Astronomy & the Solar System — Science

  1. Grade 9 — Earth, Moon & Sun: Moon Phases
  2. Grade 9 — Seasons & Earth's Tilt
  3. Grade 9 — The Solar System
  4. Grade 9 — Gravity, Orbits & Tides

Apply it in a new setting

Problem 1

One full moon-phase cycle (new moon back to new moon) takes about 29.5 days. The four main phases — new, first quarter, full, third quarter — are evenly spaced around the cycle. About how many days after a new moon does the third-quarter (last-quarter) phase occur?
Hints

The four main phases split the 29.5-day cycle into four equal parts.

Third quarter is three-quarters of the way around the cycle.

Compute ¾ × 29.5.

Answer and explanation

22 days

The four main phases are evenly spaced, so each quarter of the cycle is 29.5 ÷ 4 ≈ 7.4 days.

Third quarter is three steps around: 3 × 7.4 ≈ 22.1 days (equivalently ¾ × 29.5).

So the third-quarter phase is about 22 days after the new moon.

Problem 2

Terrestrial planets are small, dense, and rocky. A rock sample from such a planet has a mass of 66 g and a volume of 12 cm³. What is its density? (density = mass ÷ volume)
Hints

Density = mass ÷ volume.

Divide the mass in grams by the volume in cm³.

A result well above 1 g/cm³ (water) confirms a dense, rocky material.

Answer and explanation

5.5 g/cm³

Density = mass ÷ volume = 66 g ÷ 12 cm³.

66 ÷ 12 = 5.5.

The density is 5.5 g/cm³ — far denser than water (1 g/cm³), as expected for a rocky terrestrial planet.

Problem 3

Mercury, close to the Sun, feels a stronger pull and races around it in about 88 days. Earth, farther out, takes about 365 days. About how many times does Mercury orbit the Sun in the time it takes Earth to orbit once?
Hints

A planet closer to the Sun completes its orbit faster (shorter 'year').

Find how many Mercury-years fit into one Earth-year.

Divide Earth's period by Mercury's period: 365 ÷ 88.

Answer and explanation

4.1 times

Closer planets orbit faster, so Mercury laps the Sun many times per Earth year.

Number of orbits = Earth's period ÷ Mercury's period = 365 ÷ 88.

365 ÷ 88 ≈ 4.1 times.

Problem 4

Earth is actually closest to the Sun (perihelion) in early January, yet that is winter in the Northern Hemisphere. What is the best explanation?
  1. The Sun simply burns cooler in January
  2. Seasons come from Earth's 23.5° axial tilt; in January the Northern Hemisphere tilts away, getting glancing sunlight and short days
  3. Winter happens because Earth is farthest from the Sun in January
  4. The Moon blocks sunlight from the Northern Hemisphere in winter
Hints

Seasons are not caused by Earth's changing distance from the Sun.

What matters is how directly sunlight strikes a hemisphere, set by the axial tilt.

In January, is the Northern Hemisphere tilted toward or away from the Sun?

Answer and explanation

Seasons come from Earth's 23.5° axial tilt; in January the Northern Hemisphere tilts away, getting glancing sunlight and short days

Earth's distance barely changes and is actually smallest in January, so distance can't cause Northern winter.

Seasons come from the 23.5° axial tilt: in January the Northern Hemisphere leans away from the Sun, so sunlight arrives at a low, glancing angle with short days — winter.

The correct explanation is B.

Problem 5

A solar eclipse can only happen at new moon, yet we do NOT get a solar eclipse at every new moon. What best explains why eclipses are comparatively rare?
  1. The Moon's orbit is tilted about 5° to Earth's orbit, so usually the Sun, Earth, and Moon don't line up exactly
  2. The Moon stops orbiting during most new moons
  3. Clouds block most eclipses from ever forming
  4. The Moon is only sometimes lit by the Sun
Hints

A solar eclipse needs the Moon exactly between the Sun and Earth.

The Moon's orbit isn't in the same flat plane as Earth's orbit around the Sun.

Most months the Moon passes a bit above or below the perfect line.

Answer and explanation

The Moon's orbit is tilted about 5° to Earth's orbit, so usually the Sun, Earth, and Moon don't line up exactly

A solar eclipse requires a near-perfect Sun-Moon-Earth alignment at new moon.

But the Moon's orbit is tilted about 5° relative to Earth's orbit, so most new moons the Moon passes slightly above or below the Sun-Earth line and its shadow misses.

Only the few times they line up exactly do we get an eclipse — A.

Problem 6

Tonight there is a full moon, so the Sun, Earth, and Moon are lined up in a row. What kind of tides are expected, and why?
  1. Neap tides, because the Sun and Moon pull at right angles
  2. Spring tides, because the aligned Sun and Moon pull along the same line and their effects combine
  3. No tides at all, because the pulls cancel out
  4. Only one high tide per day instead of two
Hints

Spring tides (biggest range) happen when the Sun and Moon act together; neap tides when they work against each other.

At full moon, are the Sun and Moon aligned with Earth or at right angles?

Aligned pulls add together.

Answer and explanation

Spring tides, because the aligned Sun and Moon pull along the same line and their effects combine

At full moon the Sun, Earth, and Moon are in a straight line, so the Sun's and Moon's gravitational pulls act along the same line and combine.

Combined pulls make extra-high and extra-low tides — spring tides (which also occur at new moon).

So the answer is B.

Problem 7

Astronomers find a new planet: small, dense, and rocky, with a solid surface, no rings, and only a thin atmosphere, orbiting close to its star. Into which group does it best fit?
  1. Gas giant, like Jupiter or Saturn
  2. Ice giant, like Uranus or Neptune
  3. Terrestrial planet, like Mercury, Venus, Earth, or Mars
  4. A comet
Hints

Match the described traits to the planet groups you know.

Gas and ice giants are large, low-density, and orbit far out; terrestrial planets are small, dense, rocky, and orbit close in.

Which group is small, dense, rocky, with a solid surface?

Answer and explanation

Terrestrial planet, like Mercury, Venus, Earth, or Mars

The planet is small, dense, rocky, solid-surfaced, and orbits close to its star.

Those are exactly the traits of the terrestrial (inner, rocky) planets, not the large low-density gas or ice giants.

So it best fits the terrestrial group — C.

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