

      It is useful to know some of the special terms for various parts of the sky. As seen by an observer, half of the celestial sphere is invisible, below the horizon. The point directly overhead is known as the zenith, and the (invisible) one below one’s feet as the nadir. The line running from the north point on the horizon, up through the zenith and then down to the south point is the meridian. This is an important invisible line in the sky, because objects are highest in the sky, and thus easiest to see, when they cross the meridian in the south. Objects are said to transit, when they cross this line in the sky.

      In this book, reference is frequently made in the text and in the diagrams to the standard compass points around the horizon. The position of any object in the sky may be described by its altitude (measured in degrees above the horizon), and its azimuth (measured in degrees from north 0°, through east 90°, south 180° and west 270°). Experienced amateurs and professional astronomers also use another system of specifying locations on the celestial sphere, but that need not concern us here, where the simpler method will suffice.

       Measuring altitude and azimuth on the celestial sphere.

      The celestial sphere СКАЧАТЬ