Global choices – desirable features for classroom globes

September 29, 2010

Maybe it was the fall Sun angle coming into my greenhouse that got me thinking about globes. A while back I decided to replace my old globe. After all, it still has the USSR on it. It is a Cram’s Imperial World Globe, and it has two features that I want in a globe, the analemma and the ecliptic.

The best features for a globe depend on what you want to show with it. It would be hard to find a globe that has everything. A classroom probably needs at least two globes, one for the Earth in space and the physical features of the Earth, and another that shows the countries, longitude, latitude, and time zones clearly. Of course, beginners need a simple globe, like the ones found in many Montessori early childhood classrooms that show the oceans and continents without labels. Elementary and older children will likely enjoy the complex globes with more in-depth geography information or with the ecliptic and analemma.

The analemma and the ecliptic are important for studies of the Earth in space and the relationship between the Earth and Sun. Before I get into what these two markings are, I found a couple of other interesting globes in my search.

Replogle Globes has a “blank” political globe that can be labeled with a dry erase marker. The act of labeling it would certainly help children consolidate their geographic information. This globe is called the Geographer. Replogle also has a relief globe, called the Atlantis, that has ocean features indented, along with raised elevations.

Back to markings for study of Earth in space, let’s start with the simpler one, the ecliptic. The tilt of the Earth’s axis means that the Sun’s rays strike the Earth at different places as the Earth rotates. If there was no tilt, the Sun would always shine directly at the equator. As it is, that only happens twice a year, at the equinox times.

Now, in late September, the tilt of the axis has begun pointing the Southern Hemisphere at the Sun, and the point at which the rays shine directly on the Earth is moving south, toward the Tropic of Capricorn. At December solstice, the Sun’s rays will shine directly on this tropic. In the Southern Hemisphere, the sun will be more overhead. In the Northern Hemisphere, the Sun will appear to move south and hang low in the sky. As the year progresses from December solstice to June solstice, the Sun will appear to move to the north and will be more directly overhead in the Northern Hemisphere in June, as its rays strike directly on the Tropic of Cancer.  

The ecliptic marking on a globe shows where the Sun’s rays are hitting directly on the Earth for any day of the year. On September 29th, this point is just south of the equator, as we have just passed September equinox. Traditionally the ecliptic is placed on the globe so that it crosses the equator at the Prime Meridian (zero degrees longitude) and the International Date Line (180 degrees longitude). The ecliptic marks the apparent path of the Sun and its plane is the plane in which the Earth orbits. When the Moon is near this imaginary line, eclipses happen, hence the name “ecliptic.”

This is the analemma on my old Cram Imperial globe. The diagonal line near the bottom is part of the ecliptic. It shows where the Sun shines directly overhead in the month of November.

If the orbit of the Earth was circular instead of elliptical, then the ecliptic marking would be all we need on a globe to show the relationship of the Earth to the Sun. As it is, Earth’s elliptical orbit makes the analemma necessary. The analemma is the odd figure 8-like object that is usually printed in the Pacific Ocean. That’s where there is enough “blank” space to put it. Its pattern results from two things, the elliptical orbit of the Earth plus the tilt of the Earth’s axis. If you had a nail or rod in a board, and if you marked where the shadow of the nail head lay on each day at noon, the dots would make a shape like the analemma. The name “analemma” comes from a Greek term that essentially means “something that sticks up” and refers to a sundial.

Kepler’s laws of planetary motion say that the planets have elliptical orbits, and the Sun is at one focus of the ellipse. Furthermore, if you drew a line from the planet to the Sun, the line would mark equal areas over equal time periods. What this means is that the Earth moves faster when it is in the end of the ellipse nearer the Sun and slower when it is farther away. You can see this in an almanac – there are five more days from the March to the September equinox than from the September to March one.

As the Earth’s orbit speeds and slows, our clocks run at a constant rate, which makes them get out of sync with the Sun. The Sun sometimes reaches its zenith before the clock says it is noon, and sometimes the Sun doesn’t get directly overhead until afternoon. The clock and the Sun agree only 4 times a year, at the solstices and around mid-April and early September. You can read the difference between the clock and the Sun on the analemma.

Knowledge of the relationship of the Earth to the Sun gives us a greater appreciation of our wonderfully complex planet. Globes with the ecliptic and analemma help children learn more about this.

It looks like I will be keeping my old globe for a long time. In my quest for a new globe, I spoke with a cartographer with Cram globe makers (now a subsidiary of Herff Jones Education) who told me that the ecliptic was removed from globes when they were converted to digital images, over 10 years ago. He also said that many Cram globes still have the analemma. If you are looking for a new globe, I suggest that you see it before you buy or else be able to return it if it doesn’t have the features you want.

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