Celestial Navigation in Australia

Sunday, Feb 22, 2015 at 16:14

equinox

It is handy to know which way is North and South when travelling around the countryside. In these modern times of electronic navigation sometimes the basic navigation skills that may be required that have been used for centuries have generally been forgotten. The reliance upon these electronic devices may be a detriment if they suddenly fail to operate, such as a local device failure or a failure of an entire satellite system.

A standard compass is a good way to find your direction too, however even this device has its pitfalls; for example if you are standing on a magnetic anomaly.

Using Celestial Navigation one can determine which way is north, south, east and west. For the beginner this will take some practise, however using various methods one will be able to determine any of these directions within seconds. This option of navigation is free, and requires only your eyes, some simple spatial reasoning and a clear sky.

I have written this blog with the objective of keeping the details of basic Celestial Navigation with an Australian emphasis as simple as possible so the casual reader can understand at least some of the concepts.

Using the stars is a very good way to help with navigation. As stars are very, very far away they do not change position relative to each other very quickly. Their position will change over hundreds and thousands of years depending how close the star is and how fast it is moving through space however this can be considered irrelevant to this topic as generally the movement is far less than an unequipped human can detect in their lifetime. Therefore they act as very good precise unchanging reference points.

If all the stars remained fixed in their positions in the sky relative to where you are, similar to a Geo-stationary satellite, then navigating would be very simplistic. We could pick out a star that was in a southerly or easterly direction for example and wherever we happened to be we could find this star and use it as a guide; “That star is always east, go that way and we will head east.” However unfortunately, although Celestial Navigation is easy – it’s not that easy. The reason for this perhaps is obvious, being that the earth spins around once on its axis relative to the stars once about every 23 hours 56 minutes so the stars appear to move their positions in the sky.

The earth is constantly spinning on it axis in a clockwise direction as viewed from an imaginary observation point hovering above the South Pole. So to the observer in Australia this means that the stars will appear to rise in the eastern hemisphere and set in the western. Of course the stars are actually stationary; they just seem to move due to the rotation of the earth and the observer upon its surface. As clocks are set to a 24 hour daily cycle and the earth spins around once relative to the stars every 23 hours and 56 minutes, the stars will appear to rise four minutes earlier every night at the same geographic location; or to put it another way the stars will appear in the same spot in the sky on one night, four minutes earlier than they did the night before when viewed from the same geographic location.

NOTE: When describing a star or pole as visible, it means that the star is in the current field of view of the observer. When it is daytime the star or pole will not actually be visible to the naked eye. Likewise, when describing a star or pole as not visible means that it is not currently in the observer’s field of view.

As a consequence of the earth’s rotation the stars as viewed from an observer in Australia will appear to rotate around two opposing points in line with the axis of the earth. These are the celestial poles called plainly, the South Celestial Pole and the North Celestial Pole. The South Celestial Pole is always visible from all locations in Australia. Stars will appear to rotate around this pole in a clockwise direction. The North Celestial Pole is never visible from Australia as it is always below the northern horizon; the stars to the north will appear to rotate around it in an anti-clockwise direction. Although it is not visible, it can be mentally perceived by relating certain rotating adjacent marker stars to it and the pole can be visualised below the horizon. Remember, the stars will appear to complete an entire revolution of the poles in 23 hours and 56 minutes.

I will attempt to explain the poles, the Celestial Sphere and the Celestial Equator by way of simple visualisation.

The earth’s equator is the only part of the of the earth’s surface that is perpendicular to the earth’s axis, therefore all stars are visible from this band around the earth at 0° latitude. Over the course of 23 hours and 56 minutes the observer here will be able to see all visible stars rising from the eastern hemisphere. The South and North Celestial poles will be exactly south and north on the horizon from the observer respectively. Looking to the east from the equator the stars will essentially appear to rise and rotate upwards from the horizon and eventually set exactly opposite symmetrically downwards below the horizon into the western hemisphere. – Try and visualise this.

Imagine for a moment that all the stars are contained on the inside boundaries of an imaginary sphere – call this the “Celestial Sphere” as this is what is defined as the visible stars and objects surrounding an observer on Earth. This can be visualised by imagining you are in the very centre of a large circular ball, and all the stars are painted on the inside of the walls of the ball. Pretend that a large skewer is going through the ball horizontally through the centre from left to right (just missing you in the centre of course). Someone is turning the skewer and hence the ball, with their fingers towards themselves, the top of the ball rolling toward them. It takes 23 hours and 56 minutes for one revolution. Pretend that the stars below the level of your eye are below the horizon – From your viewpoint in the centre of the ball, you can see the stars in front of you rolling upwards and over the top of you and the stars more towards the sides rolling from the front to the rear - less distance is covered by the these ones. The stars at your extreme left and right will just circle around the skewer. In this example the skewer represents the spinning axis of the earth and penetrates the ball at the both the celestial spheres. This is analogous to what the visible stars will look like when viewed from the equator looking east.

The Celestial Equator is an imaginary line that follows the path on the Celestial Sphere that is directly above the equator on earth. Just as the earth equator splits the North and South Poles on Earth at 0° Latitude exactly between the two poles, so does the Celestial Equator with the South and North Celestial Poles. Declination is the term used to describe the angle of celestial positions, stars and other objects from the Celestial Equator, parallel to it. Just as the term Latitude describes the angle of surface positions on parallel bands derived from either sides of the equator on earth. The Celestial Equator is assigned as declination 0° and the Celestial Poles are at 90° Declination. (Declination is usually positive in the northern hemisphere and negative in the southern hemisphere however in this blog I will use positive declinations for the southern hemisphere.)

The Celestial Equator remains fixed from the observers viewpoint at the same location on Earth however changes its altitude when you move north and south by changing your latitude. The Celestial Equator above the horizon always starts (or finishes) due east or 90° and finishes (or starts) due west or 270°. It reaches its highest altitude at the point when it crosses the northern central meridian or to put it another way a point that is on an imaginary line that starts at the horizon due north and extends to the point directly above the observer (the Zenith). Please note that this method does not work at the South Pole, where the Celestial Equator follows the horizon exactly. The altitude in degrees of the point above the horizon on the northern central meridian where the Celestial Equator crosses is equal to 90° minus your current latitude. As an example let’s say you are at Tennant Creek in the Northern Territory. It is at a Latitude of about 19.6° South. The Celestial Equator starts at a point due east and follows a line along the celestial sphere to a point at an angle of 70.4° (90 – 19.6) above the horizon due north and continues in a line to a point due west.

Note: The celestial sphere is broken up into 88 sections known as constellations. The constellations are figures depicted by drawing imaginary lines between certain bright stars. Some commonly known constellations are Orion, Crux, Scorpio and Andromeda. By learning and identifying certain constellations you can easily find significant stars within these constellations.

If there is a star that is on or near the Celestial Equator such as Mintaka in the constellation of Orion then this star will be exactly east when it rises and exactly west when it sets. When Mintaka reaches the northern central meridian and you subtract its altitude in degrees above the horizon from 90 then you will have your current latitude however that requires a clinometer so is a bit beyond the scope of what I’m saying here.

Let’s go back now to the imaginary sphere, except this time the skewer is vertical. We are in the centre of the ball again and someone is rotating the ball towards themselves from the left hand side. Again pretend that the stars on the wall inside of the ball that are below the level of your eye are below the horizon. This time the stars will appear to rotate around the skewer clockwise directly overhead and the stars that are directly in line with your eye will rotate to the left in a horizontal fashion representing the celestial equator. This is what the celestial sphere will look like from an observer’s viewpoint standing on the South Pole.

So using the analogy of the imaginary ball one can visualise what the celestial sphere, the south celestial pole and the celestial equator appear to the observer at the South Pole and the equator. At the South Pole the south celestial pole is directly overhead at 90 degrees altitude (Zenith) and for every degree of latitude you travel north along the curvature of the earth from the pole, the pole decreases in altitude by 1 degree until you reach the equator when the pole is on the horizon at zero degrees. So therefore it figures that the south celestial pole is above the southern horizon in Australia by the current latitude that you are on. As an example, using the previous example of Tennant Creek which is at southern latitude of 19.6°, the south celestial pole will be at an angle of 19.6° above the point that is exactly south on the horizon. At South East Cape in Tasmania the south celestial pole will be 43.6° above the horizon and at Cape York in Queensland it will be 10.6° above the horizon.

In the Northern Hemisphere there is a star called Polaris, in the constellation of Ursa Minor that is almost exactly on the north celestial pole. When Polaris is identified, North can be found by simply imagining a line dropping from it, straight down to the horizon and this is where north lies. Polaris is not visible from Australia so this method cannot be used there and other methods are required.

The most common and popular method of finding the south celestial pole, and subsequently south in Australia is using the constellation of Crux (or Southern Cross). Before I describe how this is achieved I will endeavour to explain how stars follow lines parallel to the celestial equator. The previously mentioned star Mintaka in Orion is close to the celestial equator so is at 0° declination. This star will always appear to rotate clockwise around the south celestial pole but 90° from it. This is analogous to a point on the earth’s equator, which is 90° from the earth’s South Pole and as the equator is the largest circle of latitude, a point on this line of latitude has the greatest distance to travel in one revolution of the earth. So to Mintaka, on the celestial equator, has the greatest distance to travel on the celestial sphere.

As an example when looking at the Southern Cross the star to the left of the horizontal axis of the cross is called beta Crusis. Stars in constellations are officially named by their level of brightness using a letter of the Greek alphabet followed by the genitive form of the parent constellation’s Latin name with Alpha being the brightest and Beta the second brightest and so on. (Some also have common names. Mintaka for instance is called delta Orionis) beta Crusis has a declination of 59.75° however for the purposes of simple explanation we shall say it is 60°. As the south celestial pole is at 90° it means that beta Crusis will be 30° from the pole and rotate around it a circle, in a clockwise direction. Imaginary circles that surround the pole to the equator are called “Parallels of Declination”. In the following diagrams I have called them simply “circles” preceded by their declinations. As previously mentioned the earth rotates relative to the stars every 23 hours and 56 minutes so beta Crusis, or any star for that matter will appear to follow its circle and return to its original point after this time period.

When photographs are taken of the night sky with the shutter open for a period of time the parallels of declination become apparent, with stars leaving “curved trails” following their circles, the closer to the south celestial pole, the more curved the trails are.

Taking this concept a bit further, keeping in mind that the south celestial pole is at an equivalent angle above the southern horizon to your current latitude, some stars, and certain parallels of declination will always be visible in the sky and never dip below the horizon. Stars that never go below the horizon are called circumpolar. They appear to continually rotate around the celestial pole (remember that you will generally not be able to see them in the daytime however will still be above the horizon.) Stars, and parallels of declination are circumpolar if their declination plus your current latitude is greater than 90. Similarly, northern hemisphere stars will never rise above the horizon if their opposite declinations plus your current latitude is greater than 90.

Using our example of beta Crusis, and we will now revert to using its proper declination of 59.75° it can be calculated that it will be circumpolar to all latitudes south of latitude 30.25° (90-59.75). So it will always be visible in Sydney, Broken Hill, Port Augusta, Kalgoorlie and Perth. In Brisbane, Coober Pedy, Bourke and Geraldton it will not be visible all the time.

The constellation of Crux has 5 main stars, 4 of which are at the termini of the horizontal and vertical axis’s of the cross (The Southern Cross) and another, the less bright epsilon Crucis is situated (with the cross vertical) between the bottom and right stars delta and alpha Crusis. The horizontal axis of the cross is not exactly perpendicular to the vertical axis, is lies with a slight downwards lean to the left. By coincidence, the long axis of the cross almost points directly towards the south celestial pole in a direction below the cross. Remember that the cross could be aligned in any direction to the observer, depending on its current position on the circle. Gacrux, at the top of the cross has a declination of about 57° and Acrux, at the bottom is about 63° so the long axis of the cross spans a angular distance of about 6°. As the south celestial pole is at 90° there is an angular distance of 27° between it and Acrux. Therefore the south celestial pole is about 4.5 times the length of the long axis of the cross below Acrux. So to estimate the position of the south celestial pole it is only a matter of imagining a line extending 4.5 times the length of the long axis of the cross from Acrux. As mentioned the long axis is not exactly aligned with the south celestial pole, however this method will approximate the position to within 2°.

To give an idea of how wide an angular distance of declination is you can compare to the sun and moon which are both approximately one half of a angular degree wide.

When the south celestial pole is identified or approximated it is simply a matter of extending an imaginary line directly from the pole directly downwards towards the horizon. Where this imaginary line intersects the horizon is true south.



Using Orion as a guide:



Using Ursa Major as a guide:



This blog is ongoing and I will update from time to time.....

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