Experiment

In the How Long Is the Day activity, we focused on how the length of the daylight hours is different for different places on Earth. We also looked at how the times of sunrise and sunset change throughout the year.

Let’s take a look at the sunrise and sunset times for December 21, the shortest day of the year in the Northern Hemisphere. At a latitude of about 40° north of the equator (near the Mediterranean Sea, the central part of the United States, or the Yellow Sea, for example), sunrise would be about 7:40 a.m., with sunset at about 4:55 p.m. (16:55 military time).This gives a total daylight time of 9 hours and 15 minutes for this latitude.

You might think that this short day would be caused by the latest sunrise and the earliest sunset; however, that is not the case! The day with the latest sunrise, around 7:43 a.m., is January 4, and the earliest sunset takes place on December 7 at around 4:52 p.m. (16:52 military time). How can this be?

Two factors produce this curious situation. One is the tilt of the Earth’s axis of rotation. The other is the fact that the Earth’s orbit around the Sun is not quite circular. Instead, it is slightly elliptical, or oval. Together, these two things produce a shift in the apparent path of the Sun throughout the year. This shift creates the surprising situation mentioned above.

If you could mark the position of the Sun in the sky each day at the same time, what would be the result? You might think that the Sun would appear at the same place in the sky at the same time each day. This would be true if the Earth had no tilt and if its orbit were a perfect circle. As we know, this is not the case. Let’s look at the tilt of the axis and the shape of the orbit in a bit more detail.

The tilt of the Earth changes how high the Sun appears to be in the sky throughout the year, as seen from a specific place on the surface of the Earth. When viewed from a location in the Northern Hemisphere, as in the diagram here, the Sun is at its lowest position in the sky at noon in December and at its highest position in the sky at noon in June. For the Southern Hemisphere, the opposite is true. For locations at least 20° in latitude north or south of the Equator, this vertical movement during the year is quite apparent.
The tilt of the Earth also has an effect on the Sun’s horizontal position during the year as seen at the same time each day. First, let’s look at the path of the Sun across the sky on a typical day. The diagram here shows this movement as seen from a location in the Northern Hemisphere. This apparent movement is caused by the rotation of the Earth. At the noon position, the Sun is at its highest point in the sky for any given day.
The tilt of the Earth causes the Sun to reach its noon position (its highest position in the sky on a given day) a little before or a little after 12:00 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 in the sky a few minutes before local noon.
This means that at 12:00 in other months the Sun will not have yet reached the same noon position in the sky as the June or December sun. These Sun positions at 12:00 are shown in the diagram below. Similarly, between March and June and between September and December, the Sun reaches its highest point in the sky after local noon. In this diagram and the one below, the horizontal distances are exaggerated so that you can see the highlighted effects clearly. In the actual sky, the horizontal distances are not this great; however, they are easily noticeable. Can you trace out the general path of the sun in the diagram here through the course of a year? How would you describe its shape?
The orbit of the Earth is nearly circular, but it is elliptical (oval) just enough to cause the orbital speed of the Earth to change throughout the year. This changing speed affects the position of the Sun in the sky. Earth is closest to the Sun in January. During the period from around October to February, the Earth’s orbital speed is much faster than at any other time of year.
This increased speed causes the Sun to reach its noon position in the sky earlier in January and later in November than the time caused by the Earth’s tilt alone. In the other half of the year, as the orbital speed of the Earth is slower, similar effects can be observed as well. The diagram here shows in lighter colors the Sun positions due to the tilt alone. The positions shown in darker colors take into account the effects of both the tilt and the changing speed due to the shape of the orbit.

It is easier to understand the effect of the tilt and slightly elliptical orbit when we look at it graphically. In the course of a year, the apparent location of the Sun—viewed from the same location at the same time each day—traces out a path shaped like a figure eight in the sky. Thisinvisible path is called an analemma. The height of the analemma is due to the tilt of the Earth, and its width is due to both the tiltand the elliptical nature of Earth’s orbit. You can see the shape of the analemma in this famous photograph taken by Anthony Ayiomamitis, an amateur astronomer in Athens, Greece. Over the course of a year, he took a photograph of the sun about once a week from exactly the same place and at the same time of day (in this case, 12:18), with his camera fixed in the same position. If he had taken the photographs at an earlier time in the day, the analemma would have been tilted to the left.

Anthony Ayiomamitis

At the top of the photograph are shown the positions of the Sun in the sky in June and July, while the positions in December and January are shown at the bottom.

Putting It All Together

The combined effects of the tilt and the orbit shape produce the analemma which in turn, helps us to understand why the earliest sunset is in early December and the latest sunrise is in early January. The early morning Northern Hemisphere analemma (created by observing the position of the Sun at the same time early each morning) is tilted to the left, while the late afternoon analemma is tilted to the right.

Are you ready to find out what the analemma looks like where you live? It’s not difficult to do. Instead of a camera, you can use either reflected light or ordinary shadows. Since this is a yearlong project, you may find it easier to work indoors, depending on your climate. If you live in a region that does not a lot of snow or rain during the year, you may wish to work outdoors instead. We suggest that you read through the directions for both projects and then decide which approach would work best for your particular climate and setting.

Review the Indoor Project and Outdoor Project instructions. Then get started on charting your local analemma.