Over the year, the sun follows a path known as an annalemma. This is a figure eight shape and is caused by the combination of the Earth's rotation around the Sun and its tilt relative to the plane of the equator.
The analemma can be observed by pointing a camera at the sun, and at the same time each month, take a picture and over lay the images:
Taking a picture every day for a month (in September) would mean that you'd see it lower in the sky, curving slightly as it followed the figure of eight shape of the analemma.
Note:
An analemma can be different on different planets. For example, a Martian analemma is different because its tilt and shape of orbit is different to that of Earth's:
- Mercury: Because orbital resonance makes the day exactly two years long, the method of plotting the Sun's position at the same time each day would yield only a single point. However, the equation of time can still be calculated for any time of the year, so an analemma can be graphed with this information. The resulting curve is a nearly straight east-west line.
- Venus: There are slightly less than two days per year, so it would take several years to accumulate a complete analemma by the usual method. The resulting curve is an ellipse.
- Mars: teardrop
- Jupiter: ellipse
- Saturn: technically a figure 8, but the northern loop is so small that it more closely resembles a teardrop
- Uranus: figure 8 (Uranus is tilted past "sideways" to an angle of 98 degrees. Its orbit is about as elliptical as Jupiter's and more elliptical than Earth's. )
- Neptune: figure 8
For more, read this.
