The Southern Cross is a constellation that is often shown to those learning about the night sky. It can easily be recognised by the obvious cross shape and by the 2 bright pointer stars to one side.
However, without understanding how the stars rotate in the southern skies, it can be very difficult to recognise this tiny constellation (it is the smallest of all the 88 constellations) a few months later or even a few hours later. If for example the Southern Cross was pointed out when the pointers were below the cross, 12 hours or 6 months later the pointers would be above the cross, making the constellation unrecognisable or at least confuse the viewer.
The Earth turns around an axis. This means that the stars apparently pass overhead in great circles. The closer to axis (north or south) the star is, the smaller the circle it scribes, until at the point in the sky where the imaginary axis extends to, the stars do not move at all. This is why the Pole Star in the northern hemisphere is an ideal pointer to the north - it is at the end of the northern axis. There is no such convenient star in the southern hemisphere and so the Southern Cross is a convenient pointer to find south, as is explained below.
A model can demonstrate how the Southern Cross moves during course of the night. Of course, it continues to move during the day but then it cannot be seen.
The model shown in the photograph has stars in the shape of the Southern Cross and the two pointers on a disk which can be rotated around its center. The pole around which the disk rotates represents the Earth's axis. Rotate the disk and see how the Southern Cross changes its orientation. One complete rotation represents 24 hours. Note how in 12 hours ( half a rotation) the Southern Cross turns around by 180 degrees.
If using the model under the night sky, point the model axis towards the south and upwards at an angle of a bit less than 30 degrees. Rotate the disk until the model cross aligns with the real cross. Rotating the disk clockwise shows how the Southern Cross will move during the night.
Note that the long axis of the cross always points to the center of the disk. That point is called the South Celestial Pole (SCP) and is vertically above South. But in the real sky there is no marker to the SCP. We know that the long arm of the cross points towards the SCP, but how far away is it?
If you were to extend the length of the long arm of the cross another 4 and a half times in the direction of the bottom end of the cross, the end of that extension is where the SCP is. Directly below that estimated point in the sky is the direction in which true south is.
The Starwaders Southern Cross Motion model can be purchased for R150.
Send an email to firstname.lastname@example.org with your contact and postal address details. State which models you are buying and the total price.
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