Collins Night Sky - Wil Tirion

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the ‘inside’ of the celestial sphere, care is needed. Looking south in the northern hemisphere, west is always to the right (and east to the left), as might be expected. Looking towards the northern sky, however, we have to be a bit more cautious. The direction of ‘west’ changes depending on where the objects lie relative to the North Celestial Pole. Going west you circle the Pole in an anticlockwise direction – to the right below it, and to the left above it.

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      Using compass directions to describe positions on the sky becomes difficult in circumpolar areas, so the terms ‘preceding’ and ‘following’ are used instead.

      Partly to avoid this confusion, astronomers also use the unambiguous terms ‘preceding’ and ‘following’ when describing the relative positions of objects. These are related to an object’s right ascension. It two stars have the same declination, the star that rises (and sets) first, precedes the other. In general, except perhaps near 0h (24h) RA, it has a lower value of right ascension. Looking south in the northern hemisphere, it lies to the right. The opposite criteria obviously apply to ‘following’. The two terms are nearly always combined with the directions north and south, to divide the sky around the reference object into four quadrants: north preceding, north following, south following, and south preceding, usually abbreviated ‘Np’, ‘Nf’, ‘Sf’, and ‘Sp’, respectively.

      Occasionally you may come across the ‘position angle’, which is an accurate method of describing an object with reference to another. It is particularly use in connection with double and multiple stars and, as such, is described here: DOUBLE AND MULTIPLE STARS.

      STAR-HOPPING

      The easiest method of finding things on the sky is to use the time-honoured method known as ‘star-hopping’. This simply consists of using stars, or patterns of stars, that you know, to help you find less familiar ones. It is the method everyone uses to learn the constellations, and may be applied just as successfully to fainter objects, whether using the naked eye, binoculars, or a telescope.

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      Storm Dunlop

      As this photograph of Perseus shows, the eye tends to travel outwards from the brighter stars by recognizing lines and specific patterns of stars.

      Generally, one works from bright, known stars or patterns to the fainter ones. When faced with finding an unknown object from a star chart, start by locating some stars that you know or can find easily on the sky. Use these to extend a line across the sky; to form the base of a triangle; the start of a curve of stars; perhaps even an ‘arrow’ pointing in the right direction; or some similar sort of pattern to take you to another star, pair or group of stars, closer to your destination. You can use this next reference point to repeat the process. It all sounds more complicated than it is in practice, because pattern recognition is so innate in everyone that it soon becomes second nature to apply it to stars and star charts.

      One important aspect of astronomy is the question of time. Because observers are scattered across the globe, they lie in different time zones, and an event (such as the eclipse of one of Jupiter’s satellites, for example), may happen in daylight for one observer, and in darkness for another. Then there are the problems caused by the use of summer time (daylight-saving time) in certain countries, especially when (as in the United States) not all parts of a country adopt it.

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      Chart of time zones

      The solution is for all astronomical events and observational reports to be given in Universal Time, which is the same for everyone, everywhere. Universal Time (UT) is the time of the Greenwich meridian, originally known as Greenwich Mean Time (GMT). It runs from 00:00 to 24:00 hours, starting at midnight. It is used for all the astronomical phenomena that are listed in the standard yearbooks and handbooks (such as those listed under ‘Further Reading’). In tables of planetary positions, for example, the Right Ascension and Declination of planets are normally given for 00:00 UT (i.e., midnight on the Greenwich meridian).

      LOCAL TIME

      Because the periods of day and night that you experience depend upon your position on the Earth, you do need to take local time into account in determining whether an event will be visible (unless you live close to the Greenwich meridian).

      The map shows the different time zones around the world, and the amount by which each is in ahead or behind Universal Time. Note that the boundaries of the time zones are generally chosen to coincide with political boundaries, and are highly irregular. For exact calculations, allow a difference of one hour for every 15° east or west of the Greenwich meridian, 4 minutes for every degree, etc.

      SUMMER TIME / DAYLIGHT-SAVING TIME

      It is necessary to take summer time (daylight-saving time, DST) into account when planning observing sessions. Because most people observe in the evening, the monthly charts in this book are drawn for 22:00 local mean time (10 p.m. LMT), which is 23:00 local summer time (11 p.m. LST). This is a compromise, because at high latitudes in summer twilight persists throughout the night and even at 23:00 summer time it may not be really dark.

      NOTE

      At the time of writing, Hawaii, Saskatchewan and parts of Indiana and Arizona do not employ daylight-saving time.

      In Europe, summer time is in force from the last Sunday in March to the last Sunday in October. The change (forwards in spring, backwards in autumn) takes place at 01:00 GMT (UT) on the dates given. In the United States and Canada, daylight-saving time (DST) starts on the second Sunday of March, and ends on the first Sunday of November.

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      THE DATE

      The other important point to bear in mind is that of the date. For astronomers world-wide, this changes at midnight (24:00 hours) UT. This may cause confusion, because observers in a different time zone, east of the Greenwich meridian, will have already changed their civil date, and for observers to the west, their civil date has yet to alter. The simplest solution is to have a clock that permanently shows Universal Time, preferably a digital one that also shows the date. All serious observers in the Greenwich time zone also do the same, because it avoids any potential confusion over the use of summer time.

      How should you record the date and time? The best way is to use the standard scientific method, in which the elements are given in descending order of size: Year, Month, Day, Hour, Minute, and Second (and in very precise cases, decimal fractions of a second). Again, to prevent confusion, it is best to record the month as an abbreviation rather than a number. So we might have a date and time given as (say) 2001 Oct. 05, 01:35:07 UT. This is unambiguous, and in many countries (such as Great Britain) is a legally recognized way of specifying the date and time.

      If you happen to observe an important event – perhaps a brilliant daytime fireball, for example – when a watch or clock showing Universal Time is not immediately to hand, then write down the observation using the current local time (even summer or daylight-saving time), making sure that you record that fact as well. A correction to UT may be made later. Don’t try to work out the correction there and then, because this takes your attention away from the event in question and is liable to error. It is all too easy to add an hour in ‘correcting’ summer time, for example, instead of subtracting one.

      Another method to reckoning the date (the Julian Date) will be described later, when discussing variable stars.

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