The word 'influenza' comes from the word 'influenced', and refers to the fact that people once believed your health was influenced by the positioning of the stars. Today we understand more about bacteria and viruses and know the stars have no direct influence over anybody's health or wellbeing. They can still tell us a lot about our planet, though.
You will need
Notepad
Ruler
Watch (with seconds)
Chalk
Compass or map
What to do
Go out one night when the stars are out. Find a place with a clear view of at least one star and mark where you are standing with an 'X' so you know precisely where to stand the next night (you can use any object to mark that spot, such as a stone or a chair).
Find east using your compass, or map and a landmark. Choose a star in that half of the sky and align it with a landmark such as the horizon, a hill or the roof of a nearby building. Note the time on a notepad as precisely as possible, down to the nearest second.
Hold the ruler straight out at arm's length and measure in millimetres the distance your star is above the landmark. Write this number down.
Return to the spot the next night at the same time. Repeat the measurement, writing down the exact distance.
Repeat this over a series of three nights. Don't worry too much if you miss a night due to bad weather. Just start counting again next time you have another clear night.
What do you notice about the star's position each night? How much does it change by?
What's happening?
One complete rotation is 23 hours 56 minutes. As Earth moves in its orbit, the sun changes its relative position, meaning that for it to be in the same place at the same time each day (such as straight above us at noon), the Earth needs to rotate for another four minutes.
As the night progresses, stars appear to move across the sky, rising in the east and setting in the west just like the sun. This is due to the Earth rotating on its axis. You can't see the stars during the day because the sun's glare overpowers their own light.
A full turn of the Earth is what we describe as a single day. As this is a full 360 degree turn, you should face the exact same point at the same time every day, meaning the stars should be in exactly the same place at the same time. However, one full day is not exactly 24 hours! It is actually 23 hours 56 minutes. When you measure the star each night at the same time on your watch, the star's position is running four minutes behind the previous night as your watch is running on 24 hour time (not 23 hour 56 minute time).
If we were just using the stars, we'd be using 'sidereal' (from Latin for stars) time.
So why do we count 24 hours in a day, then? That's because we don't use the stars for time, but rather our sun, which is at its highest in the sky once every 24 hours.
After one sidereal day (one complete rotation), the Earth still needs to rotate further – for another four minutes – for the sun to be at the same position as it was at that time the previous day. This is because the Earth's position relative to the sun changes slightly as the Earth moves around the sun in an orbit.
Applications
Using the stars to make calculations about our place in the universe goes back to ancient times. One function of great structures such as Stone Henge in England was probably as landmarks to map how the stars changed positions with the seasons. As you can see in the activity, simple comparisons between how the stars move with how the sun moves can give very precise measurements about time and how our planet moves in relation to the sun, planets and other stars.
Navigators throughout history have used a mix of the star's positions, compasses and time to tell precisely where they were in an ocean devoid of landmarks. Sextants, rather than rulers, were instruments which measured the position of a star from the horizon.
A full orbit does not take exactly 365 days, but rather 365 days and nearly six hours. This means every four years during a leap year an extra day is added to February to catch up. It's not quite that simple, however. I said 'nearly' six hours. So every 100 years (the next being 2100), when it should be a leap year, it won't be.
