Star Spy: Sky in rotation

22:48, Nov 11 2012
Morning sky
Morning July sky in Central Otago.
Morning sky
Morning July sky in Central Otago
Morning sky
Morning November sky in Central Otago
Morning sky
Morning November sky in Central Otago


You may recall this beautiful image of the pre-dawn sky that Fraser took about four months ago on a chilly July morning. You may also recognise that same sky from the photo he took about midnight just two weeks ago. 

Spot the differences. Jupiter has changed position with respect to the background stars, and Venus is not hidden behind the Church, it is gone. What happened in the intervening four months?

First, the stars have been rotating slowly from east to west across the sky. In July this scene had just risen in the east right before sunrise. Now in November it is well up and heading around toward the north in the middle of the night. 

If you could watch the stars rotate around the South Celestial Pole for 24 hours, you would see that they make one complete rotation plus almost an extra degree.

This is because in that same 24 hours Earth has moved almost a degree around the Sun; 360 degrees in a circle, 365 days in a year. 


This means that the stars rise and set almost four minutes earlier each night, which means that the summer sky that was rising about 5am in July and then disappearing in the daylight sky, is now, four months later rising about eight hours earlier and crossing the sky at night. 

It means that the sky slowly changes throughout the year; constellations come and go and come back again next year. 

The positions of planets in the sky are also affected by Earth’s rotation, but they also move separately from the stellar background because they are in orbit with Earth around the Sun, some faster and some slower. 

Jupiter has moved very slowly eastward against the background of stars, while Venus has remained close to the Sun and is still there in our pre-dawn sky.  Watch the sky to the north as this view moves into our early night for summer. 


The Moon, as we know, is a tiny fraction of the size of the Sun, but because it is so much closer to us, it appears as large as the Sun in our sky.  It wasn’t always so, nor will it always continue to be so. 

The Moon used to be closer to us than it is now, and would have appeared larger in the sky than the Sun. As it moves farther away from Earth at a rate of about 4cm per year, it is getting smaller in the sky every year. 

Not that you will notice that difference in your lifetime. Even if you live to be 100, the Moon will have moved only 4m farther from Earth than it was when you were born. 

Such a small shift in distance will only begin to make any significant difference in the Moon’s apparent size over many millions of years. It will appear to have shrunk about one per cent in 100 million years.

However, because we live during an epoch when the Moon and Sun are about the same size in the sky, we occasionally get to experience amazing total eclipses of the Sun in which the Moon fully blocks light from all but the million-degree outer layer of the Sun’s atmosphere, the corona. 

The path of a total eclipse across the Earth is narrow, generally about 100km wide. However observers in a much wider band, either side of the total eclipse, see a partial eclipse. 

The farther you are from the narrow band, the smaller the part of the Sun that will be covered.       

On Wednesday morning, all of New Zealand will be in the wide region of a partial Solar Eclipse. 

The path of total eclipse will move across small parts of northern Australia and then across the southern Pacific Ocean toward Chile, but will not make landfall again. 

Wherever you are in New Zealand, your time of maximum eclipse will occur between 10.21 and 10.36am.

In Invercargill a maximum of 58 per cent of the Sun will be blocked at 10.35am. In

Whangarei the eclipse will reach 89 per cent at 10.25am.   

Of course, the really important thing to remember is not to look at the Sun without an approved filter, not even an eclipsed Sun. 

You probably have a local Astronomy Club you can contact for advice.

 They may even have special solar telescopes set up for public viewing.