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Capítulo 3 de 10/5 min de lectura

The moving sky

Why stars rise and set, and why constellations change with the seasons.

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EsteSurOesteculminación15° por hora
Mirando al sur desde el hemisferio norte.

Earth spins from west to east. That is why we see the sky turn the opposite way: stars rise in the east, culminate in the south (from the northern hemisphere) and set in the west, just like the Sun.

  • The sky turns about 15° per hour: one and a half fists.
  • Stars close to Polaris never set: they are called circumpolar.
  • Each star rises about 4 minutes earlier every night, because a sidereal day lasts 23 hours and 56 minutes.

The seasons of the sky

Those 4 minutes a day add up to two hours in a month. So, at the same clock time, the sky changes with the seasons: in winter Orion dominates, in summer Scorpius and the Summer Triangle.

The ecliptic

The Sun, the Moon and the planets always move along the same band of sky, the ecliptic, which crosses the zodiac constellations. A bright "star" far from this band is almost certainly a star, not a planet.