Hence, the pyrocentric model of Philolaus and the Pythagoreans is a pioneering and significant contribution to astronomy, in that they first envisioned accurately that the universe is neither geo- centric nor heliocentric, and that the earth and the sun are merely two heavenly bodies orbiting around a central fire.

The geocentric model says that the earth is at the center of the cosmos or universe, and the planets, the sun and the moon, and the stars circles around it. The early heliocentric models consider the sun as the center, and the planets revolve around the sun.

We know today that this explanation was completely wrong. In the 1500s, Copernicus explained retrograde motion with a far more simple, heliocentric theory that was largely correct.

The heliocentric model was generally rejected by the ancient philosophers for three main reasons: If the Earth is rotating about its axis, and orbiting around the Sun, then the Earth must be in motion. Nor does this motion give rise to any obvious observational consequences. Hence, the Earth must be stationary.

It was embraced by both Aristotle and Ptolemy, and most Greek philosophers assumed that the Sun, Moon, stars, and visible planets circle the Earth. Christianity taught that God placed the earth in the center of the universe and this made earth a special place to watch human life unfold.

The first big problem with the geocentric model was the retrograde motion of planets like Mars. His model has the planets moving around the Sun in circular orbits. This can explain retrograde motion, but his model doesn’t fit all the planetary position data that well.

Heliocentrism, a cosmological model in which the Sun is assumed to lie at or near a central point (e.g., of the solar system or of the universe) while the Earth and other bodies revolve around it.

Nicolaus Copernicus

The most highly developed geocentric model was that of Ptolemy of Alexandria (2nd century ce). It was generally accepted until the 16th century, after which it was superseded by heliocentric models such as that of Nicolaus Copernicus.

Although you can jump very high on the moon, you’ll be happy to know that there’s no need to worry about jumping all the way off into space. In fact, you’d need to be going very fast – more than 2 kilometres per second – to escape from the moon’s surface.

Gravity on Mars is only about 38% of Earth’s. So, if you weighed 100 pounds on Earth, you would only weigh about 38 pounds on Mars. And if you can jump one meter (3.3 feet) high on Earth, you would be able to jump 2.64 meters (almost 9 feet) high on Mars.

The Martian moon Phobos orbits through a stream of charged atoms and molecules that flow off the Red Planet’s atmosphere, new research shows. Many of these charged particles, or ions, of oxygen, carbon, nitrogen, and argon, have been escaping Mars for billions of years as the planet has been shedding its atmosphere.

NASA’s new modeling efforts show that Phobos’ grooves could be caused by tidal forces. NASA/JPL-Caltech/University of Arizona. Unlike Saturn, Mars doesn’t have many moons to spare.

This moon is doomed. Phobos orbits so close to Mars that gravitational tidal forces are dragging it down. In 100 million years or so, Phobos likely will be shattered by stress caused by the relentless tidal forces, the debris forming a decaying ring around Mars.

Many scientists believe that Phobos and Deimos are captured asteroids from the outer solar system, but there are other scientists who counter this idea. Both moons have a low density, which means that they cannot be made of pure rock, but rather a mixture of rock and ice.

So an observer on the Martian surface would see Deimos rise in the east, but the moon would then move across the Martian sky at a very slow pace. And since Phobos’ west-to-east motion is much faster than Mars’ rotation period, the satellite would appear to rise in the west and set in the east.