Treading Among the Stars: How We’ve Found Our Way

Planetary Models: Celestial Globes and Orreries

Globes and other 3-D models help to put objects in space into perspective, and serve as scientific instruments, ornamental showpieces, and physical illustrations of the astronomy beliefs of the day. Over time, as scientists gather more and more data they use it to improve their models and make them more and more accurate.

After 2-dimentional representations of the sky achieved some sophistication, celestial globes and orreries incorporated a newly understood concept of the shape of the sky and the universe, that of depth and volume in space beyond Earth.

You may be familiar with Terrestrial globes which detail geographical features of the Earth, but there are also globes that illustrate the physical features of planets, like the moon or Mars. Celestial globes, are spherical maps of the visible sky. Orreries incorporate these various globes into 3-D mechanical models of our Solar System. 

These new physical representations of the sky allowed even an untrained user to begin to comprehend the dimensional connection between the Earth as a globe and its physical presence in an expanding conceptual understanding of the solar system and the universe.

This artifact has a glass globe representing the celestial sphere encircling an orrery inside. The glass globe shows the brighter stars and constellations, while the interior shows the motion of the planets and their moons through the use of gears. Though the sizes of the planets and their satellites and the distances between them are not to scale, the mechanism was constructed to rotate in proportion to the planets they represent.

Erhardt Weigel, a professor at the University of Jena in Germany, built this celestial globe in 1699. The larger holes you see in the bottom of the globe provide a space to put a lamp inside. The light shines through the small holes identifying each constellation, making this one of the earliest known optical projecting planetariums. To make astronomy more relatable at the time, Weigel linked existing constellations to European royal families and countries. For example, Ursa Major was replaced with the elephant representing Denmark and the constellation Orion is a double-headed Austrian eagle.
The word "planetarium" can mean any apparatus or model representing the planetary system, but you may have visited an entire room that is dedicated to projecting stars and planets on the walls like the Fels Planetarium in Philadelphia.

A significant evolutionary step in the groundwork of a more accurate representation of the universe emerged when the planetarium experience debuted in the early 1920’s. Under the artificial sky, a presenter could now guide an audience through a wide variety of tutorials while visually illustrating the complexity of the cosmos. Current planetarium technology employs state-of-the-art computer graphic image capability to allow immersive, virtual travel excursions across the universe, across time. 

Tellurians are astronomical instruments that show the rotation of the Earth in its axis and its orbit around the sun. This movement demonstrates the day and night, as well as the seasons. The model works something like a clock, except with planets instead of hour and minute hands. Table-top models with gears and levers, like this one, were used in the 1800s in schools and public lectures to demonstrate astronomical events. Turning the crank sets the gears in motion. The gears are carefully designed so that the planets all stay properly aligned with one another.

This orrery was owned by Joseph Priestley who was a scientist and philosopher credited with the discovery of oxygen. Priestley’s orrery depicts Mercury, Venus, and Earth rotating around the sun. The moon, with phases engraved below, orbits the Earth.

The sun is not moving; it remains in its fixed location. Planet Earth revolves around the sun once every 365 days and rotates on its axis once every 24 hours. At any moment in time, Earth is both rotating and revolving. Likewise, the earth’s moon is revolving around the earth while rotating. As the moon orbits Earth, the sun lights up whichever side of the moon is facing it. To the sun, it’s always a full moon! We see the moon from the center of its orbit, so we see different portions of the lit side of the moon, or phases of the moon. 

The historical models you see here, all have informed predictions about various astronomical events, like solar eclipses.

What is a solar eclipse? A solar eclipse is an astronomical event observed on Earth during which light from the sun is blocked by the moon. Eclipses are all about the alignment of these three objects, which of them is blocking the sun’s light, and the interesting shadows they create. When the moon lines up directly between the Sun and the Earth, it blocks sunlight from reaching a narrow strip of Earth for a few hours. There’s a time element involved because the earth and the moon are in constant motion. This event is called a total solar eclipse; the sun is ‘eclipsed’ (completely blocked) by the moon. If the moon doesn’t exactly line up in front of the sun, only part of the sun is eclipsed, and the event is a called partial solar eclipse. Then there’s the very special case of an annular eclipse, where the moon lines up directly in front of the sun, but isn’t big enough to completely cover the sun. In that case, a ring of sunlight encircles the moon as it stands in front of the sun. 

Sound complicated? It really isn’t. For the casual observer, it’s just light and shadows. Try this experiment: Stand outside in bright sunlight. Allow your shadow to fall on a nearby person, plant or pet. Your body, blocking light from the sun, creates a shadow, causing a solar eclipse for whatever is in your shadow. 

Likewise, the moon, in blocking light from the sun, creates a shadow, causing a solar eclipse for whatever is in its’ shadow. Get it? That’s all an eclipse is! But that’s not to say eclipses aren’t complicated and even fascinating as individual events – in fact, for scientists who study eclipses, they are incredibly complicated and each solar eclipse has its own signature.