Weekly Online Lesson Online Lesson Archive Grade Level: 4-10 Subject: Astronomy

Our Solar System

For the past several weeks, and for the next week or two to come, you will be treated to a rare astronomical sight simply by gazing up at a clear westward sky an hour or so after sunset. Five naked-eye planets — Mercury, Venus, Mars, Jupiter, and Saturn — have lined up in an almost straight row relative to Earth, and during the first week of May they will appear to be coming closer together.

An alignment like this happens only once a century, when the planets are on the same side of the sun and their paths join up. In fact, there will be only three other chances in the coming 100 years to see five planets so tightly grouped, in September 2040, July 2060, and November 2100 (maybe your grandkids will catch that last show).

In this week's online lesson you will learn about the coincidental parade of planets marching by, and you will see dramatic views of our solar system.

Space.com

For the best coverage of the planetary alignment, visit Space.com's Spacewatch page (note: if a quiz pops up, ignore it and close the window—it's not part of this lesson). Look for the most recent Backyard Astronomy feature (the latest at the time of this writing was April 26).From this page you can click on the Images links on the right, one to see a Sky Map, and the other to see a view of the planets' orbital positions. The first shows you where the planets are when you look out to the west — use this as a guide at night to help you find the planets. The second image shows you why the planets seemed lined up in a row.

If you like multimedia, watch the Space.com report on the planetary alignment by clicking Rare Planetary Alignment. Choose the settings that match your system, and then click Play.

For another view of how the planets shift positions from night to night during the alignment, see the animated graphic at Sky and Telescope.

Scale Solar System

How would you like to build a scale model of the solar system? Better be ready for a long walk or bike ride. If you tried to make a scale model small enough to fit inside your bedroom, the planets would all be so tiny you couldn't even see them.

Visit the Build a Solar System page from Exploratorium. Read the directions, and then, under the heading Solar System Model, enter the diameter (width) of your sun in inches. For example, if you want your sun to be the size of a standard basketball, which is 9 1/2 inches, enter 9.5 in the red box (don't enter anything in the green box). Now click Calculate. The chart will show you the relative size of the nine planets. With the sun this size, some of the planets are just a fraction of an inch. Earth, for example, is just over 1/12 of an inch, about the size of a pinhead. But if you look to the column on the right, you'll see that your Earth pinhead needs to be 85 feet from your basketball sun to be the right scale. That's nothing. Neptune would be nearly half a mile away.

A few communities have actually built scale models of the solar system. The largest one in the world is in Peoria, IL. The sun is located at the Lakeview Museum of Arts and Sciences, while the rest of the planets are spread out around the community. You can learn about it at the Bradley University site. Click What is the Community Solar System first, and read about the project. Then click the Scale Solar System page and take a visual tour of the model. You don't have to hike at all, just scroll down the page to see a photo of each planet.

Solar Views

Now that you've learned about the solar system and this week's planetary alignment, spend some time visiting each of the planets. For this lesson, turn to NASA's Planetary Data System site. Click on each of the planets to learn more about it, starting with the five aligned planets. You will see a series of images from various sources (click the down arrow to see more). Click an image to see more detail, to read a description, or to listen to the description read aloud (either Real or MP3 Audio). Click the magnifying glass button for a closer look.

Which of the planets is close to the size of Earth? Which planets are made up (primarily) of gas? Which planet is hottest? Which has the most varied landscape?

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