Skip to content
Spark & Whetstonethe grown-up commons

← Finding Your Way by the Stars

Finding Your Way by the Stars

Three short lessons on the oldest navigation: the star that stays put, measuring its height with your hand or a card on a string, and reading your latitude from the noon Sun.

Spark & Whetstone · sparkwhetstone.org/learn/finding-your-way-by-the-stars · CC BY-NC-SA 4.0. Answers are at the end, so you can try each question first.

Lesson 1. The star that stays

As the night goes on, the stars wheel slowly round one point in the northern sky. Polaris, the North Star, sits less than a degree from that point, so it hardly moves.

Its height above the horizon tells you how far north you are. At the North Pole it is straight overhead (90 degrees); at the equator it sits on the horizon (0 degrees). Halfway between, at 45 degrees north, it is halfway up.

From south of the equator Polaris cannot be seen, and navigators there use other stars.

Question. You measure Polaris at 40 degrees above the horizon. About how far north are you?

  1. 40 degrees north.
  2. 50 degrees north.
  3. You cannot tell from one star.

Say it back. In your own words: why does Polaris tell you how far north you are?

________________________________________________

________________________________________________

Lesson 2. Measure with your hand

You do not need instruments to measure the sky roughly. Hold your arm straight out: a closed fist covers about 10 degrees of sky, and the width of your little finger about one.

Sailors on the Indian Ocean used a kamal: a small board on a knotted cord. Holding the knot in their teeth and the board at arm's length, they lined its bottom edge up with the horizon and its top with Polaris. Each knot marked the star's height at a known port.

Question. At arm's length, Polaris is about four fists above the horizon. Roughly how high is it?

  1. About 4 degrees.
  2. About 40 degrees.
  3. About 90 degrees.

Say it back. Say it back: how did a kamal let a sailor find a port?

________________________________________________

________________________________________________

Lesson 3. The noon Sun

By day, navigators measured the Sun at its highest, at true noon. On the two equinoxes, in late March and late September, the Sun is over the equator, and your latitude is simply 90 degrees minus the Sun's noon height.

On other days the Sun is north or south of the equator by a known amount that sailors read from a table, and they added or subtracted it.

Question. On an equinox, the noon Sun is 50 degrees above the horizon. What is your latitude?

  1. 50 degrees.
  2. 40 degrees.
  3. 140 degrees.

Say it back. In your own words: how do you find latitude from the noon Sun on an equinox?

________________________________________________

________________________________________________

At the table: Make a kamal

Players: a grown-up and a child, 8 or older, on a clear night. You'll need: a piece of card about 5 by 8 cm, a metre of string, and a dark, clear sky.

  1. Make a small hole in the middle of the card and tie the string through it.
  2. Find Polaris: follow the two stars at the end of the Big Dipper's bowl upwards.
  3. Hold the string's end to your chin and slide the card out until its bottom touches the horizon and its top touches Polaris.
  4. Tie a knot where the string meets your chin. That knot is your home's latitude; check it on a map.

Answers

  1. 40 degrees north. — The height of Polaris is close to your latitude, so 40 degrees up means about 40 degrees north.
  2. About 40 degrees. — A fist is about 10 degrees, so four fists is about 40 degrees.
  3. 40 degrees. — On an equinox, latitude is 90 minus the noon height: 90 − 50 = 40 degrees.