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What you will learn in Constellations and Celestial Navigation

1. The sky tonight
The sky above you depends on where you stand and when you look, and nothing else. Every point on this dome is a measured star, placed for your latitude and your hour.
2. Constellations
A constellation figure is a pattern seen from one place. Orion's stars are scattered along the line of sight from 26 to about 2 300 light-years, and from anywhere else the figure is not there.
3. Why the sky turns
The stars do not move across the sky in a night; the ground you stand on turns under them. The proof is in the clock: the stars run 3 minutes 56 seconds a day ahead of the Sun.
4. Why the constellations change
Four minutes a night is two hours a month and the whole sky in a year. That is why Orion is a winter evening constellation in the north, and why some constellations never set and others never rise.
5. Finding your way by the stars
The height of the pole star is your latitude, which made latitude a matter of geometry for centuries. Longitude needed a clock, and four seconds of clock error is a nautical mile.
6. The sky is not permanent
Earth's axis sweeps a circle every 25 772 years. Polaris is the pole star for now; Thuban was for the pyramid builders, and Vega will come close in about 12 000 years. The stars themselves drift too.
7. What light pollution takes
Under a pristine sky about 11 000 stars are visible above the horizon at once; in a city centre, about 150. The stars have not gone anywhere. The sky in front of them has got brighter.

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1. The sky tonight
The sky above you depends on where you stand and when you look, and nothing else. Every point on this dome is a measured star, placed for your latitude and your hour.
2. Constellations
A constellation figure is a pattern seen from one place. Orion's stars are scattered along the line of sight from 26 to about 2 300 light-years, and from anywhere else the figure is not there.
3. Why the sky turns
The stars do not move across the sky in a night; the ground you stand on turns under them. The proof is in the clock: the stars run 3 minutes 56 seconds a day ahead of the Sun.
4. Why the constellations change
Four minutes a night is two hours a month and the whole sky in a year. That is why Orion is a winter evening constellation in the north, and why some constellations never set and others never rise.
5. Finding your way by the stars
The height of the pole star is your latitude, which made latitude a matter of geometry for centuries. Longitude needed a clock, and four seconds of clock error is a nautical mile.
6. The sky is not permanent
Earth's axis sweeps a circle every 25 772 years. Polaris is the pole star for now; Thuban was for the pyramid builders, and Vega will come close in about 12 000 years. The stars themselves drift too.
7. What light pollution takes
Under a pristine sky about 11 000 stars are visible above the horizon at once; in a city centre, about 150. The stars have not gone anywhere. The sky in front of them has got brighter.