Astronomy · Rung 4 of 12

The naked-eye sky

Finding your way up there is like using a map: you need two coordinates and a fixed point. The fixed point is the celestial pole, and Polaris sits less than a degree from it. Its height above the horizon is your latitude: from Stockholm, 59°. Anything within 59° of the pole is circumpolar and never sets (the Plough, Cassiopeia, Draco). Constellations are not groups of neighbouring stars but figures we draw over stars at wildly different distances; they are a street map, not physics. And one line matters more than the rest: the ecliptic, where the Sun, the Moon and the planets travel.

Where this sits

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The sky from here
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Instruments

Exercises from this rung

Generated by the app, from easiest to hardest

  1. From Stockholm, Polaris sits 59° above the horizon. What does that number tell you?

    • Your latitude
    • Your longitude
    • The time
    • The season

    Why The height of the celestial pole above the horizon is exactly your latitude, and Polaris sits under a degree from the pole. It is the oldest navigation method there is: with a sextant and a clear night you know your parallel. Longitude, by contrast, needed accurate clocks and was not solved until the 18th century.

    easier

  2. Since 1922 the 88 official constellations are, technically…

    • Regions of sky with borders
    • The ones visible to the naked eye
    • Groups of neighbouring stars
    • Drawings approved by the IAU

    Why The International Astronomical Union stopped defining them as figures and turned them into regions with borders drawn in coordinates, like countries on a map. Every point of sky belongs to one and only one, which is why a very distant galaxy “is in” Ursa Major even though it has nothing to do with those stars.

    in between

  3. 4,800 years ago the pole star was not Polaris but Thuban. Why does it change?

    • The Earth's axis traces a slow cone
    • The definition of north changed
    • The stars move a great deal
    • The pole drifts with the continents

    Why The precession of the equinoxes: the Earth is a flattened top, and the pull of Sun and Moon on its equatorial bulge makes its axis trace a slow cone, 26,000 years around. In 12,000 years the north star will be Vega. It is also what shifted the zodiac by almost a whole sign relative to the inherited dates.

    harder