Understanding Sky Charts

Fig 1: Sky looking south at 20h GMT on April 01

There has been a request by some users of the site for further explanation of the sky charts.

For large field work a scale is used that will show the horizon over an azimuthal arc of 100º and up to an altitude of 75º. (This may vary a little depending on how the horizon is positioned.) In Fig. 1, for example, we have the sky looking due south at 20h 00m on April 01.

The horizon is shown curved with 180 in the middle. [The degree sign, º , is omitted from the charts. The cardinal points are: N (0), E (90), S (180), W (270).] The point overhead (zenith) would be 90º but this is not possible to show on the chart at this scale. The altitude of 75º in fact is uncomfortable to view and may give the impression of being almost directly overhead. (I use a deck-chair when observing with binoculars in such positions.)

In becoming familiar with the night sky there are certain important points to note. First, the constant movement from east to west of all the stars (the diurnal motion as a result of the Earth's rotation). Second, the passing of the seasons which causes stars to rise and set earlier by approximately 4 minutes of time each day. (Except for circumpolar stars, which remain above the horizon.)

My many years experience as an extra mural lecturer has convinced me that the comprehension of scale is all important; once grasped, the task becomes far easier.

Orion as a group is easy to identify but other constellations are not so readily identified. Using the charts, gauge the relative distances between the stars of Orion and apply this as a "measuring stick" for other groups. Thus, Leo to the east (left) will be seen to resemble a rampant lion; but for many the figure is much larger than they might expect from a cursory look at a star map. Bear in mind, what is represented on the flat page of a chart or computer screen is an approximation to the impression one has of looking up into the real sky where everything appears to be studded on the inner surface of an enormous sphere.

In contrast to the winter night skies, spring has fewer bright stars and this makes the task easier in some respects. Always work from the familiar and try to add new groupings to your repertoire as the seasons pass. Leo is a useful "base" for the spring sky. (Note the familiar Plough is overhead when the body of Leo is due south.) The three brilliant stars Arcturus, Capella and Vega should become familiar as "staging posts" for different times of the year.

Thus, Capella is roughly overhead in our winter skies, Arcturus is high up in the night skies of late spring, and Vega is very high in the late summer and early autumn. And remember, from Orkney both Vega and Capella are circumpolar.

In order not to overcrowd the charts I generally omit the connecting lines frequently used on maps to denote the constellations. (The intention is to feature an article on the constellations later in the year.)

There are any number of aids to identifying the constellations both in books and through the internet. In my opinion a good star atlas is essential. For the serious observer Norton's 2000 (based upon earlier versions of Norton's Star Atlas) is not only a fine atlas but it is also a mine of general astronomical information. For the less ambitious observer, The Times Night Sky Companion can be recommended.