16.1 The Solar System and the Earth-Moon-Sun System
Key Takeaways
- The solar system has eight planets divided into four inner terrestrial (rocky) planets — Mercury, Venus, Earth, Mars — and four outer Jovian planets: Jupiter and Saturn (gas giants) and Uranus and Neptune (ice giants).
- Seasons arise from Earth's 23.5° axial tilt; solstices mark the longest and shortest days of the year, and equinoxes mark days of nearly equal daylight and night everywhere on Earth.
- Lunar phases are caused by the changing geometry between the Sun-lit half of the Moon and the viewer on Earth, NOT by Earth's shadow on the Moon — Earth's shadow only causes lunar eclipses.
- A solar eclipse occurs when the Moon is between Earth and the Sun; a lunar eclipse occurs when Earth is between the Sun and the Moon; both involve the umbra (dark central shadow) and penumbra (partial shadow).
- Earth is habitable because of liquid water, a protective atmosphere, a magnetic field generated by a molten iron core, a stable Sun at a suitable distance, and an axial tilt that prevents extreme climate swings.
Celestial Objects in the Solar System
The solar system contains more than the eight planets. Texas 4–8 students should know the major categories:
- Planets: large bodies that orbit the Sun, are roughly spherical, and have cleared their orbit of other debris.
- Moons (natural satellites): objects orbiting planets; Earth has one, Mars has two, the gas giants have dozens.
- Asteroids: rocky or metallic bodies, mostly in the asteroid belt between Mars and Jupiter.
- Comets: icy bodies that form glowing comas and tails when they approach the Sun; the tail always points away from the Sun.
- Meteors: streaks of light from meteoroids burning in Earth's atmosphere; a meteorite is any piece that reaches the ground.
- Dwarf planets: orbit the Sun and are spherical but have not cleared their orbit; Pluto, Ceres, Haumea, Makemake, and Eris.
Inner (Terrestrial) vs. Outer (Jovian) Planets
| Property | Terrestrial (Mercury, Venus, Earth, Mars) | Jovian (Jupiter, Saturn, Uranus, Neptune) |
|---|---|---|
| Composition | Rock and metal | Hydrogen, helium, ices (Jupiter/Saturn = gas giants; Uranus/Neptune = ice giants) |
| Size | Small to medium | Large (Jupiter could hold ~1,300 Earths) |
| Moons | Few or none | Many — Jupiter and Saturn each have 50+ |
| Rings | None | All have ring systems; Saturn's are most visible |
| Position | Closest to Sun, inside the asteroid belt | Outside the asteroid belt |
| Atmosphere | Thin or (Mercury) almost none | Thick and massive |
A common TExES probe asks which feature distinguishes the inner planets as a group: the answer is rocky, metal-rich composition and relatively small size.
The Earth-Moon-Sun System: Seasons
Earth's 23.5° axial tilt is the key to the seasons. Because the axis stays pointed in the same direction as Earth orbits the Sun, each hemisphere tips toward the Sun for part of the year and away for the other part.
- June solstice: Northern Hemisphere tipped toward the Sun; longest day of the year north of the equator; Sun directly overhead at the Tropic of Cancer (23.5° N).
- December solstice: Northern Hemisphere tipped away; shortest day north of the equator; Sun directly overhead at the Tropic of Capricorn (23.5° S).
- March and September equinoxes: the axis is perpendicular to the Earth-Sun line; day and night are about 12 hours each everywhere on Earth; the Sun is directly overhead at the equator.
Addressing the misconception explicitly: Earth is at perihelion (closest to the Sun) in January — during Northern Hemisphere winter — so distance cannot explain why it is summer in Texas in July. The angle of sunlight and the length of daylight do.
Lunar Phases
The Moon always has half of its surface lit by the Sun. What changes is how much of that lit half we can see from Earth as the Moon orbits us. Phases are NOT caused by Earth's shadow on the Moon — Earth's shadow only falls on the Moon during a lunar eclipse.
| Phase | What we see from Earth | Moon's position relative to Earth-Sun line |
|---|---|---|
| New | Dark face toward us | Moon between Earth and Sun |
| Waxing crescent | Thin sliver growing | Between new and first quarter |
| First quarter | Half lit on the right side | 90° east of the Sun |
| Waxing gibbous | More than half, growing | Between first quarter and full |
| Full | Entire near side lit | Moon opposite the Sun |
| Waning gibbous | More than half, shrinking | Between full and last quarter |
| Last quarter | Half lit on the left side | 90° west of the Sun |
| Waning crescent | Thin sliver shrinking | Between last quarter and new |
A useful rule: if the lit side is on the right and growing, the phase is waxing; if on the left and shrinking, it is waning.
Eclipses
- Solar eclipse: the Moon passes between Earth and the Sun and casts a shadow on Earth; occurs only at new moon. The umbra is the dark central shadow; the penumbra is the lighter outer shadow where only part of the Sun is blocked.
- Lunar eclipse: Earth passes between the Sun and the Moon and casts its shadow on the Moon; occurs only at full moon. During totality the Moon often looks red (the "blood Moon") because sunlight bent through Earth's atmosphere projects red wavelengths onto the lunar surface.
Eclipses do not occur every month because the Moon's orbit is tilted about 5° to the Earth-Sun plane, so the shadows usually miss.
Tides
Tides are caused mainly by the Moon's gravitational pull and, to a lesser degree, the Sun's. The ocean bulges toward the Moon and, on the opposite side, bulges away due to inertia — producing two high tides and two low tides per day in most locations.
- Spring tides: Sun, Moon, and Earth aligned (new and full moon); gravity adds together; higher high tides and lower low tides.
- Neap tides: Sun and Moon at right angles (first and last quarter); gravity partially cancels; smaller tidal range.
What Makes Earth Habitable
Earth is the only known planet with abundant liquid water on its surface. Other factors that make Earth habitable and that a TExES item may ask about:
- Atmosphere rich in nitrogen and oxygen that protects surface life and moderates temperature.
- Magnetic field generated by the molten iron outer core, shielding the surface from harmful solar radiation.
- Distance to a stable star in the "habitable zone," where water can remain liquid.
- Axial tilt and rotation rate that prevent one side from permanently freezing and the other from overheating.
- Active plate tectonics that recycle carbon and regulate long-term climate.
A student says the Moon's phases happen because Earth's shadow falls on the Moon. What is the best correction?
Which statement correctly describes the conditions for a solar eclipse?
Why are the inner planets grouped together as terrestrial planets?