2.1 Earth–Sun–Moon System

Why the Earth–Sun–Moon System Matters

Space Systems is the most heavily weighted strand on the Earth and Space Sciences Regents (20–31%), and the Earth–Sun–Moon system is its everyday core. Cluster questions rarely ask you to recall a single fact; they give a diagram, a photo of a Moon phase, or a tide graph and ask you to reason from geometry. If you understand how three moving bodies cast light and shadow, you can answer most of these without memorizing tables.

Earth's Rotation

Rotation is the spin of Earth on its axis, an imaginary line through the geographic North and South Poles. One full rotation takes about 24 hours and produces day and night. Because Earth turns from west to east (counterclockwise viewed from above the North Pole), the Sun, Moon, and stars appear to move from east to west across the sky. This apparent motion is about 15° per hour (360° ÷ 24 h), a value you can use to estimate time of day from the Sun's position.

Two classic demonstrations prove rotation:

• The Foucault pendulum swings in a fixed plane while the floor (Earth) turns beneath it, so its swing direction appears to rotate over a day.
• The Coriolis effect deflects freely moving objects — winds, ocean currents, projectiles — to the right in the Northern Hemisphere and left in the Southern Hemisphere, steering global wind belts and storm rotation.

Earth's Revolution and the Axial Tilt

Revolution is Earth's orbit around the Sun, taking about 365.25 days (one year). Earth's axis stays tilted 23.5° from the vertical and points toward the same spot in space (near Polaris) all year. This constant tilt with changing position is the engine of the seasons.

What Actually Causes the Seasons

A frequent Regents trap: seasons are NOT caused by Earth being closer to or farther from the Sun. (In fact, the Northern Hemisphere has winter in early January, when Earth is nearest the Sun.) Seasons happen because the 23.5° tilt changes two things for a given hemisphere:

1. Insolation — the intensity of incoming solar radiation. When sunlight strikes at a high angle (closer to vertical), energy is concentrated and the surface heats more.
2. Duration — the length of daylight. Longer days deliver more total energy.

At the solstices, one hemisphere is tilted most toward the Sun (summer) while the other tilts away (winter). At the equinoxes, the tilt is sideways to the Sun, so the whole Earth gets roughly 12 hours of day and 12 of night.

Moon Phases

The Moon does not make its own light; we see the half lit by the Sun. As the Moon revolves around Earth every ≈29.5 days, the angle between Sun, Moon, and Earth changes, so we see different fractions of the lit side:

• New moon — Moon between Earth and Sun; lit side faces away; Moon not visible.
• Waxing — lit fraction growing (crescent → first quarter → gibbous).
• Full moon — Earth between Sun and Moon; entire near side lit.
• Waning — lit fraction shrinking back to new moon.

Eclipses

An eclipse happens when one body's shadow falls on another. The Moon's orbit is tilted about 5°, so eclipses occur only when the Moon crosses Earth's orbital plane — not every month.

• A solar eclipse occurs at new moon: the Moon passes between Earth and Sun, casting its shadow on Earth and blocking the Sun.
• A lunar eclipse occurs at full moon: Earth passes between Sun and Moon, and Earth's shadow falls on the Moon.

Tides

Tides are the regular rise and fall of ocean water caused mainly by the gravitational pull of the Moon (and, to a lesser degree, the Sun). The Moon raises two bulges — one facing it and one on the opposite side — so most coasts see two high and two low tides per day.

• Spring tides (greatest tidal range) occur at new and full moon, when the Sun and Moon align and their pulls add together.
• Neap tides (smallest tidal range) occur at the first and third (last) quarter, when the Sun and Moon pull at right angles and partly cancel.

The word spring here means "to spring up," not the season. Connecting tide range to Moon phase is a common cluster question.

Reasoning Through Cluster Questions

Regents Space Systems clusters usually hand you a diagram or photo and ask you to deduce position. A few reliable reasoning moves:

• Given a Moon phase photo, picture where the Moon sits relative to the Sun: a full Moon means Earth is between Sun and Moon; a crescent means the Moon is nearly between us and the Sun.
• Given a Sun-path diagram, a high, long arc means summer (high insolation, long day); a low, short arc means winter.
• Given a shadow diagram, ask whose shadow lands where: the Moon's shadow on Earth is a solar eclipse, Earth's shadow on the Moon is a lunar eclipse.

Because every phenomenon here comes from the same three moving bodies, you can usually reconstruct the answer from geometry instead of recalling an isolated fact — exactly the reasoning the NYSP12SLS exam rewards.