Earth, sun and moon: phases, eclipses and seasons
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課程筆記
Moon's Orbit and Rotation
- The Moon orbits Earth once every 27.3 days.
- The Moon rotates on its axis once every 27.3 days (same as its orbital period).
- Because rotation and revolution are synchronized, we always see the same side of the Moon (nearside).
- The Moon's orbit is elliptical, so its distance from Earth varies; average distance is about 385,000 km.
- The Moon moves anti-clockwise around Earth when viewed from above the North Pole.
- The Moon's diameter is just under one-third of Earth's diameter.
Phases of the Moon
- The Moon does not make its own light — we see it because it reflects sunlight from the Sun.
- The Moon is always half lit by the Sun. As the Moon orbits Earth, different portions of that lit half face Earth, which is why its shape appears to change.
- The 8 phases, in order, are: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, and waning crescent.
- At new moon, the lit side faces away from Earth. At full moon, the lit side faces directly toward Earth.
- This cycle of phases repeats about every 28 days.
Phases of the Moon
Moon's Surface
- The Moon's surface is covered with regolith (lunar soil), which lacks organic matter.
- Dark areas on the Moon are called maria (seas), which are basaltic plains.
- Bright areas are called highlands, which are higher in elevation.
- The nearside has more maria than the farside.
- The farside was first photographed by the Soviet spacecraft Luna 3 in 1959.
Solar Eclipses
- A solar eclipse occurs when the Moon passes directly between the Sun and Earth, blocking the Sun's light.
- Solar eclipses happen only when the Sun, Moon, and Earth are aligned in the ecliptic plane.
- The Moon's shadow has two parts: the umbra (inner, fully dark) and the penumbra (outer, partially dark).
- A total solar eclipse happens when the Moon completely covers the Sun, visible only from within the umbra.
- A partial solar eclipse occurs when only part of the Sun is blocked, visible from the penumbra.
- Solar eclipses are rare and brief because the Moon's shadow covers a small area and Earth rotates rapidly.
- During totality, the Sun's corona and solar prominences become visible.
Solar eclipse geometry
Lunar Eclipses
- A lunar eclipse occurs when the Earth passes between the Sun and the Moon, casting its shadow on the Moon.
- Lunar eclipses happen only during a full moon when all three bodies are aligned in the ecliptic plane.
- Earth's shadow also has an umbra and penumbra.
- A total lunar eclipse occurs when the Moon passes completely into Earth's umbra.
- A partial lunar eclipse occurs when only part of the Moon enters the umbra.
- Lunar eclipses can last for hours and are visible from anywhere on Earth's night side.
- Lunar eclipses do not occur monthly because the Moon's orbit is inclined about 5° to Earth's orbit.
Lunar eclipse geometry
Seasons: Cause and Effect
- Seasons are caused by the 23.5-degree tilt of Earth's axis and its revolution around the Sun.
- Earth's axis always points in the same direction, toward Polaris (the North Star).
- When a hemisphere is tilted toward the Sun, it receives more direct sunlight and has longer days, causing summer.
- When a hemisphere is tilted away from the Sun, it receives indirect sunlight and has shorter days, causing winter.
- The distance from the Sun does not cause seasons; in fact, Earth is closest to the Sun during Northern Hemisphere winter.
- Seasons are reversed in the Northern and Southern Hemispheres.
Earth's revolution around the Sun showing the seasons in the Northern and Southern Hemispheres.
Solstices and Equinoxes
- The summer solstice (around June 21) is when the Sun's rays hit the Tropic of Cancer most directly; it's the longest day in the Northern Hemisphere.
- The winter solstice (around December 21) is when the Sun's rays hit the Tropic of Capricorn most directly; it's the shortest day in the Northern Hemisphere.
- At the equinoxes (around March 21 and September 22), the Sun shines directly on the equator, and day and night are nearly equal.
- The vernal equinox occurs in spring, and the autumnal equinox occurs in fall.
- Solstices and equinoxes mark the start of seasons in many calendars.
Earth's axial tilt relative to its orbital plane
Key Vocabulary
- Orbit: The path an object takes around another object.
- Rotation: The spinning of an object on its axis.
- Revolution: The movement of one object around another.
- Axis: An imaginary line through the center of an object around which it rotates.
- Ecliptic: The plane of Earth's orbit around the Sun.
- Umbra: The inner, darkest part of a shadow.
- Penumbra: The outer, lighter part of a shadow.
- Solstice: The time when the Sun reaches its highest or lowest point in the sky at noon.
- Equinox: The time when the Sun crosses the celestial equator, making day and night equal.
投影片
練習題
免費預覽——45 題中的 8 題。註冊即可查看全部。
1.What is the time taken for the Moon to complete one orbit around the Earth?
Easy- A24 hours
- B27.3 days
- C365.25 days
- D28 days
2.Which of the following is the correct order of the Sun, Earth, and Moon during a total solar eclipse?
Easy- ASun, Moon, Earth
- BSun, Earth, Moon
- CMoon, Sun, Earth
- DEarth, Sun, Moon
3.The Moon has an atmosphere.
EasyTrue or false?
4.Why do we always see the same side of the Moon from Earth?
Easy- ABecause the Moon does not rotate on its axis.
- BBecause the Moon rotates on its axis at the same rate as it revolves around the Earth.
- CBecause the Moon revolves around the Earth in the opposite direction to its rotation.
- DBecause the Moon is tidally locked to the Sun.
5.What is the name of the outer part of the Moon's shadow during a solar eclipse?
Easy- Aumbra
- Bpenumbra
- Ccorona
- Dmaria
6.During a lunar eclipse, the Earth is between the Sun and the Moon.
EasyTrue or false?
7.Why is it hotter at the equator than at the poles?
Easy- ABecause the equator is closer to the Sun.
- BBecause sunlight strikes the equator more directly.
- CBecause the equator has more volcanoes.
- DBecause the equator rotates faster.
8.Which of the following are true about the Moon? (select all that apply)
Medium- AIt takes 27.3 days to orbit the Earth.
- BIt has a substantial atmosphere.
- CIt always shows the same face to Earth.
- DIts surface is covered with regolith.
- EIt is larger than the Earth.
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