Adapted with permission by SEED.
The sun rises in the east and sets in the west. This we know from personal observation. But have you ever thought about where the sun is in the sky at the same time each day?
You might think the sun would appear at the same place at the same time each day. This would be true if Earth had no tilt and its orbit were a perfect circle. But this is not the case.
The tilt of the Earth changes how high the sun appears throughout the year, as seen from a specific place on Earth’s surface. When viewed from a location in the Northern Hemisphere, for example, the sun is at its lowest noon position in December and at its highest noon position in June.
Earth’s tilt also causes the sun to reach its noon position (its highest position in the sky) a little before or a little after 12 o’clock noon, local time, on all but a few days of the year. For example, between December and March and between June and September in the Northern Hemisphere, the sun reaches the “noon position” a few minutes before local noon.
The orbit of Earth is nearly circular, but it is elliptical (oval) just enough to cause the orbital speed of Earth to change throughout the year. Earth is closest to the sun in January. During the period from around October to February, Earth’s orbital speed is much faster than at any other time of year. This changing speed affects the position of the sun in the sky and how much earlier or later the sun reaches its noon position.
The result is the analemma, a figure-eight shaped path of the sun in the sky over the course of a year. This year-long activity is designed to make visible this remarkable aspect of the Earth-sun system, one of the most striking patterns in nature. You can trace this pattern — in only a few minutes each week!
- Flagpole or other vertical structure on the south side of a flat area
- All-weather marking paint
- Clock that measures to the nearest second
- Find a flagpole or other vertical structure that casts a shadow on a flat area such as a parking lot. You will trace the path of the shadow at the same time of day for a year. Obtain permission to use the flat area for about 15 minutes once a week, and to paint marks on it.
- Select a convenient time of day for your shadow marking. Usually, the best results are obtained around midday.
- When the sky is clear at your selected observation time, paint a mark on the flat surface to indicate the position of the shadow. Do not worry if there are cloudy or rainy days. One observation every week or so is sufficient.
- If you live in an area where daylight saving time is observed, you will need to decide which time to use for your observation. The shifted time will produce an interesting offset path, while keeping the same “sun time” will give you a single coherent analemma figure.
- Discuss: As the year progresses, what do you see? Why?
- SEED offers a detailed version of this activity, including a method for producing an indoor analemma: http://www.planetseed.com/laboratory/experiment-what-path-sun
- Science Blogs offers a more detailed explanation of the shape of the analemma: http://scienceblogs.com/startswithabang/2009/08/26/why-our-analemma-looks-like-a/