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Earth and Space Science Unit 5: The solar system

Earth's motions and the Moon's orbit explain days, years, seasons, moon phases, eclipses and tides.

Covers: Earth's rotation and revolution, seasons, moon phases, eclipses, tides, planets, gravity

Study guide

Earth rotates on its axis once about every 24 hours, causing day and night, and revolves around the Sun once about every 365.25 days. Earth's axis is tilted about 23.5°, and that tilt, not the distance from the Sun, causes the seasons. When a hemisphere tilts toward the Sun, it gets more direct sunlight and longer days, so it has summer. The solstices mark the longest and shortest days, and at the equinoxes day and night are roughly equal everywhere.

The Moon orbits Earth about every 27 days, and its phases cycle about every 29.5 days. The Moon doesn't make light; we see the half lit by the Sun from different angles. The phases go new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter and waning crescent. We always see the same side of the Moon.

Eclipses happen when the Sun, Earth and Moon line up. In a solar eclipse, the Moon passes between Earth and the Sun and blocks sunlight (at new moon). In a lunar eclipse, Earth's shadow falls on the Moon (at full moon). They don't happen every month because the Moon's orbit is tilted.

Tides are caused mainly by the Moon's gravity pulling on Earth's oceans, creating two bulges, so most coasts get two high and two low tides a day. Spring tides (the biggest range) occur when the Sun and Moon line up, and neap tides when they're at right angles.

The solar system has eight planets. The inner planets (Mercury, Venus, Earth, Mars) are small and rocky; the outer planets (Jupiter, Saturn, Uranus, Neptune) are gas and ice giants with many moons and rings. Gravity holds the planets in orbit, and the closer a planet is to the Sun, the faster it orbits.

At the equinoxes the Sun rises due east and sets due west. In the Northern Hemisphere's summer it rises north of east and climbs highest at noon, and in winter it rises south of east and stays low. In New York the Sun is never directly overhead. In the Northern Hemisphere, the altitude of Polaris above the northern horizon equals the observer's latitude. Because Earth rotates, stars appear to circle Polaris counterclockwise at about 15° per hour, so long-exposure photos facing north show circular star trails.

Earth rotates 360° in 24 hours, or 15° per hour, so each 15° of longitude is about one hour of time, and places farther east are later. Evidence that Earth rotates includes the Foucault pendulum, whose swing slowly changes direction, and the Coriolis effect. Earth's axis keeps pointing the same direction in space (toward Polaris) all through its orbit, which is why each hemisphere tilts toward the Sun at one time of year and away at another. As Earth revolves, about 1° per day, the night side faces different parts of space, so some constellations are visible only in certain seasons.

Because the Moon advances in its orbit each day, high tides come about every 12 hours 25 minutes. Low tide falls roughly 6 hours after each high tide. The solar system formed about 4.6 billion years ago from a collapsing, spinning nebula. Rocky inner planets formed close to the hot young Sun and gas giants farther out. As the young planets melted, dense iron and nickel sank to their centers and lighter materials rose, creating layers.

Key ideas

• Rotation (24 hours) gives day and night; revolution (365 days) gives the year.
• Seasons come from Earth's 23.5° axial tilt, not distance from the sun.
• Moon phases come from how much of the lit half we see.
• Solar eclipse: Moon between Sun and Earth. Lunar eclipse: Earth between Sun and Moon.
• Tides come mainly from the Moon's gravity; two high tides happen each day.

Formulas and rules

Common mistakes

• Saying seasons happen because Earth gets closer to the sun.
• Thinking moon phases are caused by Earth's shadow.

Words to know

Axial tilt

Earth's 23.5° tilt, which causes the seasons.

Solar eclipse

The Moon blocks the Sun from Earth.

Lunar eclipse

Earth's shadow falls on the Moon.

Tides

Rise and fall of sea level, mainly from the Moon's gravity.

Lunar cycle

About 29.5 days, new moon to new moon.

Rotation

One spin of Earth on its axis: about 24 hours.

Revolution

One orbit of Earth around the Sun: about 365.25 days.

Inner planets

Mercury, Venus, Earth, Mars.

Solstice

The longest or shortest day of the year.

Equinox

Day and night are roughly equal everywhere.

Moon phases

New, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, third quarter, waning crescent.

Waxing / waning

The lit part growing / shrinking.

Spring tide

The biggest tidal range, when Sun and Moon line up.

Neap tide

The smallest tidal range, when Sun and Moon are at right angles.

Outer planets

Jupiter, Saturn, Uranus, Neptune: gas and ice giants.

Orbit

The path of one object around another, held by gravity.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.