How to use the stars to find your latitude

How to Use the Stars to Find Your Latitude

Ken Christison captured these glorious star trails around Polaris, the North Star. He wrote, “For the most common and often the most spectacular star trails, you want to locate Polaris and compose the image so it is centered horizontally and hopefully you can have a bit of foreground for reference.”

The North Star or Pole Star – aka Polaris – is famous for holding nearly still in our sky while the entire northern sky moves around it. That’s because it’s located nearly at the north celestial pole, the point around which the entire northern sky turns. Polaris marks the way due north.

As you face Polaris and stretch your arms sideways, your right hand points due east, and your left hand points due west. About-face from Polaris steers you due south. Polaris is not the brightest star in the nighttime sky, as is commonly believed. It’s only about 50th brightest.

But you can find it easily, and, once you do, you’ll see it shining in the northern sky every night, from Northern Hemisphere locations.

In a dark country sky, even when the full moon obscures a good deal of the starry heavens, the North Star is relatively easy to see. That fact has made this star a boon to travelers throughout the Northern Hemisphere, both over land and sea. Finding Polaris means you know the direction north.

Best of all, Polaris is readily found by using the prominent group of stars known as the Big Dipper, called the Plough in the U.K., which may be the Northern Hemisphere’s most famous star pattern.

To locate Polaris, all you have to do is to find the Big Dipper pointer stars Dubhe and Merak. These two stars outline the outer part of the Big Dipper’s bowl.

Simply draw a line from Merak through Dubhe, and go about five times the Merak/Dubhe distance to Polaris.

How to Use the Stars to Find Your Latitude

If you can find the Big Dipper, you can find Polaris. The two outer stars in the bowl of the Dipper – Dubhe and Merak – always point to the North Star.

Use the Altitude of Polaris to Find Latitude | Science project |

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How to Use the Stars to Find Your Latitude

Sailors and travelers have used Polaris, also known as the North Star, for centuries to locate their position on the surface of the Earth. Polaris is the brightest star in the constellation Ursa Minor, whose seven brightest stars form the Little Dipper. Polaris is the brightest star at the end of the tail of the Little Dipper and is useful because it is the only star that does not appear to move in relation to a specific location on Earth. Polaris cannot be seen from south of the equator.

The altitude of a star is the measurement in degrees of the angle of the star above the horizon. Flat out on the horizon is 0° and straight up in the sky is at 90°, which has a special name, the zenith.

How to Use the Stars to Find Your Latitude

  • Drafting compass with degree measurements
  • Clear, starry night
  1. Go outside on a dark, clear, starry night.
  2. Locate Polaris. It is the last star in the tail of the Little Dipper.
  3. Hold the compass out in front of you.
  4. Align the 0° edge of the compass with the horizon.
  5. Keeping the 0° edge flat against the horizon, lift one arm of the compass until it points directly at Polaris.
  6. Read off the angle. This is the altitude of Polaris from your location on Earth. This corresponds to your latitude. What would be the altitude of Polaris if you were standing at the North Pole? What would be the altitude of Polaris if you were standing at the Equator?

Use a map or the internet to determine the latitude of your hometown and see if your measurement is correct.

Polaris is so far away (about 434 lightyears) that the rays of light approach the Earth in a parallel manner.

This allows us to look at the angle between us and the star (which is the same as the angle between the horizon and the star) to locate our latitude on the Earth. Polaris is about 0.

7° from the exact North Pole, so with the rotation of the Earth it makes its own tiny circle in the sky at night as well, but it is the only star that appears fixed in the sky to us.

Polaris is also a multiple star, which is why it is so bright. It consists of alpha-Polaris, the main, brightest star, and two tiny stars very close to it, so to the naked eye they appear as one star.

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Earth moves in a motion called precession. This means our axis shifts in a circle over the span of about 26,000 years. This means Polaris hasn't always been above our North Pole as it is right now.

In ancient Egyptian times, the North Star was Thuban, from the constellation Draco, and in about 12,000 years, it will be Vega, from the Lyra constellation, which will appear to be an even brighter beacon than Polaris.

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Latitude by the stars

How to Use the Stars to Find Your Latitude

Finding your latitude by the stars. Image courtesy NASA.

How do you find your way around the world? GPS? Well what if you haven't got reception? A map? Good idea, but what if you're travelling on the high seas with no land mark in sight? That's a situation that many, many people have found themselves in over the millennia. These brave seafarers used the Sun and the stars to navigate instead. And to do this, they needed a fair bit of geometry, in particular trigonometry.

Suppose you are on the open ocean and you want to work out your latitude (see here for a definition of latitude). The Sun and most of the stars change their position in the sky over time.

But some stars always appear to be in the same place. An example is Polaris, also called the North star, which always appears to be sitting directly overhead the North pole.

It turns out that your latitude is the angle at which Polaris appears to sit above the horizon.

To see why, let’s look at a two-dimensional picture. Consider the plane that contains the North pole, the point you are sitting at and the centre of the Earth. Strictly speaking, Polaris doesn’t sit vertically above , as shown in the picture, but it is so far away that the line of sight from to Polaris is very nearly vertical, so we can pretend that it does.

How to Use the Stars to Find Your Latitude

The angle that “Polaris sits above the horizon” is the angle indicated in the figure. It’s the angle our line of sight to Polaris, call it , forms with the line that is tangent to the Earth at the point (that’s our line of sight towards the horizon).

Extending and , we see the angle again on the other side of the crossing point :

The latitude of is defined to be the angle that the line from to makes with the plane containing the equator. In our two-dimensional picture, that equatorial plane is just a horizontal line which passes through It meets the vertical line in a point and the tangent line in a point

Because is the radius of the circle and a tangent, we know that and form a right angle at And since and form an angle we know that the angle between and is

Now consider the triangle with vertices , and As we have just seen, the angle at is Since is vertical and horizontal, the angle at is . And because angles in a triangle always add up to , the angle , which is our latitude, is

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Et voilà — your latitude is given by the angle Polaris sits above the horizon. The Greek astronomer Hipparchus defined latitude in this way over 2000 years ago. He didn’t even know that the Earth was round, but our little picture here should explain why Hipparchus’ definition agrees with the modern one.

There is no equivalent of Polaris in the South, but to find your latitude if you are in the Southern hemisphere you can use a constellation called the Southern cross (illustrated on the flag of Australia) and two stars called the Southern Pointers.

Over the millennia navigators have used different devices to measure the angle at which a star appears above the horizon. These include beautiful astrolabes and sextants, which you often find in antique shops and museums.

That sorts out latitude, but how do you work out your longitude? That's another story.

How to navigate using the Stars

(These bestselling books include lots more tips on how to navigate using the stars.)

Lots of people love the idea of finding direction and navigating using the stars, but are put off because they fear it is complicated. It does not need to be complicated at all, it is something you can learn to do in minutes. In fact finding direction using the stars is much quicker and easier than using a compass. It is also a lot more fun.

Imagine one night you arrange to meet a friend under a lamppost on the other side of a gentle hill. As you walk towards your friend you would see the light appear over the brow of the hill, long before your friend became visible. You would know exactly what direction they were, even though you could not see them. This is the simple concept behind using the stars to find our way at night.

The stars can act as our lampposts.

All we need do is find a star that is directly above the place we need to get to and it will point exactly the right direction for us, from quarter of the globe away.

If you called a friend on the telephone who was in another country a few thousand miles away, and you asked them to name the star that was directly over their head, you could then find that star in the night sky and the point on the horizon directly below that would be their exact direction from you at that moment.

Unfortunately, a few minutes later that star would have moved and so you would need a new one. It would take a lot of phone calls to use this method with most stars! Fortunately there is one star in the night sky that does not appear to move. It is called Polaris, or the North Star.

The easiest method for finding the North Star is by finding the ‘Plough’, an easy to identify group of seven stars. It is known as the ‘Big Dipper’ to the Americans and the ‘saucepan’ to many others.

Next you find the ‘pointer’ stars, these are the two stars that a liquid would run off if you tipped up your ‘saucepan’.

The North Star will always be five times the distance between these two pointers in the direction that they point (up away from the pan). True north lies directly under this star.

The ‘Plough’ rotates anti-clockwise about the North Star, so it will sometimes appear on its side or even upside down. However its relationship with the North Star never changes and it will always dependably point the way to it.

The reason the North Star is so important for natural navigation is that it sits directly over the North Pole.

Something that people often forget is that whenever you are trying to find true north, you are actually trying to find the direction of the North Pole from wherever you are – even if you are only heading a few hundred metres on a gentle walk – ‘north’ is still just an abbreviation for ‘towards the North Pole’.

The constellation, Cassiopeia, is also very helpful in finding the North Star as it will always be on the opposite side of the North Star from the Plough and therefore often high in the sky, when the Plough is low or obscured.

Having found the North Star, there is something about its height above the horizon that is well worth knowing. Wherever you are in the northern hemisphere, the North Star will be the same angle above the horizon as your latitude.

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This can be measured accurately using a sextant, but an estimate can be made using an outstretched fist. We are all different shapes and sizes, but we share proportions. An outstretched fist makes an angle of close to 10 degrees for most people.

In an under a minute and with just your bare hands you can now find north and estimate your latitude.

The constellation, Orion, rises in the east and sets in the west.

Orion’s belt, the only three bright stars that form a short straight line in the whole night sky rise very close to due east and set very close to due west.

If you want to be really accurate then the first star in the belt to rise and set, called Mintaka, will always rise and set within one degree of true east and west wherever you are in the world.

Lots more information about these methods and lots of others can be found in my books.

They also contains lots of information on using the sun, moon, plants and animals to help you find your way, on land or at sea or even in the city.

How to Use the Stars to Find Your Way

Both polar (or, in the South, near-polar) locations can be used to find latitude, which indicates one's north/south distance from the (east/west-running) equator. Latitude is measured in degrees, referencing the 360 degrees that is the entire Earth.

Together with longitude, which measures east/west distance from the (north/south-running) prime meridian, latitude can tell you where you are on the planet, even if all you can see is sandy desert.

Actually, for latitude, you need to see two more things: a polar star and the horizon. Inconveniently, there are only two times in any 24-hour period when you can see both: the moment before the sun sets and the moment before the sun rises. Twilight.

To determine latitude in the short windows of twilight, you need to find the angle of elevation of a polar star. This calculation requires three points of reference: a polar star, a horizon and you. Once you have these three points, you'll draw two lines – one from you to the star, and one from you to the horizon. The angle formed at the intersection of these lines is your latitude.

At the North Pole, where the North Star is directly overhead, the latitude is 90 degrees north; at the South Pole, it's 90 degrees south; and at the equator, where the polar stars are on the horizon, latitude is 0 degrees.

There are manual tools that make the calculation easier, and more accurate. The sextant is probably the most well-known. Invented in the 1700s, it uses mirrors and a moveable arm to line up any two objects and determine the angle between them [source: Beyond the Map]. And yet, without a sextant, without even sticks, you can still get a pretty good fix on it.

At twilight, find the polar star, let's say the North Star, and the horizon, and draw an imaginary line connecting them. Extend your arm toward the horizon, and make a fist. Your knuckles should be facing outward.

Then, fist over fist, count how many of your fists it takes to get from the horizon to the North Star. Each fist-length measures approximately 10 degrees. Let's say it takes three fists.

You now know your latitude is around 30 degrees N.

Latitude, of course, is only half the positional equation. It's nice to know your desert is 30 degrees north of the equator, but the equator circles the entire planet.

You need longitude, too. And longitude is tricky.

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