Angular Hand Measurements (not to scale)
How to Use the Quick Sky Guides
OF ALL THE things that I have learnt when trying to help people find their way around the night sky the most useful is to have something like a story or interesting fact to hook on to. It is just how the mind works: give a piece of information something to cling on to and you will have a much better chance of committing it to your long-term memory. Doing this can not only bring the sky to life but also help you to retain what you have learnt about the objects in it and, more importantly, remember where they are.
My ‘Quick Sky Guides’ cover first the sky in the northern hemisphere and then the sky in the southern hemisphere. It is important to add that there is a subtle difference between the northern and southern hemispheres of the sky, and the sky as it is visible from the northern and southern hemispheres of the Earth.
To make that a little clearer: if you stood at the North Pole, then you would only see the northern hemisphere of the sky; and from the South Pole only the southern hemisphere of the sky would be visible to you. If you were to observe the sky from a point somewhere along the equator, then one half of what you could see would be a part of the northern hemisphere sky, and the other half part of the southern hemisphere sky.
This means that, unless you live at either of the poles, you will be able to see part of both hemispheres of the sky, and how much of each is visible depends on your location. Therefore, to get the most out of them, you will need to look at parts of both sky guides.
The key to making this all work is in identifying an imaginary line called the celestial equator, which is ‘visible’ from anywhere on Earth, and it is here that each hemisphere starts before heading either north or south. The great thing is that its position in the sky relative to where you are will not change, at least not for a few thousand years, so once you know roughly where it is your exploration of the night sky can begin.
A tiny piece of mathematics is needed to find the celestial equator from your location. The first thing you need to do is find your latitude, which is a measure of how far north or south of the equator you are. With the vast array of mobile devices on the market which use various technologies to pinpoint your location on Earth, it is a simple matter of reading off your latitude. It will probably be given in degrees, minutes and seconds, but it is just the degrees you need. These are degrees like the degrees of a circle you learnt about in school, and you do not need to be that accurate. If you do not have access to this kind of device, get hold of a map and use its grid reference to find your latitude – again, pin-point accuracy is not crucial. Either way, you will have a number between zero and 90; if you live near the equator then it will be towards the lower end, if you live nearer the poles, then it will be towards the higher end.
Now here is the maths bit: subtract your number from 90 and that will give you the maximum height above the horizon of the celestial equator from where you are. All you need to know now is how to translate that to the sky and you can get on with the book. There is a really useful scale using your hand which helps you to estimate distances in the sky. A hand at arm’s length with fingers spread measures about 25 degrees from thumb to little finger, and a clenched fist equals around 10 degrees. Smaller measurements can be gained from the width of the three middle fingers equivalent to 5 degrees and one finger’s width equal to about 1 degree. To make that a bit clearer, see the illustrations below.
Angular Hand Measurements (not to scale)
In the northern hemisphere you need to turn and face south (if you are not sure which direction that is, when you look towards where the Sun sets your left shoulder is pointing south), or face north when in the southern hemisphere (the direction your right shoulder points when you look towards the position of the setting Sun). Looking at the objects due south or north like this means they are in the best position to be observed, as their light passes through the smallest amount of gas in our atmosphere, distorting it the least. Looking back at your earlier calculation, use your hand at arm’s length to measure up from the horizon until you reach this value, remembering that you do not need to be too accurate. (For example, to find 38 degrees it might be easier just to measure four clenched fists and go back by two finger widths.) You are now looking roughly at the celestial equator due south or north and this is the starting point for the guides.
Now to get you looking at the right time of night. It might seem an obvious statement but astronomy relies on the sky being dark, unless you are studying the Sun of course. For this reason the charts and descriptions in this book are all set to introduce you to some of the most important objects in the sky at midnight, local time (the time shown by your watch). On any day during the month, go outside around midnight – do not forget to allow for any daylight saving scheme used in your country – and the stars will be in roughly the right position. As the months progress and you follow these guides you will notice that the stars move a little further to the west from month to month, giving a slightly different view of the sky; Arcturus, for example, was in the east at midnight in April and will be a little more to the west in June. This is because the Earth actually takes 23 hours, 56 minutes and 4 seconds to rotate once on its axis and we call this the sidereal day, although our watches measure a day as 24 hours. The effect of this is that the sky slowly gets out of synchronization with our watches.
Start with the section of the hemisphere you live in, and once finished switch to the other section and follow it until you meet your horizon. One more important thing to remember: if you live in the northern hemisphere, you must reverse the east/west references when using the southern sky guides and, likewise, southern hemisphere observers must reverse the east/west from the northern sky guides. You will also see references to magnitude in the guides as an indication of brightness. Astronomers use this scale to measure brightness, where a lower number is brighter than a higher number; for example, magnitude 6 is fainter than magnitude 2 and magnitude 2 is fainter than magnitude –1. To give you a gauge, magnitude 6 is the faintest star the average human can detect on a dark moonless night.
Now, before we get started, the following diagrams highlight the most important constellations in each hemisphere. These are not all visible at any one time, but will come into focus as we progress through each monthly guide.
