The pointer stars Merak and Dubhe point at Polaris and go once around per sidereal day, every 23 hours 56 minutes and 4 seconds.  By making a daily correction it is possible to estimate the time at night using these three stars.

One point on this is pretty much intuitive. What is the Right Ascension of Merak? Well, it’s about the same as Dubhe, because a straight line joins both stars to the axis of the celestial clock, the north celestial pole.

But what is that angle, really? Well, it’s 11 hours of Right Ascension. Or, if you use Daylight Saving Time, it appears to be 12 hours.

Thanks to “reasons”, we now spend at least 7 months of the year on Daylight Saving Time, and this helps to make the Big Dipper Star Clock easier to use. At midnight Eastern Daylight Time on September 21st (most years, September 22nd this year), the Autumnal Equinox, the line between Polaris, Merak and Dubhe points straight down to the northern horizon. At midnight Eastern Daylight Time on March 21st, the Vernal Equinox, the line between Polaris, Merak and Dubhe points straight up to the zenith.

On another day of the year, at the moment when the line joining Merak, Dubhe and Polaris is vertical in either direction, if you know the date, you can make an estimate of the time in Daylight Saving Time.

  1. Count the number of days before or since September 21 or March 21
  2. For each day before September 21st, imagine winding your mental clock forward from midnight by 4 minutes. If after September 21st, imagine winding your clock back 4 minutes per day. If your index date is March 21st, use the same directions.

So because tonight is 0 days after September 21st, at the moment tonight when the line between Polaris, Merak and Dubhe points straight down to the northern horizon, you can estimate that it’s about 0*4 minutes before midnight, Eastern Daylight Time. (copy and paste, and replace the zeroes for different dates.)

This system doesn’t work perfectly because it’s the same Daylight Saving Time in Detroit as it is in Boston, although the angles of objects in the sky are different. But it is predictable, if you know that you live east or west of the Central Meridian of your Time Zone, to expect objects in the sky to run reliably fast or slow.

Please refer to the Bradstreet Fulldome Curriculum Vol. 2 Minilesson ‘Time Zones’ and the printed lesson plan in the binder for more detail on time zones. The asterism line representing the Pointer Stars, and the Meridian to show the time when the Pointer Stars are vertical, can be drawn in with Starry Night, and in ATM-4 as well. Please refer to the FDC 2 ‘Hour Angles’ Minilesson and its printed lesson plan to interpret how to use other objects in the sky as a star clock. Because the Pointer Stars are vertical at (DST) midnight on the Equinoxes, they are terrifically useful for this, but the Minilesson might suggest stars for you that have hour angles that are also convenient to remember for relating to the dates that index our seasons. Right-click on a star and select ‘Hour Angle Lines’ to get a dynamic readout of the time since the last moment it crossed the local meridian to the south. The minilessons ‘Time LAST (Local Apparent Solar Time)’ and ‘Time LMST (Local Mean Solar Time)’ use very similar tools.

Eric Briggs

Eric Briggs

SciDome Support at Spitz, Inc.
Eric has been an expert user of the Starry Night astronomy simulation software since its original release in 1997. He joined Spitz in 2008, providing support and training for Starry Night Dome and other software on SciDome planetarium systems.

Eric is the co-discoverer of 10 supernovae (and counting).
Eric Briggs

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