SkyTalk April | A Celestial Compass, Calendar & Clock

The sky’s best-known figure (with the possible exception of the constellation Orion) is the Big Dipper.  The Dipper itself is actually not a constellation but only part of one—Ursae Major, the Great Bear.  It’s an “asterism” or a distinctive pattern made up of parts of one or more constellations.  But it surely has to be the most “useful” of any sky figure of all, as we will now see.

Compass: As the Scientifics Star and Planet Locator set for any early spring evening this month will show, the Big Dipper is perfectly placed on the meridian and riding high in the sky facing north.   Everyone seems to know that the two stars marking the end of the Dipper’s bowl are called the Pointer Stars, since following them upward in the sky points directly to Polaris the North Star.    For those of us living at mid-northern latitudes, the Big Dipper is circumpolar.  This means that the Pointer Stars are visible and available for finding North any clear night of the year—even in the winter when the Dipper’s handle “scrapes” the northern horizon!  Rotate the star wheel and see this for yourself.

Calendar:  We’ve just given a clue as how to use the Big Dipper as a calendar.  Pick a convenient time in the evening to stargaze—say 9:00 p.m.  Now slowly rotate the star wheel on the Locator and see when the month marking the start of the four seasons is at the 9 0’clock mark.  That’s March for the vernal or spring equinox, June for the summer

solstice, September for the autumnal or fall equinox, and December for the winter solstice.  Note where the Big Dipper is in the sky at those times.  Then should you find yourself for some reason outdoors at night not knowing for sure just what season it is, looking at the Dipper’s position will immediately let you know.  Continuing its use as a calendar, note that the circumference of the star wheel has all the days of each month individually marked.

Clock:  The third use of the Star and Planet Locator is a little more subtle and involved. Here’s the idea.  First, look at the Dipper’s handle as the “hand” of a huge celestial clock.  As you spin the star wheel, see it moving around the “face” of the star clock (the sky).  Now, pick a time for any given month to “set” the clock (the star wheel)—say 8:00 p.m.

Notice where the Dipper’s handle is at that time.  Then advance it one hour, to 9:00 p.m.

And again to 10:00 p.m. and then 11:00 p.m. The idea is to form a mental picture of where the Dipper’s handle is at those hours of the evening during that month.

Now the next step. After doing this several times, start again at 8:00 p.m.—but then without looking at the time on the dial, move the Dipper’s handle to where you think it should be at 9:00 p.m. And then do it again for the other times.  Having once engrained this in your mind, with practice you can go outside, look where the handle is in the sky, and then check the actual time.  You will be surprised at how accurate this can be.  And you will amaze family and friends that you can tell them the time by simply looking at the sky.   Having done this many times myself over the years, I can usually tell the time not only to the nearest hour—but also to within a matter of minutes!  Back when I was

teaching adult continuing education astronomy classes, the students would often bet me that I couldn’t really do this and asked me to prove it.  Needless to say, I won many after-class pizza sessions!

— James Mullaney

Former assistant editor at Sky & Telescope magazine & author of nine books on stargazing.   His latest, Celebrating the Universe!, is available from