Why does it matter what city I’m in?
What time it is depends upon where you are on the planet. That’s why you use a compass to orient yourself in this activity.
A compass needle (which is attracted to the magnetic field of the earth) points in a direction called magnetic north. That isn’t exactly the same as true north, or geographic north, which is the direction of the earth’s North Pole. We’ve set up our Sun Clock so that it uses geographic north as a reference point. If you don’t line up the diagram print-out with geographic north, the Sun Clock won’t give you the right time of day.
The difference between magnetic north and geographic north is called magnetic declination, and it’s different in different locations. When you position the Sun Clock according to the directions on the diagram, you are compensating for the magnetic declination of where you live. After you do this, the “Geographic North” arrow at the top of the diagram will be pointing to geographic north and your Sun clock will work just fine.
What if my city is not on the list? Can I still use my Sun Clock?
If you aren’t near any of the cities on the list, you can still use the Sun Clock. Go out at night and look for the North Star. (You may need a book of constellations to help you find it.) Mark an arrow on the ground that points toward the North Star. That’s geographic north. The next day, position your print-out of the Sun Clock diagram with the Geographic North arrow (in the top right-hand corner of the diagram) pointing in the same direction that you marked on the ground (toward geographic north). Now follow the rest of the instructions on the diagram.
How does a shadow tell time?
Shadows change direction, depending upon the time of day. A Sun Clock like this one uses a shadow’s position to tell the time.
Why doesn’t the time on my Sun Clock exactly match the time on my watch?
The time you get from your Sun Clock is solar time, not standard time. The two aren’t exactly the same.
According to solar time, it’s noon when the sun reaches its highest point in the sky. But the sun is always moving across the sky–which means that noon where you are is at a slightly different time than noon at a place a few miles to the east or west.
Back before 1883, people used solar time. Each community kept its own time, basing that time on the sun’s position in the sky. Back then, noon in one town would be four minutes later than noon in a town fifty miles to the east.
In 1883, to regulate time for the sake of railroad schedules, the United States adopted what is called standard time, designating time zones and requiring all communities within a time zone to keep the same time–even though that standard time didn’t quite match solar time.
If you are in the middle of your time zone, your Sun Clock will be fairly accurate. If you are at one edge of your time zone, the time on your Sun Clock (solar time) may differ from the time on your watch (standard time) by as much as forty minutes.
Why do I have to put the pencil on different spots for different times of the year?
The position and length of a shadow depends on the time of day–but it also depends on the season of the year. That’s because the sun’s position at a certain time of day is different in different seasons.
How does a sundial clock work, and does it matter what city I’m in? Quick answer, yes. More specifically, it matters where you are on the planet. That is why you need tools to orient yourself when you’re dealing with sundials.
First, you need to understand that magnetic north is not the same as true north, which points to Earth’s North Pole. Sundials use geographic north as its reference point. If the sundial is not calibrated to line up with its geographic north, it will not yield the correct time display.
The difference between magnetic north and geographic north, aka magnetic declination, is different in different locations. That is why you will need to position the sundial in accord with instructions so that you can compensate for the magnetic declination inherent in your location.
Note too that the position and length of a shadow depends not only on the time of day but also on the season of the year. Different seasons mean that the sun’s position at a certain time of day differs and that plays significantly into how the sundial projects its shadow.
Shadows cast by the sundial tell time because as shadows change direction, it corresponds to the changing times of day. A sundial uses a shadow’s position to tell the time. This will not match exactly what you see on your watch, which displays standard time, as opposed to solar time, which is the time given by sundials. The two measures of time aren’t exactly the same, though they of course trend in the same direction.
Prior to the invention of the mechanical clock, say prior to 1883, people predominantly used solar time. Each neighborhood maintained its own time, based on what their sundial indicated, which of course is based on the sun’s position in the sky. Of course, given the sundial’s limitations, noon in one town ended up being a few minutes off, relative to say a town twenty five miles to the east. But it worked for that time, and until the advent of the mechanical clock, it didn’t really matter that the time was off a few minutes.
Post Published: 06 September 2009
Author: Sun Staff
Found in section:
Sundial
