The little dot to the right of the sun in the picture above is Alpha Centauri, the closest star to the sun. It is a long way away. The middle stars of the big dipper were all made together and are travelling through space in the same direction. They are the closest “cluster” of stars to Earth. Dubhe and Alkaid (at either end of the dipper) are out on their lonesome, which is not true as there is another star (invisible to our eyes) that orbits Dubhe. Mizar and Alcor look to us like they’re close enough to go around each other but they’re not. Mizar is (however, actually) a system of four stars and Alcor, like Dubhe, is a two star system. Space is therefore a giant merciless collection of optical illusions and silly names.
circles – rotation, orbit and wobble
Earth is spinning and moving along a sloppy oval orbit around the sun. One full spin is a day, one full orbit is a year. Also the Earth is gyrating like a slowing down spinning top does. When a spinning top slows down the stick at the top goes around in circles. The performer at 1:24 of Ian Russell’s spinning-top circus is showing off a particularly fine example of this effect, which is called precession:
It is whilst doing all of this circle making that we look at space. Fortunately once the movement of the solar system is under control the stars become reassuringly static. There are only so many stars that a person can see with their eyes and as far as a person can see they will never change. It is very humane of our senses to create finite environments in an infinite universe. It is inhumane of the universe to make us do so many circles.
spin – the day and the stars
For this bit we are going to pretend that the sun doesn’t exist and so it is night time all the time. The stars are a fixed backdrop for the whole solar system. Different areas of this backdrop get revealed depending on which way your side of the Earth is facing when it is night time. In most of the world it is only after a full year that it is revealed what, without the sun, could be seen in a full day.
1 – Looking at space from here no space can be seen that is below because the planet is in the way. And (as the planet is a spinner that is spinning around its poles) the whole sky seems to turn around the point right overhead and the stars that can be seen are the same ones all night every night. It is like sitting in the middle of the roundabout. There is a bit at the top of the sky that doesn’t move and the whole sky spins around that bit even though it’s the roundabout that is moving. The star that is closest to that point right overhead that doesn’t move is the North Star.
2 – Here it is like being at the edge of the roundabout looking out at the playground. The playground all rushes past, none of it is still. When looking straight up at space from the equator the stars do not spin around a point, they move in a stream overhead and, when looking east, the new ones come straight up over the horizon and the old ones disappear straight down below the other horizon behind.
3 – This is like a mix of the two. Looking half way between straight above you and the horizon will be the North Star that the stars are moving around, whereas to the side and behind stars will be curving up and going down in an arch.
orbit – the year and the seasons
Now we will pretend that the sun does exist and that Earth is tilted, because those things are true. The direction of the tilt doesn’t change during the year.
^This is summer time in the north. In fact it is June 21st, the longest day of the year in the northern hemisphere and the shortest in the southern. The north is as tilted towards the sun as it can get. But what does that mean for looking up?
1 – Here it is the shortest night. Most of the time is spent in the day with the sun hiding the rest of space.
2 – It is the exact opposite here. It is the longest night of the year, June 21st. It is a great night for space, a whole bunch can be seen as the sun is invisible for most of the spin, but it is winter so everyone is sad.
The tiny red peg on the right of the Earth is where the sun is right overhead at the very top of the sky (where the North Star appears to be at the North Pole). It is the northest part of Earth where the sun can get to be at the top of the sky. The line that the sun draws here, parallel to the equator as Earth spins, is called the Tropic of Cancer. Six months on from this picture Earth will look the same but it will have moved around the other side of the sun, so the sun will be on the left. Then as Earth spins the sun will draw a line along where the blue peg is, the Tropic of Capricorn. These two times of the year, the longest/shortest days, are solstices.
Half way between solstices the Earth will be tilted neither towards nor away from the sun and all over the world day and night will be the same length. The two times of the year when this occurs are equinoxes. The yellow blob on the equator is where the sun is directly overhead during an equinox. Solstices and equinoxes are officially when the different seasons begin, although everybody has their own opinion of when the different seasons actually begin.
This image also demonstrates why you see different stars throughout the year. In this picture the stars on the left are visible, in six months the stars on the right will be visible.
plane – the solar system and the zodiac
Back in the playground the sun is sitting in the middle of the roundabout and we are the Earth, sitting on the edge of the roundabout and looking at the sun. As the roundabout goes around, the edge of the playground seems to rush past behind the sun. The sun is looking at Earth too and the edge of the playground is rushing past behind Earth. Around the edge of the playground there are twelve constellations of stars loitering like smoking parents, they are the zodiac constellations. The other planets are also on the roundabout but they are moving at different speeds so it is a special roundabout.
The imaginary disc that goes from the sun in the middle to the zodiac constellations at the edge is called the ecliptic.
This picture is things on the ecliptic. Mars, Venus and Earth are the coloured blobs. The stars are where the zodiac constellations are. I’ve made it so Earth (bottom left) is standing up straight, which has made the ecliptic tilted instead. I thought this might help to show how the sun, planets and constellations can appear to be high or low in the sky. It is still June 21st so the sun is high in the northern hemisphere and the visible zodiac constellations on the other side of Earth are low.
wobble – the precession of the poles
The Tropic of Cancer is called that because back when the tropic was named the sun appeared to be in front of the constellation Cancer on the summer solstice. These days it is in front of Taurus on June 21st. Your star sign corresponds to which constellation the sun was in front of when you were born several thousand years ago. If you were born more recently than that there’s a good chance you are not the star sign you think you are because the dates for the star signs have not changed because star signs are meaningless. Every year on the same day the sun is over a slightly different place on the zodiac because of the precession.
The North Pole is precessing around in big slow circles. The tilt of the planet is Earth appearing to be frozen whilst doing the wobble but it’s not frozen it’s just slow. It takes 26,000 years for the North Pole to do one circle. This means that the direction of the tilt is slowly moving in a circle and so the position that Earth is in on its orbit when the North Pole is tilted directly towards the sun is slowly moving in a circle and so the part of the zodiac that the sun is in front of at the summer solstice is slowly moving in a circle.
As Earth precesses the point of space that is right above the North Pole and stays still while the stars spin around it slowly draws a big circle in space. The stars remain a static backdrop so over 26,000 years different stars that sit on the path of the circle become the North Star but most of the time there is no North Star.