Venus And Her Sisters; Tell Time At Night With The Big Dipper Sky Clock

Venus and the Seven Sisters star cluster last night - what a sight! Details: 200mm at f/5.6, ISO 800 and 20-second guided exposure. Photo: Bob King

It was great to stand in the front yard last night under a clear sky and see Venus and the Pleiades. A casual glance showed a sprinkling of little stars around the brilliant planet. Binoculars gave the best view of the “8 sisters” with Venus the dominant by far. I tried to see how many Pleiads I could count with the naked eye. The five brightest were easy enough but the sixth – Taygeta – was tricky. Pleione was beyond me. It was also fun to recall that 8 years ago to the day I stood in the yard and watched Venus in almost the same position inside the cluster. Apparently I was on time for my next appointment.

A wider view of Venus and the Pleiades along with Jupiter (at bottom). 35mm at f/5.6, ISO 800 and 20 seconds. Photo: Bob King

Tonight Venus glides up and to the left of the star cluster, so if it was cloudy by you last night, you have another shot at seeing them together.

After a good swig of the western sky show, I checked out a new paperless way to tell time using the Big Dipper and North Star. In yesterday’s blog we looked at a simple star clock you can make with paper and scissors. Today we’ll pretend we’re out at night with neither watch nor guide.

The map below shows the North Star as the center of a clock face with the two Pointer Stars in the Bowl of the Big Dipper as the hour hand.  Our example shows the sky at 9 p.m. local time.

To start, face north and look high in the northeastern sky to find the Big Dipper. Draw a line through the two Pointer Stars until you arrive at the first easy-to-see star. That’s the North Star or Polaris. It’s about 5 “Pointer lengths” away. The line connecting the North Star and Pointer Stars now becomes the hour hand on our celestial clock.

To find the time using the Big Dipper and North Star we imagine the northern sky as the face of a clock. Key hours are marked. Illustrations created with Chris Marriott's SkyMap

1. Estimate the time the Pointer hour hand indicates to the nearest quarter hour (15 minutes). In our example, it’s 1 o’clock.

2. This next step is key. We now adjust the clock for the time of year. On March 7, the Pointer Stars stand directly above the North Star at midnight standard time. All you have to do is figure to the nearest quarter month how much time has elapsed since March 7. Since tonight’s April 4th, that rounds off to one (1) month. If it was April 15 instead, the number would be 1 1/4 or 1.25.

3. Add the two figures from above and then multiply by 2 as in: 1 (from 1 o’clock) and 1 (one month) = 2. Then 2 x 2 = 4. Now subtract that sum from 24, so 24-4 = 20. The result will be the time in 24-hour or military style. 20 hours military is the 20th hour of the day or 8 p.m. Add an hour for daylight-saving time and we arrive at 9 p.m. By gosh, that’s correct! Note: If your final number is greater than 24, subtract from 48 instead.

Facing north about 5:15 this morning, the Pointers were now off the left of the North Star in the 9 o'clock position. I figured the time as: 9+1=10. Multiply by 2 = 20. Subtract 24 - 20 = 4 and added one hour for DST to arrive at 5 or 5 a.m. Close enough.

It’s important to remember to add that hour for daylight time when it’s in use and to face north. With practice – and checking against a watch for accuracy – you’ll soon become a master of time at night. Be aware that where you are within your time zone will affect your time estimate. If you try this a few times, you’ll soon be able to factor that in and fine tune your time. I tried this simple method both last night and this morning before dawn and was frankly surprised how well it worked. If you’ve got some time on your hands, let us know how you fare.

4 Responses

  1. Stephan Burkhardt

    Hi Bob!
    what a great picture of Venus and the Seven Sisters! Congratulations! Unfortunately we were not able to witness it down here last night, as it rained here heavily (for the first time in three weeks – so no grudge at all), but I was able to follow Venus’s approach in binoculars during the past days. A great view! Thanks for the lovely guides and star charts.


    1. astrobob

      Hi Stephan,
      Thanks for writing and I appreciate your kind comments. We were just the opposite — cloudy all the way up to last night, when it finally cleared.

  2. john plummer

    i was wondering if you could tell me where to look for the pleiades i live in north ms nearthe al.state line i can find the big dipper and little dipper but cant
    figure out the maps i have found.can you tell me which way to look

    1. astrobob

      Hi John,
      If you’re out around 8 o’clock and look overhead you’ll see a very brilliant “star”. That’s Jupiter. Now look two fists held at arm’s length below Jupiter to find a bright reddish star called Betelgeuse. About three fists to the right (west) of Betelgeuse you’ll come across another bright reddish star, but not as bright as Betelgeuse. That’s Aldebaran and you might notice a V-shaped outline of stars below and right of it – that’s the Hyades. Now look about one fist farther to the right to find a slightly fuzzy clump of half a dozen stars very close together in the shape of a tiny Dipper. That’s the Pleiades.

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