The Moon: Phases, Eclipses and Tides
The Moon is the Earth's only natural satellite and the closest celestial body to us. Examinations ask this topic through the distance and period of the Moon, the order of its phases, the conditions for solar and lunar eclipses, and the cause and types of tides. The concepts are closely connected to the positions of the Sun, the Earth and the Moon, so a simple diagram with these three bodies makes the chapter easy to master and remember.
The Moon and its motion
The Moon is about 384,400 kilometres from the Earth and has a diameter of about 3,475 kilometres, roughly one-fourth of the Earth's diameter. It takes about 27.3 days to complete one revolution around the Earth, which is the sidereal month. The Moon also takes the same time to rotate once on its own axis, so it is tidally locked and always shows the same face to the Earth. The Moon has no atmosphere and no light of its own; it shines only by reflecting sunlight. The time from one new moon to the next is about 29.5 days and is called the synodic month or lunar month. The Moon's gravitational pull is the main cause of tides on the Earth.
Phases of the Moon
The changing shapes of the Moon, called phases, are caused by the changing angle between the Sun, the Earth and the Moon. The cycle begins with the New Moon (Amavasya), when the Moon is between the Sun and the Earth and its dark side faces us. Then come the waxing crescent, the First Quarter (half moon, growing), and the waxing gibbous, leading to the Full Moon (Purnima), when the Earth is between the Sun and the Moon and the whole bright side faces us. After the full moon the Moon seems to shrink: the waning gibbous, the Last Quarter (half moon, shrinking) and the waning crescent lead back to the new moon. The whole cycle takes about 29.5 days.
Solar and lunar eclipses
An eclipse occurs when one body comes into the shadow of another. A solar eclipse happens on a new moon, when the Moon comes between the Sun and the Earth and blocks the Sun's light; it may be total, partial or annular depending on the alignment and distance. A lunar eclipse happens on a full moon, when the Earth comes between the Sun and the Moon and the Earth's shadow falls on the Moon; it may be total or partial. The dark inner part of a shadow is the umbra and the lighter outer part is the penumbra. Eclipses do not occur every month because the Moon's orbit is tilted about 5 degrees to the Earth's orbit, so the three bodies usually do not line up perfectly.
Tides and their types
Tides are the regular rise and fall of the sea level caused mainly by the gravitational pull of the Moon and, to a lesser extent, of the Sun, combined with the rotation of the Earth. There are usually two high tides and two low tides in about 24 hours 50 minutes, so a high tide occurs roughly every 12 hours 25 minutes. When the Sun, the Earth and the Moon are in a straight line, that is at new moon and full moon, the combined pull produces the highest tides, called spring tides. When the Sun and the Moon are at right angles, that is during the first and last quarters, the pulls partly cancel and the lowest tides, called neap tides, occur. The Bay of Fundy in Canada has some of the highest tidal ranges in the world.
Quick revision
Remember that the Moon is about 384,400 kilometres away, takes 27.3 days to go around the Earth (sidereal month) and is tidally locked, always showing the same face. The phases, from new moon to full moon and back, take about 29.5 days (synodic month). A solar eclipse occurs on a new moon and a lunar eclipse on a full moon; eclipses do not happen every month because of the 5 degree tilt of the Moon's orbit. Spring tides occur at new moon and full moon, and neap tides at the first and last quarters. The Bay of Fundy is famous for its very high tides.
How to prepare this topic
- Fix the numbers first: 384,400 kilometres distance, 3,475 kilometres diameter, 27.3 days revolution and 29.5 days for the phase cycle.
- Draw the eight phases of the Moon in order, from new moon to full moon and back, and label them clearly.
- Learn the eclipse rule: solar eclipse on new moon, lunar eclipse on full moon, and remember the 5 degree tilt of the lunar orbit.
- Separate spring tides (new and full moon, highest) from neap tides (first and last quarter, lowest).
- Remember the tide cycle of two highs and two lows in about 24 hours 50 minutes, with a gap of about 12 hours 25 minutes.
Common exam traps
- A solar eclipse occurs on a new moon and a lunar eclipse on a full moon; do not reverse them.
- The Moon is tidally locked, so the same side always faces the Earth; the far side is not permanently dark.
- Eclipses do not happen every month because the Moon's orbit is tilted about 5 degrees to the Earth's orbit.
- Spring tides occur at new and full moon and are the highest; neap tides occur at the quarters and are the lowest.
- There are usually two high and two low tides each day, not one of each.
Frequently Asked Questions
Why does the Moon show phases and how long is the cycle?
The phases are caused by the changing angle between the Sun, the Earth and the Moon as the Moon revolves around the Earth. The Moon has no light of its own and only reflects sunlight, so we see different parts of its bright side. The full cycle from one new moon to the next takes about 29.5 days and is called the synodic month or lunar month.
When do solar and lunar eclipses occur?
A solar eclipse occurs on a new moon, when the Moon comes between the Sun and the Earth and blocks the sunlight; it can be total, partial or annular. A lunar eclipse occurs on a full moon, when the Earth comes between the Sun and the Moon and its shadow falls on the Moon; it can be total or partial. Eclipses do not occur every month because the Moon's orbit is tilted about 5 degrees to the Earth's orbit.
What causes tides and what are spring and neap tides?
Tides are caused mainly by the gravitational pull of the Moon, aided by the Sun and the rotation of the Earth. Spring tides, the highest tides, occur at new moon and full moon when the Sun, Earth and Moon line up. Neap tides, the lowest tides, occur during the first and last quarters when the Sun and Moon are at right angles and their pulls partly cancel.
How many tides occur in a day and where is the highest tidal range?
There are usually two high tides and two low tides in about 24 hours 50 minutes, so a high tide occurs roughly every 12 hours 25 minutes. The Bay of Fundy in Canada experiences some of the highest tidal ranges in the world.
Does the Moon have its own light and an atmosphere?
No. The Moon has no light of its own and shines only by reflecting sunlight, and it has no atmosphere. Because there is no air, sound cannot travel, temperatures are extreme, and the footprints of the Apollo astronauts have remained on the surface. Apollo 11 in 1969 first landed humans on the Moon.