Gravity, Orbits, and the Solar System

For most of history, the obvious model won: Earth sits still at the center and everything circles us. This geocentric model matches what your eyes see, and with enough added circles it even predicted planet positions tolerably. But the planets misbehave. They drift eastward against the stars and then loop briefly backward. Explaining that retrograde motion made the Earth-centered machinery absurdly complicated.

The heliocentric model puts the sun at the center, with the planets orbiting it, Earth included. That makes retrograde motion simple to explain. A faster inner planet passes a slower outer one, the same way a car you pass on the highway seems to slide backward for a moment. When Kepler replaced the circles in the model with ellipses, the predictions finally matched what astronomers actually observed. That gave us the modern picture of a sun-centered system, held together by a force the next section explains.

Gravity is the attractive force between all masses, everywhere, all the time. Two things control how strong it is, and those are mass and distance. More mass means more pull. That is why the sun, which holds more than 99 percent of the solar system's mass, controls every orbit. Gravity also weakens quickly with distance. Move twice as far away and the pull drops to a quarter of what it was.

So why does the moon not fall into Earth, or Earth into the sun? They are falling, but they are also moving sideways fast enough to keep missing. An orbiting body has inertia, which is the tendency to keep moving in a straight line. Gravity constantly bends that straight line toward the center. An orbit is the balance between the two. Throw a ball sideways fast enough and the curve of its fall matches the curve of the planet. Remove gravity and the moon would sail off in a straight line. Remove its sideways motion and it would drop straight down. An orbit needs both.

Orbits are not circles. Every orbit is an ellipse, which is an oval built around two inside points called foci. The sun sits at one focus and nothing sits at the other. Eccentricity measures how stretched an ellipse is, and you now need to know the formula by heart. Eccentricity equals the distance between the foci divided by the length of the major axis. An eccentricity of 0 is a perfect circle. The closer the value gets to 1, the more stretched the ellipse.

Earth's eccentricity is just 0.017, so our orbit is nearly circular. Earth is only slightly closer to the sun at perihelion in January than at aphelion in July. Mercury, at 0.206, has the most eccentric planetary orbit, and comets run to extremes near 1. Distance from the sun also sets speed. A body moves fastest at perihelion and slowest at aphelion. That is why a comet spends centuries crawling through deep space and mere weeks whipping around the sun.

Build an ellipse the way the Regents does, using two measurements. You need the distance between the foci and the length of the major axis. The lab runs the formula and compares your ellipse to real orbits. Push the foci together and watch the orbit round out into a circle.

The whole system began as a huge, slowly turning cloud of gas and dust. About 4.6 billion years ago it collapsed under its own gravity. Most of the material fell to the center and became the sun. What was left flattened into a disk and clumped together into planets. That is why all the planets orbit in the same direction and in nearly the same plane.

The result splits neatly in two, as the Solar System Objects Data Table on page 2 of your Reference Tables shows. The four inner, terrestrial planets, Mercury, Venus, Earth, and Mars, are small, dense, and rocky, built from the materials that could survive close to the young sun. The four outer Jovian planets are Jupiter, Saturn, Uranus, and Neptune. They are enormous, gaseous, and so low in density that Saturn would float in a big enough bathtub. Between them and around them travel more than 200 moons, and the chart's columns of data, distance, period, diameter, eccentricity, are a Regents favorite for practice reading tables.

The planet-building process left debris. Asteroids are rocky bodies, most of them orbiting in the asteroid belt between Mars and Jupiter, leftovers that Jupiter's gravity never let assemble into a planet. Comets are mountains of ice and dust on extremely eccentric orbits. When one swings near the sun, its ices vaporize into a glowing tail. That tail always points away from the sun, pushed by sunlight and solar wind, whichever way the comet is moving.

Small fragments have their own vocabulary, and it is tested often. A meteoroid is the fragment while it is still in space. A meteor is the streak of light it makes burning through the atmosphere. A meteorite is the piece that survives and reaches the ground. Impacts matter at every scale, from the craters that cover the moon to the asteroid strike 66 million years ago that ended the Mesozoic Era. That last one connects to the extinction record you read about in Unit 7.

The heliocentric model beat the geocentric one because ellipses and a moving Earth explain the sky more simply, including the retrograde loops. Gravity gets stronger with more mass and much weaker with distance. It works together with inertia to make an orbit, which is really endless sideways falling. Every orbit is an ellipse with the sun at one focus. Eccentricity measures how stretched it is, and it equals the distance between the foci divided by the major axis. Bodies move fastest at perihelion, the closest point. The system formed from one spinning cloud into four rocky inner planets and four low-density gas giants. The leftovers are the asteroids, the comets with tails that point away from the sun, and the meteoroids, meteors, and meteorites that still occasionally hit Earth.

Ten Regents-style questions, one at a time in a focused view, each with an instant explanation. The set reshuffles when you reach the end, so you can keep practicing as long as you like.