Jul 222010


Pole-Star-Light-TrailsAnd before those of you in the Southern Hemisphere jump all over the title, with its clear Northern Hemisphere slant, don’t worry, I’ll be talking about the Southern Cross (Crux) as well.  There just wasn’t room in the title to fit in all of that.

Let’s start off with the ‘why’ and then move to the how.  For those of you well versed in astronomy this may all seem very simple to you and you’re welcome to skip right over.  But as I’ve traveled I’ve been amazed at how many people, while sitting around a campfire or on a beach, can’t find the North Star or Southern Cross.  I’ll admit, for most people, it doesn’t matter.  They just like seeing stars overhead, making up constellations and looking for shooting stars.  And that fine.  But if you are looking to take pictures of the night sky, it can be helpful to find either the North Star (Polaris) or Southern Cross.

The reason is that it will tell you how light trails from the Earth’s rotation will show up in your picture.  Are you looking for streaks across the sky when using a long shutter speed or stacking photos?  Or do you want that cool circular effect?  It can be fun to play with different directions and foreground objects and unless you’re familiar with finding the right locations, it can be tricky.  First, some explanation.

A Pole Star is a star that sits near the point of rotation for the axis of the Earth directly above either pole.  In the case of the North Pole, it’s the North Star. In the case of the South Pole there currently is not a single star close in that is easy to spot with the naked eye, so the Southern Cross is often referenced.  Unless you are on the equator, you can only see one or the other (or very high up a mountain near the equator, like Mt. Kilimanjaro).  When you point a camera at either Polaris or the Crux and leave the shutter open for a while, you’ll get a swirled pattern as you see in the photo above (shot in Arches National Park, Utah, USA).  If you point the camera away from the pole, you’ll get more gradual curves depending on your location.  The photo at the bottom of this post was taken near the equator, in Serengeti National Park, Tanzania, while pointing Southwest.

Speaking of location, let’s find the North Star.  It’s fairly simple and you only need to know one constellation, the Big Dipper or Ursa Major.  I usually reference this constellation rather than the Little Dipper (Ursa Minor), even though Polaris is part of the Little Dipper, because the Big Dipper has ‘brighter’ stars (I know I’m disturbing some astronomers out there with a few colloquial terms and I apologize).  Once you have found the Big Dipper, connect the dots from the bottom of the outside of the dipper (farthest from the handle) to the top of the outside of the dipper.  Now follow that line until you meet the next bright star.  That’s Polaris and it is at the tip of the handle of the Little Dipper.

Serengeti Star Trails As I have practically no experience finding the Southern Celestial Pole, I’m going to borrow from Wikipedia (this is where the astronomers can jump in and give some pointers (har har har)) “Since the southern sky lacks an easily visible pole star, Alpha and Gamma (known as Acrux and Gacrux respectively) are commonly used to mark south. Tracing a line from Gacrux to Acrux and extending it for approximately 4.5 times the distance between the 2 stars leads to a point close to the Southern Celestial Pole. Alternatively, if a line is constructed perpendicularly between Alpha Centauri and Beta Centauri, the point where the above mentioned line and this line intersect marks the Southern Celestial Pole. The two stars of Alpha and Beta Centauri are often referred to as the “Southern Pointers” or just “The Pointers”, allowing people to easily find the asterism of the Southern Cross or the constellation of Crux.”

A helpful reminder when finding either celestial pole is they are as far off the horizon, North or South, as you are from the equator. In other words, the photo at the top was taken in Utah at approximately 38.7°North Latitude, so the North Celestial Pole is 38.7° off the horizon to the North.  This is helpful for visualizing a trip in advance.  If you know your latitude, you can guesstimate what your star pattern will look like.

And that’s the how and why of finding either celestial pole.  Armed with that knowledge, and the DPS post 4 Steps to Creating Star Trails With Stacking Software, it’s time to head outside when the sun goes down and see what you can create!  Feel free to post some of your memorable star trail photos in the comments section.

PHOTO NOTES: The top photo is a stack of 36 images taken over 51 minutes. Each image is 75 seconds in duration shot at 16mm, ISO 1250 f/6.3. The second photo is one single image lasting 618 seconds, f/2.8, ISO 400, 16mm.

Post from: Digital Photography School – Photography Tips.

How To Find The North Star And Why You’d Want To
Peter West Carey
Thu, 22 Jul 2010 14:07:52 GMT