Sundial at the Equator

[EmilyParticipant]

We live just about on the equator at -2 degree latitude. How would this affect activities like making a sundial? Is it necessary to align it to the earth’s rotational axis or will it work even more or less perpendicular to it? Obviously we can’t track much change in day length and things will be a little more complicated with true north, but I’m hoping we can still do some sort of sundial!

[Bernard NebelKeymaster]

You are quite right. A traditional sundial will not work at the equator. During the summer months (spring equinox to fall equinox) the arc of the sun’s path will go to the north of you making shadows fall to the south and the opposite during the winter months. However, you can still make a modified sundial. Mount a pole vertically in the ground (or straw vertically on poster board). Each day its shadow will start as long to the west, gradually shorten to zero when the sun is directly overhead at solar noon, then gradually lengthen to the west. You can calibrate the length of shadow with cross lines for each hour. These hour lines should remain consistent throughout the year, although the sun will proceed to make a low arc south and north of directly overhead.

This will not permit the determination of north as described in the text. However, a line from the tip of the first morning shadow to the tip of the last evening shadow will be an almost perfect east-west line. A perpendicular line, of course, will be north and south. Thanks for the question. Please ask further.

[EmilyParticipant]

Thanks for your response! We will do a modified sundial as you suggested. As long as this lesson isn’t conducted too close to an equinox, wouldn’t we still be able to determine true north (or south depending on the time of year) since the sun isn’t exactly directly overhead except for those times?

[Bernard NebelKeymaster]

Yes, quite so. It will be interesting to observe how, over the period of the equinox, the noon shadow (what there is of it) will shift from north to south, or vis versa.

In the whole process, don’t let kids loose track of basic phenomenon–the shift is due to the Earth maintaining the same degree and direction of tilt as it orbits the sun.

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