Most people can point out the asterism known as the Big Dipper. No, the dipper is not a true constellation, rather it is an asterism, a familiar group of stars located within a constellation. The Big Dip is actually part of the constellation Ursa Major, the Great Bear. It is a group of stars that at our latitude, and anywhere north of here, is visible all night, every night of the year. That makes it handy for things like telling time, and with some practice, you can gauge time to within 30 minutes by observing the Dipper.
The dipper and the stars around it are called circumpolar stars, meaning that they circle the pole. At our latitude, and anywhere north of here, these stars are visible all night long in the northern sky. Other constellations that you may know, Orion, Leo, or Scorpius are seasonal and are only visible during part of the year.
Telling time by the Big Dipper's position is based on the idea that all stars rotate around the north pole. The north star, Polaris (of Little Dipper fame) is very close to the north pole. Its height above the horizon is also equal to our latitude (for here, about 40 degrees.) Polaris then, will be the center of our celestial clock. The Big Dipper circles once around the pole every 24 hours.
To find Polaris, use the pointer stars in the end of the Big Dipper's bowl (see diagram) a line drawn through them to Polaris. This line will become the hour hand of our celestial clock..
Now, there are
a few differences between the clocks we are familiar with and the celestial
clock. There are 24 hours on the celestial clock instead of 12, and it
also runs backwards! One other difference is that the celestial clock runs
fast, by 24 hours a year, but it just takes a few adjustments to compensate
The reason the celestial clock runs fast is because the Earth makes one trip around the Sun in 365 days, but a circle only has 360 degrees. This makes the Earth move ahead about 1 degree in its orbit every day, which in turn, makes the hour hand of our clock move fast counterclockwise about 1 degree every day. That may sound confusing, but all it really means is that our celestial clock runs about a half hour fast per week, two hours per month.
There is one date where our celestial clock agrees with real time exactly. On March 8th, at midnight, the Big Dipper is directly overhead. The hour hand points from Polaris to the Dipper overhead at midnight, real time.. So March 8th serves as calibration time for the rest of the year. .
So, our celestial clock looks like this. Midnight (the 24 hour mark) is when the Big Dipper is straight overhead. The hour hands points from Polaris, to the Dipper, pointing overhead to the south. 12:00 noon would point straight down to the northern horizon. 6PM (18 on our clock) would be when the Dipper was to the right (east) of Polaris and 6AM would have the Dipper to the left (west) of Polaris.
Now, to tell the time
by the Dipper, face north and imagine our clock face transplanted to the
sky. Find the Dipper and draw the imaginary hour hand, from Polaris to
the Dipper's pointer stars. Figure out what time this reads on our celestial
clock, now all we need to do is compensate for our clock's faster running
time. Subtract 2 hours for each month from March 8th. Subtract 30 minutes
for each week, and , if you want to be more accurate, subtract 4 minutes
per remaining day. Oh, and don't forget to ADD one hour for Daylight Savings
Time. Your answer will be local real time, accurate to within 30 minutes!
It may sound complicated, but once you try It a few times, it gets quite easy. Easy, that is, until the clouds set in?..!
Copyright © 2001 Kathy A. Miles and Charles F. Peters II