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BAA Journal 2018 April

Absolute Beginners No 13. The Pole Star

Journal issue: 2018 April
Pages: 72–73

1 Introduction

 If you go outside on a clear night, point a camera upwards and take a long exposure photograph, you'll find that the resulting image shows star trails.  Even an exposure time of a a minute or so is enough to show the effect and stars closer to the horizon have longer trails compared to those higher in the sky.

 Star trails showing the rotation of the sky.

Star trail photographs nicely illustrate the illusion that, from down here on the ground, that the entire sky seems to be rotating about a single point in the sky (indeed this is what the ancients used to believe.) In reality, the rotation of the sky is due to the fact that the earth is rotating on its axis.  The axis of rotation is an imaginary line passing through the centre of the earth which the planet appears to revolve around, and if we extend it outwards (figure 2) you can see that in both cases the axis will point towards that part of the sky where everything appears to rotate about.

Figure 2 The Earth's rotational axis points to the Pole StarIn the northern hemisphere this is the north celestial pole, and in the south this is the south celestial pole.  The star which lies at the north celestial pole is called the north polar star or 'Polaris', similarly the one which lies at the southern celestial pole is called the south polar star. Knowing the location of the pole star is very important in astronomy- particularly if you have a telescope fitted with an equatorial drive (see absolute beginners 4).  In the northern hemisphere, the pole star is called Polaris, and it is the brightest star in Ursa Minor.  It is easy to find- simply locate the constellation Ursa Major (which is visible all year in the northern hemisphere).  Part of this constellation forms an 'asterism' (an unofficial name for a group of stars) looks rather like a saucepan- take the two stars at the end of the saucepan (Merak and Dubhe) and draw a line between them, then continue the  line upwards until you come to Polaris (figure 3). 

 

Figure 3 Using the two pointer stars in Ursa Major to locate the pole star.Polaris does not lie exactly at the north celestial pole- it is about 1 degree away, so if you look closely on star trail photographs you will see that it describes a little circle.  The southern hemisphere by contrast has no bright star near to the south celestial pole- the star closest to the southern pole is a rather faint +5.4 magnitude star called σ-Octantis.

 

2 The Pole Star is not Permanent

 It would appear that, night after night, year after year our night skies remain the same.  Although the Moon waxes and wanes and the planets come and go, overall the stars tend to remain a permanent fixture.  This is in fact an illusion, the stars do have their own motion, but because they are so far away, in general it takes many millions of years for this motion to become apparent.

Figure 4 Due to precession, the earth's axis does not constantly point in one directionThere are other effects which cause the stars to slowly change their position in the night sky- for example there is the phenomenon of 'precession'.  The combined gravitational pull of the sun and the Moon causes the the earth to 'wobble' on it's axis rather like a spinning top.  As a result, the earth's axis of rotation slowly describes a circle over time, taking 25,700 years to complete one circuit (see figure 4).The effect of precession means that the rotational axis does not point in the same direction, and so after some time the pole star changes.  In 3000BC, the star Thuban in Draco the Dragon would have been the northern pole star.  In the year 14000, the brilliant star Vega in Lyra will be the pole star for observers in the northern hemisphere.

 

3 Using the Pole Star.

Polaris is a useful tool for amateur astronomers and for navigation.  The altitude of  Polaris above the horizon depends on your latitude.  In Leicester, the Pole star is some 52° 36' above the horizon- this means the latitude for Leicester is also 52° 36'.  The further north you go, the higher the altitude of Polaris becomes- from the centre of the north pole it will of course be right overhead at the zenith.

If your telescope sits on an equatorial mount, the pole star is essential in getting it properly equatorially aligned.  To align your telescope, you will need to make sure it is set to the correct latitude (this is usually indicated by a dial which you can adjust for your latitude).  Most mounts today have a 'polar scope' fitted, this is a very small telescope inside the mount.  If you look through this telescope, you'll find it contains a circle- the idea being that you bring Polaris into the field so it lies on the circle which is about 1 degree away from the north pole.  Your telescope will then be equatorially aligned- your declination axis will be the same as the Earth's rotational axis, and so the sky will 'rotate' around your telescope just as it seems to rotate around the pole.  If your telescope is undriven, this means you will only need to make changes in RA.

 If you fit a drive to your telescope, the drive will slowly rotate your telescope in RA at the same rate as the earth turns- this is called Sidereal time, and one sidereal day is 23 hours 56 minutes 4.095 seconds (slightly shorter than 24 hours).  If you wish to do long exposure photography or visual observations of the Moon and planets, a drive is essential as you will need to keep your target stationary in the telescope field for a long period of time.

 In the northern hemisphere, we are very lucky to have a bright star marking the position of the north celestial pole in the the night sky.  Even if you don't use Polaris to align your telescope, it is still important to know where it is and how to locate it as it is a very important marker in the night sky.