Astronomy & Earth's Motions
Key facts about rotation, revolution, seasons, moon phases, eclipses, and tides.
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Questions Covered in This Set
12 cards to master
What does Earth's rotation cause?
Day and night (~24 hours), the apparent rising/setting of Sun, Moon, and stars in the east/west, and the Coriolis effect.
What is revolution, and how long does it take?
Earth orbiting the Sun, once every ~365.25 days — the extra quarter day gives us leap years (Feb 29).
Perihelion vs. aphelion
Perihelion = closest to Sun (~147 million km, early January); aphelion = farthest (~152 million km, early July).
What actually causes the seasons?
Earth's 23.5° axial tilt, which stays pointed the same direction (toward Polaris), so hemispheres lean toward or away from the Sun — NOT distance from the Sun.
Two effects of a hemisphere tilted toward the Sun
Sunlight strikes at a higher (more direct) angle, concentrating energy, and days are longer, giving more hours of heating.
Where is the Sun directly overhead on the June solstice?
The Tropic of Cancer, 23.5°N — the longest day in the Northern Hemisphere (~June 21).
What happens on an equinox?
The Sun is directly over the equator (0°) and day length is about 12 hours everywhere (~Mar 21 and ~Sep 23).
Why do we always see the same face of the Moon?
Synchronous rotation (tidal locking): the Moon's rotation period equals its orbital period.
Why are there moon phases?
Viewing geometry — half the Moon is always sunlit, and we see different fractions of that lit half. It is not Earth's shadow.
Solar vs. lunar eclipse
Solar eclipse: at new moon, the Moon's shadow falls on Earth. Lunar eclipse: at full moon, Earth's shadow falls on the Moon.
Why don't eclipses happen every month?
The Moon's orbit is tilted about 5° from Earth's orbital plane, so it usually passes above or below the exact line-up.
What causes tides?
The Moon's gravity pulls the ocean into two bulges (one toward the Moon, one on the far side); Earth rotates through them, giving ~two high and two low tides daily.
