Seasons, lunar phases and eclipses
In Dublin, a summer evening in June can stay bright until after 10 p.m. In December it is dark before 5 p.m. The Moon changes shape from week to week, and once in a while the Sun or Moon is briefly blotted out. What causes all of this? By the end of this page you will be able to explain it using one simple model: the Sun, the Earth and the Moon, each moving in its own way.
The Earth's movements and the seasons
The Earth makes two movements at the same time:
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Rotation: it spins once on its axis in about 24 hours. This gives us day and night.
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Revolution (orbit): it travels around the Sun once in about $365\frac{1}{4}$ days, which is one year.
The Earth's axis is not upright. It is tilted at about $23.5^\circ$ to the vertical, and it always points in the same direction in space (towards the Pole Star) as the Earth orbits the Sun.
When the northern hemisphere (where Ireland is) is tilted towards the Sun, we have summer. When it is tilted away, we have winter. Two things change:
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Day length. In June, Dublin is tilted towards the Sun, so we spend more of each 24-hour spin in sunlight: about 17 hours of daylight. In December it is about 7.5 hours.
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Angle of the Sun's rays. In June the Sun is high in the sky at noon. Its rays hit the ground almost from above, so the energy is concentrated on a small area. In December the Sun is low, and the same rays are spread over a larger area, so the ground heats less.
Summer therefore has longer days and more concentrated sunlight. Both make it warmer. The seasons are reversed in the southern hemisphere: when it is summer in Ireland, it is winter in Australia.
The Earth's axis is tilted at about $23.5^\circ$ to the vertical as it orbits the Sun. a) State how long the Earth takes to complete one orbit of the Sun. b) State which…
The Earth's axis is tilted at about $23.5^\circ$ to the vertical as it orbits the Sun.
- a) State how long the Earth takes to complete one orbit of the Sun.
- b) State which season Ireland has in June and which season it has in December.
- c) Name the feature of the Earth that causes the seasons.
In Dublin, the Sun is high in the sky at noon in June and low in the sky at noon in December. Day length is about 17 hours in June and about 7.5 hours in December. a)…
In Dublin, the Sun is high in the sky at noon in June and low in the sky at noon in December. Day length is about 17 hours in June and about 7.5 hours in December.
- a) Calculate the difference in day length between the two months.
- b) Explain, using the tilt of the Earth's axis, why the days are longer in June.
- c) Explain why the Sun's rays heat the ground more effectively when the Sun is high in the sky.
A student claims: "We have seasons because the Earth is closer to the Sun in summer and further away in winter." The Earth is about 147 million km from the Sun in early…
A student claims: "We have seasons because the Earth is closer to the Sun in summer and further away in winter." The Earth is about 147 million km from the Sun in early January and about 152 million km from the Sun in early July.
- a) Use these data to show that the student's claim is incorrect for Ireland.
- b) Explain the real cause of the seasons, referring to the tilt of the axis, the angle of the Sun's rays and day length.
- c) Predict the season in Australia when it is summer in Ireland, and justify your answer.
The Moon and its phases
The Moon does not make its own light. We see it because it reflects sunlight. At any time, the half of the Moon facing the Sun is lit. The Moon orbits the Earth about once every 27.3 days. As it moves, we see different amounts of its lit half. These are the phases of the Moon.
Follow the four main phases in order:
| Phase | Where the Moon is | What we see |
|---|---|---|
| New Moon | Between the Sun and the Earth | The lit half faces away from us: we see (almost) nothing |
| First Quarter | A quarter of the way round | Half of the face lit (right-hand side, seen from Ireland) |
| Full Moon | On the far side of the Earth from the Sun | The whole face is lit |
| Last Quarter | Three-quarters of the way round | Half of the face lit (left-hand side) |
Between New Moon and Full Moon the lit part grows: this is waxing (a crescent, then First Quarter, then a gibbous Moon). After Full Moon it shrinks: waning. The whole cycle from one New Moon to the next takes about 29.5 days, roughly a month.
Why do we always see the same face?
The Moon rotates on its axis once in the same time as it takes to orbit the Earth once (about 27.3 days). So the same side always faces us. The "far side" is not permanently dark: it gets sunlight too, we just never see it from Earth.
Name the four main phases of the Moon in the order they appear, starting with the New Moon, and state approximately how many days pass between one New Moon and the next.
Name the four main phases of the Moon in the order they appear, starting with the New Moon, and state approximately how many days pass between one New Moon and the next.
A student in Galway looks at the Moon on a clear night. She sees that the right-hand half of the Moon's face is lit and the left-hand half is dark. a) Name this phase of…
A student in Galway looks at the Moon on a clear night. She sees that the right-hand half of the Moon's face is lit and the left-hand half is dark.
- a) Name this phase of the Moon.
- b) Describe the positions of the Sun, Earth and Moon at this phase.
- c) Explain why we always see the same face of the Moon from Earth, using the fact that the Moon rotates once per orbit.
- d) Explain why the Moon is visible at all, given that it does not produce its own light.
Eclipses
An eclipse happens when the Sun, Earth and Moon line up so that one body casts its shadow on another.
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Solar eclipse: the Moon passes between the Sun and the Earth. This can happen only at New Moon. The Moon's shadow falls on a small part of the Earth. In the darkest part of the shadow (the umbra) the Sun is totally covered: a total eclipse. In the lighter outer part (the penumbra) it is only partly covered.
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Lunar eclipse: the Earth passes between the Sun and the Moon. This can happen only at Full Moon. The Moon moves into the Earth's shadow.
(Sizes and distances are not to scale.)
Why not every month?
There is a New Moon and a Full Moon every month, but eclipses are rarer. The Moon's orbit is tilted by about $5^\circ$ to the plane of the Earth's orbit. Most months the Moon passes slightly above or below the Sun–Earth line, so its shadow misses the Earth (or the Earth's shadow misses the Moon). Eclipses happen only when New or Full Moon occurs while the Moon is crossing that plane.
Solar versus lunar eclipse
A lunar eclipse can be seen from anywhere on the night side of the Earth where the Moon is above the horizon, because the whole Moon is in the Earth's shadow. The Moon's shadow on Earth is tiny, so a total solar eclipse is seen only along a narrow path. The Moon often looks dull red in a total lunar eclipse, because some sunlight is bent (refracted) through the Earth's atmosphere and reaches the Moon.
a) Name the type of eclipse that happens when the Moon passes between the Sun and the Earth. b) Name the type of eclipse that happens when the Earth passes between the…
- a) Name the type of eclipse that happens when the Moon passes between the Sun and the Earth.
- b) Name the type of eclipse that happens when the Earth passes between the Sun and the Moon.
- c) State which of these two eclipses happens only at New Moon and which only at Full Moon.
A solar eclipse is visible from part of the west of Ireland. A Moon of diameter 3,474 km is about 384,000 km from the Earth. The Sun has a diameter of about 1,391,000 km…
A solar eclipse is visible from part of the west of Ireland. A Moon of diameter 3,474 km is about 384,000 km from the Earth. The Sun has a diameter of about 1,391,000 km and is about 150,000,000 km from the Earth.
- a) Calculate the ratio of the Sun's diameter to the Moon's diameter, to the nearest whole number.
- b) Calculate the ratio of the Sun's distance to the Moon's distance, to the nearest whole number.
- c) Use your answers to explain why the Moon can just cover the Sun's disc in the sky.
The Moon's orbit is tilted by about $5^\circ$ relative to the plane of the Earth's orbit around the Sun. A solar eclipse can occur only at New Moon, and a lunar eclipse…
The Moon's orbit is tilted by about $5^\circ$ relative to the plane of the Earth's orbit around the Sun. A solar eclipse can occur only at New Moon, and a lunar eclipse only at Full Moon, yet there is a New Moon and a Full Moon every month.
- a) Explain why eclipses do not happen every month.
- b) Explain why a lunar eclipse can be seen from every place on Earth where the Moon is above the horizon, whereas a total solar eclipse is visible only along a narrow path.
- c) During a total lunar eclipse the Moon often appears dull red rather than disappearing. Suggest an explanation.
Why a lunar month is longer than an orbit
The Moon takes about 27.3 days to orbit the Earth once, measured against the stars. But from one Full Moon to the next takes about 29.5 days. The difference is $29.5 - 27.3 = 2.2$ days.
The reason is that the Earth is also moving around the Sun. After the Moon completes one orbit, the Earth has moved on along its own orbit, so the Sun is now in a slightly different direction. The Moon must travel a little further, about 2 more days, to get back to the same position relative to the Sun and Earth, and so show the same phase.
Back to the start: a June evening is bright for long because Ireland is tilted towards the Sun; the Moon changes shape because we see different parts of its lit half; and eclipses occur when the three bodies line up.
Between two consecutive Full Moons about $29.5$ days pass, but the Moon takes only about $27.3$ days to orbit the Earth once relative to the stars. a) Calculate the…
Between two consecutive Full Moons about $29.5$ days pass, but the Moon takes only about $27.3$ days to orbit the Earth once relative to the stars.
- a) Calculate the difference between these two periods.
- b) Explain, with reference to the Earth's movement around the Sun, why the time between two Full Moons is longer than the orbital period of the Moon.
- c) A Full Moon occurs on 1 March. Estimate the date of the next Full Moon and of the next New Moon after 1 March.