Now Boarding The Circle Tour To Polaris!

The North Star, left of center, is nearly stationary as the rest of the stars circle around it in this 45-minute time exposure. The farther a star is from Polaris the larger the circle it makes. Stars near Polaris never set, but ones further away get their 'circles' cut off by the horizon, causing them to rise and set. Photo: Bob King

It’s enough to make you dizzy. If you could speed up time that is. Take a look north any clear night. The Big Dipper sails nearly overhead and its two “Pointer” stars reliably guide the eye to Polaris the North Star. Unnoticed by the casual observer, every star in the northern sky circles around Polaris once every 23 hours and 56 minutes. This interval is called a sidereal day. The 24-hour day we use to set our clocks and TiVos is called a solar day.

Finding Polaris is easy. Face north at nightfall and look way up high to find the Big Dipper. A line extended through the "Pointer Stars" in the bucket will take you there. The North Star is similar in brightness to the Dipper stars. Created with Stellarium

Why not an even 24 hours, the time it takes for Earth to spin once on its axis? Since our planet’s also moving around the sun, every single day we travel 2 million miles on our year-long orbital journey. This orbital motion causes the stars to rise in the east 4 minutes earlier each evening.

Polaris happens to in the direction our north polar axis points. The south end points to the dim star Sigma Octantis visible in the southern hemisphere. Illustration: Bob King

Time adds up. After a week, 4 minutes becomes 28, and after a month, stars are up in the east fully two hours earlier than the month before. That’s why Saturn, Virgo and the Big Dipper rise higher each night, while Orion slinks away into the west. In a year’s time – one full orbit later – the stars are back in their same places and ready to begin another cycle. It’s like the shampoo label: wash, rinse and repeat.

The spinning Earth resembles a twirled umbrella. The shaft is like the polar axis. A spun umbrella stays pointed in the same direction, while 'stars' on its underside circle around the 'axis'. Photos: Bob King

Polaris lies very close to the north celestial pole, the spot in the sky where the Earth’s north polar axis points. As the Earth spins, its axis stays pointed in the same direction, so Polaris remains nearly stationary. Just as the cities and towns dotting the globe describe circles around the stationary axis as the Earth spins, so the stars appear to describe circles around the stationary North Star over the course of a sidereal day. Of course, the stars only appear to be moving – it’s really our spinning planet that creates the illusion of motion.

In a time exposure with a telephoto lens, you can see that Polaris (bright dot) is not quite at the north celestial pole, but still close enough barely notice the difference. Photo: Bob King

The North Star is not precisely at the pole. Very astute sky watchers might notice that it describes a tiny circle 1.5 degrees across. I’ve never paid that close of attention; I let the camera do the work for me instead.

Among other fascinating tidbits about Polaris is that it’s height above the horizon is equal to your latitude. Here in Duluth at 47 degrees north, the North Star is 47 degrees high or about midway between the horizon and zenith. At the North Pole (90 degrees north), it shines directly overhead!

Polaris not only tells us where north is. It’s also the brightest star in the Little Dipper or Ursa Minor. Sad to say, the Little Dipper is nowhere near as bright as the Big version. Count yourself lucky if you’ve see its complete outline. Only Kochab (KO-cab), the star at the end of the bucket, is bright enough for city dwellers to see.

Before we leave the North Star, you should know that it’s farther away than many of the naked eye stars.  While the Big Dipper stars range from 68 to 210 light years; Polaris is 430. The only reason this little twinkle of light is easy to see is because it’s 45 times the size of the sun and 2500 times brighter.

Tomorrow among other topics, we’ll check in on T Pyx, the repeat nova, and see what it’s been up to the past week.

7 Responses

  1. Mike

    Hi Bob!
    Please tell me more of how those polar axis photos were done.
    What aperture? What lens? Time of night?
    Sorry to bug you with questions! Just curious to learn.

    1. astrobob

      Hi Mike. I probably should have included that in the caption info. Anyway, I used a 16mm lens at f/2.8 for the top 45 minutes exposure at ISO 200. The telephoto view was taken with a 200mm lens at f/2.8, ISO 400 and a shorter exposure – between 5 and 10 mins. I don’t remember the time of night but both were in the evening hours.

  2. Ramon Miro

    The spinning umbrella with circumpolar constellations seems like a great tool for astronomy outreach. I’m going to give that a try.

    1. astrobob

      Ramon – all you need a few shiny stars to paint or tape on the inside of an umbrella and you’re all set.

  3. Hi Bob,

    Great pics and info! I am pastor in El Cajon, CA and I’m doing a sermon using celestial navigation as an analogy. May I use your “finding Polaris” photo in a powerpoint presentation for the sermon? Should you approve I will appropriately credit your page as the source. Thanks for your consideration.


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