2.2 The Solar System & Orbital Motion

From Earth-Centered to Sun-Centered

For centuries the geocentric model placed a motionless Earth at the center of the universe, with the Sun, Moon, and planets revolving around it. It struggled to explain retrograde motion — the times when a planet like Mars appears to move backward against the stars. The heliocentric model, advanced by Copernicus and supported by Galileo's telescope observations, puts the Sun at the center with planets orbiting it. Retrograde motion is then simply an apparent effect: as faster, inner Earth overtakes a slower outer planet, that planet seems to slip backward, just as a passed car seems to drift backward relative to you.

The heliocentric model is the foundation of everything else in this section.

Kepler's Laws of Planetary Motion

Johannes Kepler described how planets actually move:

1. Law of Ellipses (First Law): Each planet orbits the Sun in an ellipse, with the Sun at one focus. An ellipse has two foci; the orbit is not a perfect circle.
2. Law of Equal Areas (Second Law): A planet sweeps out equal areas in equal times, so it moves faster when nearer the Sun (perihelion) and slower when farther away (aphelion).
3. Law of Periods (Third Law): Planets farther from the Sun take longer to complete one orbit. Neptune's year is far longer than Mercury's.

Eccentricity — How Elliptical an Orbit Is

Eccentricity measures how stretched an ellipse is. The Earth and Space Sciences Reference Tables give the formula:

eccentricity = distance between the foci ÷ length of the major axis

Key points for the exam:

• A perfect circle has eccentricity = 0 (the two foci sit at the same point).
• As an ellipse becomes more elongated, eccentricity approaches 1.
• Eccentricity has no units — it is a ratio of two lengths, so the units cancel.
• Example: if the foci are 2.0 cm apart and the major axis is 8.0 cm long, then eccentricity = 2.0 ÷ 8.0 = 0.25. Always round to the appropriate number of digits.

Most planetary orbits, including Earth's (about 0.017), have low eccentricity — nearly circular. Comets and Mercury have noticeably higher values.

Gravity: Why Orbits Exist

Isaac Newton explained that every object attracts every other object through gravity. The strength of this gravitational force depends on two things:

• It increases as mass increases — more massive objects pull harder.
• It decreases as the distance between objects increases, specifically with the square of the distance. Doubling the distance makes the force one-fourth as strong (an inverse-square relationship).

Gravity is what keeps planets bound to the Sun and moons bound to planets: each body is continually "falling" toward the Sun while moving sideways fast enough to stay in orbit.

Reading the Solar System Data Table

The reference tables include a Solar System Data table. Regents questions often ask you to compare planets using its columns:

Terrestrial (inner) planets — Mercury, Venus, Earth, Mars — are small, rocky, and dense. The gas giants — Jupiter, Saturn, Uranus, Neptune — are large, low in density, and far from the Sun. A density question is a favorite: Saturn's density is less than water's, so the table lets you rank planets by mass-to-volume ratio without any outside knowledge.

Distance, Speed, and the Tilt Trap (Again)

Because this strand mixes seasons with orbits, expect a question that tries to link the seasons to a planet's changing distance in its slightly elliptical orbit. On Earth, the small eccentricity means distance barely affects climate — the 23.5° axial tilt, covered in Section 2.1, is the real cause of seasons. Use the data table for size, mass, and period comparisons, but keep the tilt rule firmly in mind when the question shifts to seasons.

Other Solar System Objects

Beyond the eight planets, the Solar System holds smaller bodies that appear in Regents questions:

• Asteroids — rocky bodies, most in the asteroid belt between Mars and Jupiter.
• Comets — icy bodies on highly eccentric orbits; as a comet nears the Sun, heated ices form a glowing tail that always points away from the Sun.
• Meteoroids, meteors, and meteorites — a meteoroid is a small fragment in space; it becomes a meteor ("shooting star") when it burns in the atmosphere, and a meteorite if it survives to reach the ground.
• Dwarf planets such as Pluto, which orbit the Sun but have not cleared their orbital paths.

Putting Orbital Motion Together

A strong test-taker links the three big ideas of this section. Gravity supplies the inward pull that bends a moving planet into a closed path. Kepler's laws describe the shape (an ellipse), the speed (faster near the Sun), and the timing (longer year for farther planets) of that path. Eccentricity quantifies how stretched the ellipse is, using the reference-table ratio of foci distance to major-axis length. When a cluster gives you a scale drawing of an orbit, measure the two foci spacing and the long axis, divide, and you have the eccentricity — no memorized planet values required.