Basic Astronomy Series: Navigate by Starlight

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Navigating by the stars is actually quite easy, and if you happen to be away from civilization and an important gadget fails, you can use the stars and the sun to find north or south! When you are building a house, and need quick reference for drawing plans, it can help. If you have directions that refer to facing a particular direction to proceed, you don't have to find a gadget and power it up! You can work out the best to place to park your car, so it is shaded for a longer period. These are simple and practical uses for such knowledge.



The stars have fascinated mankind before civilization came to be, and our ancestors mapped pictures of Mythology and day to day life amongst the stars. Today, within the realms of science, the sky is divided into 88 mapped areas, located using coordinates that relate to latitude and longitude. These geographical concepts have been translated upwards into the sky. Each mapped area is called a constellation and most are named and mapped from their position and the pictures they reflect. A few more recent additions reflect modern day life too; an example is the constellation Microscopium.



Probably the most well-known constellation of our southern skies is the Southern Cross or Crux, as it is otherwise known. It is located in a region of sky where the stars never set when seen from Melbourne. These Circumpolar stars highlight an important concept, which is the Celestial Sphere. In order to describe this imaginary model, let us consider the lines of latitude and longitude used to find a place on earth. When these lines are imprinted in the night sky, which we think of as a black sphere enveloping the earth, then we have created our Celestial Sphere. The lines of latitude are named lines of Declination and these are measured in degrees from north to south. While bearing in mind that a sphere can be broken up into 360 degrees of compass points, the numbering of Declination lines runs from 0 degrees at the Celestial Equator and up to negative 90 for the southern hemisphere and positive 90 for the northern hemisphere. While considering east-west, we divide the earth into hours of longitude as it rotates once in near 24 hours. It then becomes the same for the sky using instead hours, minutes and seconds of Right Ascension, in place of the degrees of longitude. Since the earth rotates one 360 degree revolution in close to 24 hours, each hour of rotation is approximately 15 degrees (15 x 24= 360!) To distinguish between real time and the divisions of degrees of arc, Astronomers will refer to "Minutes and seconds of arc," and the relative size of an observed object or distance between two objects will be expressed as seconds of arc. The International dateline runs through Greenwich, England, and this corresponds to our 0 hour for the 24 lines of time divisions of Right Ascension.



Refer to the photo of an analogue watch above. It is 3pm and I have pointed the hour hand at the sun. Then drawing an imaginary line from the 12th hour mark, I take a line half way between the hour hand and the imaginary line, and that halfway line points north. To complete the picture, face north and west is left, east is right, and south is behind you. Most maps are generally orientated so north is at the top of the map. If you are located in the northern hemisphere, you would find south using this method; by that I mean the midday-way line on the watch would indicate south.



The next concept to grasp is the celestial poles. Imagine you are standing at the North Pole and all of the stars will rotate around a point directly overhead. This point is the North Celestial Pole and as you stand there, it rotates anti-clockwise about you. It is interesting to note that the earth's axis is 23 degrees off the vertical plane, which is perpendicular to our orbital plane around the sun. You can imagine why the Polar Regions experience around 6 months of daylight and night. The South Pole is the same except the stars rotate clockwise. Here you will see all of the stars you will ever see at this location, at any given time. You could say nearly all the stars are circumpolar. If you are at the equator, the whole sky rolls over like a rolling barrel, the axis being the poles. None of the stars are circumpolar, they all set and rise. From the equator, you see all of the stars seen from earth, but only half of them at any given time. The last scenario of course is for middle latitudes and from Melbourne for example; the circumpolar stars are seen all year round and the rest for a given time of night, can be seen only at certain times of year.

In the next Astronomy article we shall explore easily found objects, star brightness and star catalogues. Try a visit to Melbourne's Planetarium a fun introduction into the night sky.

References:
Celestial sphere diagram;
www.physics.csbsju.edu/astro/CS/CS.16.html

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