Astronomy
Astronomy studies the celestial bodies and the Universe: the position and motion of stars and planets, the light that reaches us, and the models that describe the Cosmos. In Orb: On the Movements of the Earth, it is the ground of the forbidden pursuit of heliocentrism, from naked-eye observation to the astrolabe.
Entries in this discipline
The astrolabe
In Orb: On the Movements of the Earth, Rafal owns an astrolabe. We explain what this ancient astronomical instrument is, what it does and how it works.
The Earth's rotation and revolution
Hubert explains it in Orb: the Earth moves twice. We separate rotation (day and night) from revolution (seasons), and why we feel nothing.
Ptolemy's geocentric model
Before heliocentrism, the Earth sat at the center of the world. We explain Ptolemy's geocentric model, its epicycles, and why it lasted over a thousand years.
Heliocentrism
Hubert defends a forbidden idea in Orb: the Sun at the center, the Earth turning. We explain heliocentrism, its history, and why it caused such an uproar.
Observing the sky with the naked eye
Hubert picks a precise observation spot in Orb. We explain why altitude, a clear sky and the absence of the Moon are the right conditions.
The constellation Orion
Orion's Belt is a clue in Orb, centered on the star Alnilam. We explain this constellation, its three aligned stars and how to spot it.
Star magnitude
Rafal marvels at seeing a magnitude-six star in Orb. We explain what magnitude is, why the scale runs backwards, and the limit of the naked eye.
Using an astrolabe
Rafal observes the sky for Hubert in Orb. We walk you through, step by step, how to use an astrolabe to measure a star's altitude and read the time.
Why measure a star with an astrolabe
The stars are already engraved on the astrolabe, so why sight one? The instrument is a calculator that derives the time, your orientation and your latitude.