The Starry Sky

The Earth is quite large. If you try to walk from Helsinki to Hämeenlinna, at some point you will probably feel like buying a train ticket. Although Hämeenlinna is far away, the Earth is tiny compared to our solar system. If the Earth were imagined as a 1 mm grain of sand, the Sun would correspond to a 10 cm orange. On this scale, the Earth would orbit the orange at a distance of 10 meters, and the Moon would be a 0.3 mm particle orbiting the Earth at a distance of 3 centimeters. The distance to the nearest next star, Proxima Centauri, would on this scale be about 3000 kilometers. If a local train from Helsinki to Proxima Centauri traveled at 100 km/h, it would arrive in 45.6 million years.

Our solar system and Proxima Centauri are two stars in a galaxy, a collection of stars called the Milky Way. The Milky Way is estimated to contain 100–400 billion stars. The number is imprecise because stars are difficult to count. I know, because I tried it myself.

If we imagine the Milky Way as a couple of hundred billion oranges floating in space, the cluster of oranges would spread across an area 74 million kilometers in diameter. This already becomes difficult to comprehend. If we reduce the scale and imagine that the distance between the Sun and Proxima Centauri is only one millimeter, then the diameter of the Milky Way would be 23 meters. You can imagine the Milky Way as a disk 23 meters across, filled with tiny points of light — stars — spaced 1 millimeter apart.

The nearest next galaxy, containing another few billion point-like stars spaced a millimeter apart, is the Andromeda Galaxy, 536 meters away. In reality, the distance is 2 million light-years, which means that light takes 2 million years to travel from Andromeda to Earth through the cold and dark emptiness of space. After such a journey, a ray of sunlight would gladly stop in a troll’s cave to warm itself. The true diameter of our own Milky Way is 100,000 light-years. The Milky Way, like galaxies in general, is flat and only about 1,000 light-years thick.

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