Celestial Navigation - Tracking the Big Dipper

Week 6: We tracked the big dipper's movement across the sky.

Celestial Navigation is a technique used by pilots, ship captains and other navigators to determine their location on the earth using the horizon, sun, moon, stars and/or other celestial bodies.

Carry On, Mr. Bowditch is a fictionalized book about Nathanial Bowditch, an actual 18th century celestial navigator. Despite challenges throughout his life, he was able to self-educate himself and develop new and improved navigation techniques.

One of the first steps to learning celestial navigation is understanding the how the Earth, moon, sun and stars move around each other. After finishing an astronomy unit in which we became familiar with the phases of the moon and learned to recognize several constellations, we tracked the big dipper's movement across the sky.

Because of the Earth's rotation, the sun appears to travel from east to west in a circular orbit across the sky. The same is true of the big dipper, except that it is closer to the center axis or the North Star. (At least it is for us living in Germany, during the winter months.)

The drawing shows the sky as it would be seen from laying on the ground. The center point is Polaris or the North Star. The lowest big dipper is sketched as was seen at 9:00. The green big dipper is the location it was seen at 11:00. The red dipper was drawn at 1:00 and the second purple dipper was drawn at 2:00.

The completed sketch looks like a clock which shows 24 hours. Since it is only dark about one half of the night the other half of the clock is blank. The oval shows the portion of the sky that was visible to us at 9:00 and was drawn so we could talk about the location of the sun at that time.

This activity was completely new to me and I learned several things that should probably have been obvious. I suppose I had just never thought about them.

• The time of night can be approximated based on the location of the big dipper just as the time of day can be approximated based on the location of the sun.
• The big dipper and all the stars follow the same path as the sun does across the sky. Everything moves in a circular counter-clockwise rotation about the North Star.
• The big dipper moves from east to west during the months of February and March just as the sun does.
• In the summer the big dipper moves from west to east, but still rotates counter-clockwise.
• All the stars move in a circular counter-clockwise rotation. If the big dipper moves from east to west Cassiopeia moves from west to east because they are on opposite sides of Polaris.
• The big dipper is visible all year round. Some constellations appear and disappear during the course of the year.

Instead of depriving the kids of sleep for an entire night, the drawings were created over the course of several nights. Each night rising a few hours later to take a peak at the stars.

Although this activity could be done with any constellation at anytime of year, the winter months of January and February turn out to be the best ones for several reasons. First of all, it gets dark much earlier, so kids don't have to stay up so late at night. Secondly, it's much easier to see that the big dipper follows the same path as the sun as it moves east to west in the winter. The big dipper is a great constellation to track because it is easy to find and is close to Polaris so is visible all year round.

Of course all of this depends on your location on the Earth. I have no idea how this works in the southern hemisphere.

This activity helped created a better understanding of the relationship between the Earth and stars. Perhaps in the future we will learn to use a sextant and celestial tables almanac to determine our location based on angles and mathematics. It will be an awesome practical application of math when we get there.




To see our other Astronomy activities, please visit our Science Page.

This post is linked to:
Sola Gratia
True Aim Education