Day and night: the Earth turns, the Sun stays put
Definition
Rotation
The spinning of an object about an imaginary line drawn through itself, called its axis.
- The Earth is a planet that rotates about its own axis once in approximately 24 hours24\ \text{hours}24 hours.
- That axis runs through the North Pole and the South Pole, and it is tilted by about 23.5∘23.5^{\circ}23.5∘ from the upright, measured against the flat plane in which the Earth travels round the Sun.
- Sunlight only ever falls on the half of the Earth facing the Sun, because the Earth is opaque and blocks light from reaching the far side.
- The lit half is having day; the half turned away sits in the Earth's own shadow and is having night.
- Because the Earth never stops turning, every place on the surface is carried out of the lit half and into the dark half, then back again.
- One full turn takes about 24 hours24\ \text{hours}24 hours, which is exactly why one cycle of day and night lasts a day: the period of the rotation sets the period of the cycle.
The Sun crossing the sky: apparent motion, not real motion
- Standing on the ground you feel completely still, so the Sun looks as though it rises in the east, climbs across the sky and sets in the west.
- This is apparent motion: the Sun is not travelling around the Earth once a day.
- The Earth spins from west to east, carrying the observer with it, so the direction you must look to see the Sun drifts steadily westwards through the day.
- Sunrise is the moment your patch of ground is carried into the sunlit half, and sunset is the moment it is carried out again.
- The same argument explains why the stars also appear to sweep across the sky at night, all in the same direction.
The seasons: a tilted axis, not a change of distance
Definition
Orbit
The repeating path that one object follows around another in space, held there by gravitational attraction.
- The Earth orbits the Sun once in approximately 365 days365\ \text{days}365 days, and that journey is what we call a year.
- All the way round that orbit the tilted axis keeps pointing in the same fixed direction in space, so it does not swing round to follow the Sun.
- That fixed tilt means each hemisphere leans towards the Sun for one part of the year and away from it half a year later.
- A hemisphere tilted towards the Sun has summer, and two separate effects make it warm.
- Sunlight arrives more directly, so a beam of a given width lands on a smaller area of ground and delivers more energy to each square metre.
- That hemisphere also spends more of each rotation inside the sunlit half, giving longer daylight hours and more time to heat up.
- A hemisphere tilted away from the Sun has winter, with both effects reversed.
- Sunlight strikes the ground at a shallow angle, spreading the same energy over a larger area, so each square metre receives less.
- Daylight hours are short, so there is less time to warm up and a long night in which to cool down.
- At the two positions in between, neither hemisphere leans towards the Sun and both receive similar amounts of sunlight, which gives spring and autumn.
- The two hemispheres are therefore always in opposite seasons: when it is July in Manchester it is midwinter in Melbourne.
- The seasons are periodic because after one complete orbit of about 365 days365\ \text{days}365 days the Earth is back at the same point with the same tilt, so the whole pattern repeats.
Common Mistake
Do not explain the seasons by saying the Earth is closer to the Sun in summer. If distance were the cause, both hemispheres would have summer at the same time, and they do not: the explanation you need is the tilted axis combined with the yearly orbit.
Phases of the Moon: how much of the lit half is turned towards you
Definition
Phase of the Moon
The shape of the sunlit part of the Moon that can be seen from the Earth at a given moment.
- The Moon is a natural satellite that orbits the Earth once in approximately one month, about 29.5 days29.5\ \text{days}29.5 days from one new Moon to the next.
- The Moon makes no light of its own; it is visible only because it reflects sunlight towards us.
- The Sun lights exactly half the Moon's surface at all times, and that lit half always faces the Sun.
- What changes through the month is how much of that lit half is turned towards the Earth, and that depends purely on where the Moon has reached in its orbit.
- When the Moon is roughly between the Earth and the Sun, its lit half points away from us and we see almost none of it: this is a new Moon.
- When the Earth is between the Sun and the Moon, the whole lit half faces us: this is a full Moon.
- At the quarter positions the Moon sits off to one side, so we see the lit half edge-on and exactly half the visible disc is bright: a half Moon.
- In between we see a thin crescent or a fat gibbous shape, and the bright fraction grows (waxing) then shrinks (waning).
- The whole sequence new, crescent, half, gibbous, full, gibbous, half, crescent, new repeats every month because the orbit itself repeats, which makes the cycle of phases periodic.
Practical
Investigation: observing the phases of the Moon
- Aim: to observe the Moon over a month, record how its phase changes and link each phase to the Moon's position in its orbit.
- Apparatus: naked-eye observation of the real Moon, a clock, a magnetic compass, and a published table of Moon-rise and Moon-set times.
- Method:
- Look up the Moon-rise and Moon-set times for your location so you know when the Moon will be above the horizon, since it is often up in daylight and often below the horizon at night.
- Go outside at a time when the Moon is up, find it, and use the compass to note the direction in which it rose or set.
- Identify the phase and make a sketch of the bright shape, marking which side of the disc is lit.
- Record the date, the time of the observation, the phase, the sketch and the rise and set times in a single table.
- Repeat every two or three days for a full month, so that no phase is missed between observations.
- Measure and record: a table of date, observed phase, sketch, and Moon-rise and Moon-set times, with the sketches laid out in date order so the cycle can be read across the page.
- Result: the Moon runs through new, crescent, half, gibbous, full and back again over about 29.5 days29.5\ \text{days}29.5 days, and the rise and set times shift later each day, confirming that the Moon is moving round the Earth rather than staying in a fixed place.
- Watch out:
- Cloud can wipe out several nights in a row, so build in spare observation dates rather than fixing a rigid schedule.
- Leaving a gap of several days can hide a whole phase, so observe more often near new Moon, when the shape changes fastest and the Moon is hardest to spot.
- Cross-check every sketch against the published rise and set times, so a mistaken date or a misremembered shape shows up straight away.
- Safety: minimal risk, but never look at the Sun while searching for the Moon in daylight.
Common Mistake
Do not say the phases are caused by the Earth's shadow falling on the Moon. The Earth's shadow only reaches the Moon during a lunar eclipse, which happens rarely; the monthly phases happen because you see different fractions of the Moon's sunlit half as it orbits.
Exam technique
- Quote the three time periods precisely, because they are usually worth a mark on their own: 24 hours24\ \text{hours}24 hours to rotate, 365 days365\ \text{days}365 days to orbit the Sun, about one month for the Moon to orbit the Earth.
- The mark that gets dropped most often is the link, so finish every explanation with the consequence: the Earth turns, therefore day and night repeat.
- Keep rotates and orbits apart in your writing, because using one for the other loses the mark even when the physics behind the answer is right.
- If a question gives you a diagram of the Earth at four points around its orbit, read the direction of the tilt first, then name the season for whichever hemisphere the question asks about.
Self review
- How long does the Earth take to rotate once on its axis, and what does this cause?
- Why does the Sun appear to move across the sky during the day?
- Give the two reasons why a hemisphere tilted towards the Sun is warmer.
- Why are the seasons periodic, repeating every 365 days365\ \text{days}365 days?
- Explain why we see a full Moon at one point in the month and a new Moon about two weeks later.
