What You See from the North Pole as the Earth Rotates

If you imagine yourself standing at the North Pole, Polaris is directly above your head and the Earth rotates beneath you from west to east. Because of this, the stars you see in the sky appear to move from left to right, from east to west. In reality, the stars remain stationary and it is the Earth beneath you that rotates.

When a star called Alnilam is visible on the horizon directly in front of you, Capella is above it, about 45° above the horizon. Another 45° upward from there is Polaris, directly above your head.

Seen from a little farther away and from the side, the previous situation looks like this.

The stars are drawn much closer here than they really are. In reality, the distance to Alnilam is about 1300 light-years, to Capella a little over 40 light-years, and to Polaris more than 400 light-years. This sheet of paper, a computer screen, or even the world’s longest sandy beach would not be enough to represent the real distances to scale if the Earth were imagined as the size of a grain of sand.

Because the Earth rotates around its axis once every 24 hours, Alnilam and Capella would appear to travel around you once every 24 hours. In reality, the Earth rotates beneath your feet and you rotate with it. Take a large cup of coffee and put on a warm fur coat. Stand facing Capella and Alnilam. Six hours later they will be directly to your right, while Polaris will still be directly above your head.

Initial situation	6 hours later

 

Twelve hours later the stars are directly behind you. By then the Earth has rotated 180°, and your nose points directly opposite compared to the initial situation.

Seen from above, the situation looks like this.

Initial situation	6 hours later. The Earth has rotated 90° eastward beneath you.
12 hours later the stars are directly behind you. The Earth has rotated 180°.	23 hours later the situation has almost returned to the starting position. Alnilam and Capella are still somewhat east of the direction your nose points. One hour later the situation is the same as in the upper left image.

C o n t e n t s

• Celestial Navigation
• The Starry Sky
• The Celestial Sphere
• What You See from the North Pole as the Earth Rotates
• How the Starry Sky Changes When You Travel to the Equator
• The Effect of Earth’s Rotation on the Starry Sky at the Equator
• The Celestial Meridian, Upper Culmination, and Lower Culmination
• Changes in Star Altitude at the North Pole, the Equator, and Between Them
• Declination
• Example of Determining Latitude
• Hour Angle, Greenwich Hour Angle (GHA)
• Aries, the Greenwich of the Celestial Sphere
• Sidereal Hour Angle (SHA)
• Local Hour Angle (LHA)
• The Relationship Between Hour Angle and Time
• Example of Determining Longitude and Latitude from the Sun at Noon
• The Relationship Between Angles, Degrees, and Nautical Miles on Earth’s Surface
• Circle of Equal Altitude
• Azimuth (A, Azimuth)
• True Bearing to a Star
• The Navigational Triangle
• Solving the Navigational Triangle, or Sight Reduction
• Celestial Navigation Using Marcq Saint Hilaire’s “Intercept” Method
• Star Identification
• Details and Practice
• Time
• Time Difference (ΔT)
• Altitude Measurement
• Example of Position Fixing
• Twilight
• Calculating Twilight Time
• Navigation Association Examination