Using the Sun & Stars to Determine Latitude & Longitude

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  • 0:05 What are Latitude and…
  • 0:56 Determining Latitude
  • 3:27 Determining Longitude
  • 5:07 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this video, you will be able to explain how to use the stars or the Sun to find out your latitude and longitude. A short quiz will follow.

What is Latitude and Longitude?

Latitude is the angular distance of a place north or south of the Earth's equator in degrees. A latitude of zero degrees is on the equator of the Earth, 90 degrees south is the South Pole, and 90 degrees north is the North Pole.

Longitude is the angular distance of a place east or west of the meridian at Greenwich, in the United Kingdom. A longitude of zero degrees means it is directly north or south of Greenwich, and a longitude of 180 degrees west means it is halfway around the world from Greenwich, measured east to west.

Every point on the Earth has a latitude and longitude, but how can you figure out your own latitude and longitude? What is your latitude and longitude right now?

Determining Latitude

You can measure your latitude using either the Sun or the stars. To find your latitude using the stars, you only need to find the North Star, called Polaris. The North Star is a star that is directly above the North Pole and appears not to be moving during the night. If you were standing at the North Pole, the North Star would be directly above you. And, if you were standing at the Earth's equator, the North Star would be right along the horizon. So, you can use this knowledge to your advantage.

If you measure the angle of the North Star above the horizon, that will be the same as your latitude. Think about it: the North Pole is a latitude of 90 degrees north, and the equator is 0 degrees north. At the North Pole, the North Star is above, which is 90 degrees above the horizon. And, at the equator, the North Star is on the horizon, which is 0 degrees above the horizon. It works!

But, what if it's not nighttime, and you need to know your latitude during the day? During the day, you can only measure your latitude by considering the position of the Sun at exactly noon. If you don't have a watch, you can find out when it's noon by looking for the moment when shadows are shortest.

Once it's noon, measure the angle of the Sun below the vertical. (Meaning straight above you is zero degrees, and on the horizon is 90 degrees.) This can be hard, because it's dangerous to look directly at the Sun. But, there are some clever ways it can be done using shadows or special equipment. However you do it, measuring the angle of the Sun above the horizon at noon is the first step.

Unfortunately, it's not quite as simple as using the North Star. This is because the Earth's tilt complicates things. The angle of the Sun below the vertical at noon is equal to the latitude, but only on March 21st and September 21st, the spring and autumn equinoxes - these are the days when there are exactly 12 hours of day and 12 hours of night. At the winter solstice (December 21st), you need to subtract 23.45 degrees from your answer because the Earth is tilted towards the Sun, moving the Sun's position in the sky. And, at the summer solstice (June 21st), you need to add 23.45 degrees. In between is... in between. You'd have to use some fractions to figure it out on other days, which makes it a bit complicated.

Determining Longitude

To figure out your longitude, you'll need to be able to communicate with Greenwich. That's because longitude is measured relative to Greenwich in the United Kingdom.

To measure the longitude, we first have to figure out how much time has passed between noon in Greenwich and noon at our location. You can figure out when noon is by putting a stick in the ground and finding when the shadows are shortest. That time is noon. It's not a good idea to use a watch or clock for this, because we use huge time zones, where everyone has the same time. This makes life easier, but it means that noon on our clock, isn't exactly the same as noon for the Sun (unless you happen to be right in the middle of a time zone).

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