Rotation and Revolution of the Earth – The Earth in Relation to the Universe (Senior 1 Geography) | Ganzaa

The Earth in Relation to the Universe

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Senior 1 · Geography

The Earth in Relation to the Universe

4 lessons

Lesson 4 of 4

Rotation and Revolution of the Earth

25 min

The week your shadow disappears

Go outside at about 11:50 in the last days of September and look down. In Kigali, around 28 September, your shadow shrinks almost to nothing, because the Sun is directly overhead. It happens again around 16 March. In London it never happens at all. Two movements of the Earth explain this, and much more: its rotation and its revolution.

Definition

Rotation: the spinning of the Earth on its axis, from west to east, once in about 24 hours (one day). Example: because of rotation the Sun appears to rise in the east over Akagera and set in the west over Lake Kivu.

Definition

Axis: the imaginary line through the North and South Poles around which the Earth spins. It is tilted at about 23½° from the upright (compared with the Earth's path round the Sun), and it always points the same way in space.

Definition

Revolution: the movement of the Earth in its orbit round the Sun, taking about 365¼ days (one year). Example: revolution, together with the tilted axis, causes the seasons.

Rotation and its effects

Figure 1. Day and night. As the Earth rotates, each place moves through the local times shown round the edge.

Day and night. Only the half of the Earth facing the Sun is lit. The line between day and night is the circle of illumination.
The Sun's apparent movement. The Sun seems to cross the sky from east to west, but it is the Earth that is turning from west to east.
Time differences. Places further east see the Sun rise first, so their clocks are ahead of places to the west.
Winds and ocean currents are deflected (to the right in the northern hemisphere and to the left in the southern hemisphere); this is called the Coriolis effect.
Tides. The Moon's pull raises the oceans; as the Earth rotates, most coasts pass through two high and two low tides a day.

Important

Longitude and time

The Earth turns through $360^\circ$ in 24 hours, so

\frac{360^\circ}{24\ \text{h}} = 15^\circ \text{ per hour} \qquad \text{and} \qquad \frac{60\ \text{min}}{15^\circ} = 4 \text{ minutes per degree}

\text{time difference (hours)} = \frac{\text{difference in longitude}}{15^\circ}

To find the difference in longitude: if both places are east (or both west) of Greenwich, subtract; if one is east and one is west, add. Places to the east are ahead; places to the west are behind.

Worked example 1: Greenwich and Kigali

It is 12:00 noon at Greenwich (0°). What is the local Sun time in Kigali (30°E)?

Difference in longitude = $30^\circ - 0^\circ = 30^\circ$ . Time difference = $\frac{30}{15} = 2$ hours. Kigali is east, so it is ahead: 12:00 + 2 hours = 14:00.

This is why Rwanda's clocks run on Central Africa Time (CAT), two hours ahead of Greenwich (UTC+2).

Worked example 2: a phone call from Kigali to Tokyo and New York

It is 09:00 in Kigali (30°E). Use longitude to find the time in (a) Tokyo, Japan (135°E); (b) New York, USA (75°W).

(a) Both east, so subtract: $135^\circ - 30^\circ = 105^\circ$ . $\frac{105}{15} = 7$ hours. Tokyo is further east, so ahead: 09:00 + 7 h = 16:00.

(b) One east and one west, so add: $30^\circ + 75^\circ = 105^\circ$ , again 7 hours. New York is to the west, so behind: 09:00 − 7 h = 02:00, in the middle of the night. (In the northern summer New York moves its clocks one hour forward, so the real gap is then 6 hours.)

Worked example 3: sunrise across Rwanda

Rwanda stretches from 28°52′E to 30°54′E. How much earlier does the Sun rise on the eastern border than at the western tip?

30^\circ 54' - 28^\circ 52' = 2^\circ 02' \approx 2.03^\circ

2.03 \times 4 \text{ min} \approx 8 \text{ minutes}

The whole country keeps the same clock time, but the Sun really does rise about 8 minutes earlier over Akagera than over Rusizi.

Definition

Time zone: a region of the Earth that keeps the same standard clock time. There are 24 main zones, each about 15° of longitude wide. Rwanda is in Central Africa Time (CAT), 2 hours ahead of Greenwich. Halfway round the world, the 180° meridian roughly follows the International Date Line: crossing it westwards you add a day, and eastwards you subtract one.

Revolution and its effects

Figure 2. Revolution and the seasons. The axis always leans the same way in space, so first one hemisphere and then the other is tilted towards the Sun.

The seasons. Because the axis is tilted and always leans the same way, each hemisphere leans towards the Sun for half the year (summer: high Sun, long days) and away for the other half (winter). Seasons are not caused by the Earth's distance from the Sun: the Earth is actually closest to the Sun in early January, during the northern winter.
The overhead Sun moves between the tropics. It is overhead at the Tropic of Cancer at the June solstice, at the Tropic of Capricorn at the December solstice, and at the Equator at the two equinoxes.
Changing lengths of day and night. Away from the Equator, days are long in summer and short in winter. Inside the Arctic and Antarctic Circles there are days in summer when the Sun never sets (the midnight Sun) and days in winter when it never rises.
Leap years. A year is about 365¼ days, so every fourth year has an extra day, 29 February, to keep the calendar in step with the seasons.

Definition

Equinox: one of the two days each year (about 20 March and 23 September) when the noon Sun is overhead at the Equator and day and night are about equal in length all over the world.

Definition

Solstice: one of the two days each year (about 21 June and 22 December) when the noon Sun is overhead at one of the tropics, giving the longest day in one hemisphere and the shortest in the other.

Rotation or revolution?

	Rotation	Revolution
Movement	Spinning on its axis	Travelling round the Sun
Time taken	About 24 hours (a day)	About 365¼ days (a year)
Direction	West to east	Anticlockwise, seen from above the North Pole
Speed	About 1,670 km/h at the Equator	About 107,000 km/h along the orbit
Main effects	Day and night; the Sun's daily path; time differences; deflection of winds; tides	Seasons; the overhead Sun moving between the tropics; changing day length; leap years

Important

Case study: a year in Rwanda, next to the Equator

Place	Latitude	Shortest day	Longest day
Kigali	1°57′S	about 12 h 01 min	about 12 h 14 min
London	51°30′N	about 7 h 50 min	about 16 h 38 min

In Kigali day and night are almost equal all year, and the Sun always rises at about six in the morning and sets at about six in the evening. In London the difference between summer and winter days is almost nine hours.
The noon Sun is directly overhead in Kigali twice a year, around 16 March and 28 September, as it moves between the tropics.
Rwanda has no hot summer and cold winter. Instead it has wet and dry seasons. The belt of rising air and rain that follows the overhead Sun crosses the region twice a year, bringing the long rains (March to May) and the short rains (October to December), separated by a long dry season (June to September) and a short one (January to February).

How Does the Earth Move? Crash Course Geography #5, by CrashCourse (11 min). Watch for how the tilt of the axis, not the distance from the Sun, explains the seasons.

Discuss

Your cousin in Canada says: "Summer is when the Earth is closest to the Sun." Using Figure 2 and the fact that Rwanda's rainy seasons follow the overhead Sun, explain why he is wrong and why people in Rwanda talk about wet and dry seasons instead of summer and winter.

Tip

Rotation = Round and round (one day). Revolution = a Very long trip (one year). For time: "East is ahead, West is behind": 15° = 1 hour, 1° = 4 minutes.

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Practice

How long does the Earth take (a) to rotate once; (b) to revolve once round the Sun?
Give three effects of the Earth's rotation.
When it is 10:00 at Greenwich (0°), what is the local Sun time at a place 45°E?
It is 15:00 in Kigali (30°E). What time is it at a place on 60°W?
Two towns have a time difference of 3 hours 20 minutes. What is the difference in their longitudes?
On what dates is the noon Sun overhead at (a) the Tropic of Cancer; (b) the Equator?
Explain why seasons are not caused by the Earth's distance from the Sun.
Explain why Kigali has days of almost 12 hours all year, while London's days change greatly.

Show answersHide answers

(a) About 24 hours; (b) about 365¼ days.
Any three: day and night; the apparent movement of the Sun from east to west; time differences between places; deflection of winds and ocean currents; tides.
$\frac{45}{15} = 3$ hours ahead: 13:00.
Difference = 30° + 60° = 90°; $\frac{90}{15} = 6$ hours; the place is to the west, so behind: 09:00.
3 h 20 min = 200 min; $\frac{200}{4} = 50^\circ$ .
(a) About 21 June (June solstice); (b) about 20 March and about 23 September (the equinoxes).
The Earth is closest to the Sun in early January, which is winter in the northern hemisphere. Seasons are caused by the tilt of the axis: the hemisphere tilted towards the Sun gets a higher Sun and longer days (summer).
Kigali is almost on the Equator, where the circle of illumination always cuts the parallel in half, whatever the tilt, so day and night stay nearly equal. London is at 51°N, where the tilt makes much more than half of its parallel lit in June and much less in December.

Remember

Rotation: west to east in about 24 h; causes day and night, the Sun's daily path, time differences, deflected winds and tides.
15° of longitude = 1 hour; 1° = 4 minutes; east is ahead, west is behind.
Revolution: 365¼ days with a tilted axis; causes seasons, the overhead Sun moving between the tropics (equinoxes and solstices), changing day length and leap years.

1The week your shadow disappears

2Rotation and its effects

3Revolution and its effects

4Rotation or revolution?

5Discuss

The week your shadow disappears

Rotation and its effects

Revolution and its effects

Rotation or revolution?

Discuss