2017 November 1
The turning sky
Sitting at my computer, writing this article, I would appear to be fairly stationary, hardly moving at all. Nothing could be further from the truth. The Earth, turning on its axis, is spinning me around at over 1000 kph (in the latitudes of the British Isles). As our planet circles the Sun in its yearly orbit, it carries me along at well over 100,000 kph. On top of that, the Sun itself is circling the centre of the galaxy at around 780,000 kph. Lastly our galaxy, along with its neighbours, is flying through space towards something called ‘The Great Attractor’ at no less than 3,600,000 kph.
In this article we will discuss how the first two of these motions affects our view of the sky, hour by hour, night by night and season by season. We will see that although the Earth turns on its axis in 24 hours with respect to the Sun, it actually turns through 360 degrees in less time than this. Lastly we will look at how different parts of the heavens are visible from different latitudes.
The Turning Earth
As day turns to night and back again it is easy to see that the Sun and Moon move across the sky, rising in the East, appearing at their highest on the meridian, the imaginary line joining the North and South points of the sky, and setting in the West. Less obvious, unless you know your constellations, is the fact that the stars also participate in this nightly procession. All of these movements are caused by the same thing, the Earth turning on its axis from West to East.
Figure 1 shows the Earth as seen from above the North Pole. Early in the evening, looking in direction “1”, the red stars in the lower part of the diagram are overhead, the white ones in the centre are just rising and the green ones in the top part are below the horizon.
As the Earth turns and we reach midnight, facing in direction “2”, the red stars are setting, the white ones are overhead and the green ones are rising.
By the morning hours, direction “3”, the green stars are more or less overhead, the white ones are beginning to set and the red ones have now sunk completely below the horizon.
Let’s imagine a simple experiment. Suppose you went out at a certain time during the day and carefully checked the position of the Sun with regard to a nearby object, perhaps when it is directly over a prominent building. Note down the time. If you repeat the experiment the next day and once more note down the time you will find that exactly 24 hours have elapsed – one full Solar Day. Actually because the Earth’s orbit is not quite circular there may be a slight difference but for our purposes we can ignore that here.
Now consider repeating the experiment but at night using a bright star instead. When you compare your two timings you will find that they are not 24 hours apart but very close to four minutes less – 23h 56m. Why is this? Because from one day to the next the Earth has moved round in its orbit and our planet has to turn a little bit extra to catch up and replicate the apparent position of the Sun. Figure 2 shows this.
Imagine you are standing where the red dot is. On Day 1 the Sun is due south at noon. By Day 2, with respect to the stars, the Earth has turned 360 degrees in 23h 56m (white circular arrow) but the Sun is still short of where it was in the sky the day before. The Earth needs to turn a little bit more (red arrow), about one degree or 4 minutes of time, to bring the Sun back overhead and complete one full solar day.
The period of about 23h 56m is called the sidereal day. Indeed there is a whole time system called sidereal time based on a day of 23h 56m which is used to determine which stars are in the sky at any given time.
This combination of the Earth’s rotation on its axis and its orbit around the Sun creates one ‘extra’ rotation of the Earth with respect to the stars as opposed to the Sun. In other words there is one more sidereal day in the year than there are solar days.
What this means in practical terms is that from night to night any given star will rise about 4 minutes earlier, cross the meridian 4 minutes earlier and then set four minutes earlier. All this adds up to about two hours a month or 24 hours a year. In the next section we explore in more detail how this affects the changing appearance of the sky.
Around the Sun
As we have just seen, the combination of the Earth’s rotation and its orbit around the Sun causes the stars to move slowly with respect to the Sun in the sky. Let’s explore what this means in more detail by considering a specific example as seen from the latitudes of the UK.
- In August the bright star Betelgeuse in Orion rises shortly before the Sun and can be seen low in the eastern sky during morning twilight. Preceding the Sun across the sky it sets while it is still daylight.
- As the Earth continues its journey around the Sun, Betelgeuse rises and sets four minutes earlier each day. Go out a couple of months later and you will find Betelgeuse now rises in a dark sky and is high up by the start of morning twilight. It continues to set in daylight.
- By early January Betelgeuse clears the horizon around sunset, reaches the meridian well before midnight and sets in the west in morning twilight.
- Rising and setting earlier each day it ultimately slips into the evening twilight and finally disappears once more into the Sun’s glare during April.
- It remains hidden from view close to the Sun until once more it reappears in summer’s morning twilight starting the cycle over again.
If you have a smartphone with a star chart app you can replicate this motion very easily. Set the display to show the sky at a particular time of night. Then set the time increment to one day and run it forward. You will see the constellations processing slowly from East to West. Figure 3 shows the well-known constellation of Orion at the same hour of the night in three different months.
So from season to season, at any given time, a different set of constellations will be on view. In the northern winter Orion the hunter dominates the evening sky. By spring he will be close to the western horizon and Leo will take his place. Come northern summer, Orion has completely disappeared, Leo is setting in the west and the so-called Summer Triangle, composed of the brightest stars of the constellations Lyra, Cygnus and Aquila, hold centre stage. With autumn these constellations are heading west and in the south stands the great square of Pegasus. If you look low towards the eastern sky there will be a small fuzzy patch of light that careful examination will resolve into individual stars. This is the lovely star cluster of the Pleiades or Seven Sisters. Its appearance in the northern hemisphere’s autumn sky is a sure sign that winter will soon be upon us and harbingers the return of Orion once more. Indeed by midnight in the middle of October the mighty hunter is once more clear of the eastern horizon bringing the sky and the seasons round full circle.
The Moving Stars
Let’s now look a bit more closely at how the stars move across the sky as so far we have taken a rather simple view. Thinking about it, it is clear that not every star could rise exactly in the East and set precisely in the West. Rather, stars will rise at all different points along the horizon in the eastern half of the sky and set at different points along the western half.
As the Earth turns, its axis points to a fixed position in space, the celestial pole. In the northern hemisphere this point is closely marked by Polaris the Pole Star. Figure 4 shows this and also the fact that as the Earth turns stars will appear to rotate about a point centred on the celestial pole. Wherever you live the celestial pole will be at an altitude above the horizon equal to your latitude. So from where I live in eastern England the pole is about 52.5 degrees above the horizon which corresponds with the latitude of my location.
The closer stars are to the north celestial pole, the further north they will rise and set. Conversely looking south, stars that are more distant from the northern pole rise and set closer and closer to the south and reach progressively lower in the sky at their highest point when crossing the meridian. Eventually you reach stars that just graze the very southern horizon, stars further south than this never ever rise from our latitudes.
Stars not too far from the pole move in circles that are not large enough to reach down to the horizon. The red star in figure 4 is an example of one of these. Stars like this are called circumpolar and which ones they are depends on how high in the sky the pole is and therefore on your latitude. So from the UK the well-known constellation of Ursa Major, the Great Bear and its brightest part, better known as the Plough or Big Dipper, never sets. Travel south to say North Africa and this is not the case, much of the Great Bear will dip below the horizon for part of the night, rising and setting as many other constellations do.
From Pole to Pole
We’ve seen how the Earth moves as seen from space; let’s now consider how the view changes as we travel the globe. As we make this journey we will ignore the obscuring effects of the Earth’s atmosphere and assume that any star that sits close to the horizon is just as readily visible as it is when high up. We will also assume that the horizon is perfectly flat and free from obstructions like trees or buildings.
Let’s start by looking due north from the latitudes of the British Isles. As we have said, wherever you live the celestial pole will be at an altitude above the horizon equal to your latitude. Figure 5A shows this. The celestial pole is over half way up the sky. The red star is circumpolar and never sets. The yellow one, further from the celestial pole rises in the eastern part of the sky, setting in the western part it continues its journey below the horizon, passing beneath the North celestial pole before rising once more in the east. The green one never rises at all.
Travelling northwards, the pole star would appear higher and higher in the sky until in the arctic wastes of the North Pole (latitude 90oN) the celestial pole would be directly overhead as in Figure 5B. Now the stars travel in circles parallel to the horizon, neither rising nor setting, all stars visible in the night sky are circumpolar. Stars in the southern half of the sky are permanently hidden, never rising above the horizon.
Heading south once more the north celestial pole would sink lower in the sky. Fewer and fewer constellations would remain circumpolar while those that were low in the south would rise higher and higher and previously invisible constellations would come into view.
On reaching the equator, the north celestial pole would lie exactly on the northern horizon. Turning through 180 degrees to face south we would find the south celestial pole sitting on the southern horizon. The stars rise and set vertically and every star in the sky would in theory at some time be visible, nothing is circumpolar and every star will rise and set.
If we were then to carry on further south the north celestial pole would sink below the horizon while the southern one would rise higher and higher in the sky. In Figure 5C the green star that was never seen from mid-northern latitudes is now visible nearly all night long while the red star never rises at all.
If we carried on until we reached the South Pole (latitude 90oS) the celestial pole would then be directly overhead and, just as at the North Pole, the stars would travel parallel to the horizon never rising or setting.
Capturing the Earth’s Rotation
Returning to mid-northern latitudes and facing north once more we see Polaris a little more than half way up the sky, very close to the celestial pole. As the hours pass and the earth turns so all the stars appear to move around the sky in circles centred on the celestial pole. Polaris turns around the pole in a very tight circle, stars further away in wider ones.
It is possible to photograph this motion with a fixed camera and a time exposure. The stars are drawn out into trails by the Earth’s rotation and by pointing the camera towards the pole a series of concentric arcs will trace out our planet’s motion as it turns in space. Figure 6 is a fine example of this.
Of course you can image star trails in any direction and from any latitude but the circumpolar trails are always the most interesting and informative.
If you would like to take such an image yourself there is an article here by James Dawson giving instructions on how to do so.
You may be wondering if the other two motions mentioned at the start of this article also affect our view of the sky. The answer is not really. The motion of the Sun and other stars around the galactic centre does cause a very slight shift from year to year in stellar positions, the so-called proper motion. However, this is so small that it is not distinguishable to the unaided eye over the course of a human lifetime. As to the galaxy’s own motion through space, the distances involved are so vast that even at its truly rapid velocity any changes are essentially impossible to detect.
In this article we have reviewed how the combination of the Earth’s rotation about its axis and its orbit around the Sun provide different views of the stars at different times of the night and different times of the year. In future articles we will look at constellations, star names and brightness, how to find our way around the night sky and which ones are visible at any particular time.