how can i use a sundial to tell the time of day?

Question

I've downloaded a Southern Hemisphere sundial construction template from http://liftoff.msfc.nasa.gov/Academy/Earth/Sundial/Sundial-ConstructSimple.html
But I do not know how to use it to tell the time. How do I do it? Thanks

Answer 1

Assuming you correctly followed the instructions for setting up the sundial, all you have to do is look at where the gnomon's shadow is pointing. For example, if the shadow is pointing halfway between 9 and 10 on the a.m. side of the dial, then it's about 9:30 am local time.

However, local time is not the same as standard time. Standard time can be as much as an hour ahead of local time. The difference depends on where you live east/west within your time zone. One way to find out is to look at the sunrise/sunset times published in your local newspaper. Figure out what's halfway between those two times and see how far off from 12:00 noon it is. Also, Daylight time is exactly one hour ahead of Standard time.

Let's suppose it's summer, so you're on Daylight time, and your local newspaper says sunrise 6:13 am and sunset 9:15 pm. Noon local time is halfway between these two times: 1:44 pm Daylight time, or 12:44 Standard time. So your offset is about 45 minutes. When local time is 9:30 am, Standard time would be 10:15 and Daylight time would be 11:15 am.

You don't need to check the sunrise and sunset every day to use your sundial. Once you know the offset for your location, it will be the same all year. You just need to remember to include the extra hour for Daylight time in the summer, and don't include it in the winter.

Of course, there are some places which don't use Daylight time (like New Mexico). Indiana just started using it this year.

Answer 2

Hi. Point it at the South celestial pole and just observe where the shadow of the pointer lies.