The Earth and its movements: day, night, seasons, latitude and longitude

Why does the sun rise over Lake Victoria in the east and set in the west? Why does Uganda have two rainy seasons while countries far from the equator have a cold winter? And why is it already night in Japan when it is afternoon in Kampala? The answers come from how the Earth moves.

By the end of these notes you should be able to:

• explain how the Earth's movements and its relationship with the sun affect temperature and seasons;
• draw diagrams of the sun's rays to explain the tropical, temperate and polar regions;
• explain how rotation causes day and night;
• locate places using latitude and longitude;
• calculate time using longitude.

The Earth in the solar system

The Earth is one of eight planets that move around the sun. It is shaped almost like a ball (a sphere), slightly flattened at the poles. The Earth makes two main movements:

1. Rotation: spinning on its own axis.
2. Revolution: moving around the sun in an orbit.

The axis is an imaginary line through the North and South Poles. It is tilted at 23½° from the vertical, and always points in the same direction in space.

Rotation: day and night

The Earth rotates from west to east once every 24 hours (one day).

Effects of rotation:

• Day and night. At any moment, the half of the Earth facing the sun has day and the other half has night. As the Earth turns, places move from day into night and back.
• The sun appears to rise in the east and set in the west, because the Earth turns towards the east.
• Time differences between places east and west of each other.
• Deflection of winds and ocean currents (to the right in the northern hemisphere and to the left in the southern hemisphere).

The line separating day and night is called the circle of illumination.

Revolution: the seasons

The Earth revolves around the sun once every 365¼ days (one year). The extra quarter day is collected into an extra day, 29 February, every four years, in a leap year.

Because the axis is tilted, different parts of the Earth lean towards the sun at different times of the year. This causes:

• the overhead sun to move between the Tropic of Cancer and the Tropic of Capricorn;
• seasons (summer, autumn, winter, spring) in temperate regions;
• changes in the length of day and night.

Key positions of the overhead sun

The sun's rays and temperature zones

The angle at which the sun's rays hit the ground affects how hot a place gets.

• Near the equator, the sun is high in the sky and its rays strike the ground almost vertically. The heat is concentrated on a small area and the rays pass through a thin layer of atmosphere, so temperatures are high.
• Towards the poles, the rays strike at a low angle. The same heat is spread over a larger area and passes through a thicker layer of atmosphere, which absorbs and scatters more of it, so temperatures are lower.

These differences divide the Earth into heat zones:

Latitude and longitude

To find any place on the globe, geographers use a grid of imaginary lines.

Latitude

Latitude is the angular distance of a place north or south of the equator, measured in degrees.

• The equator is 0°. It divides the Earth into the northern and southern hemispheres.
• Lines of latitude are called parallels because they run east–west and never meet.
• Latitude goes up to 90° N (North Pole) and 90° S (South Pole).
• Important parallels: Tropic of Cancer 23½° N, Tropic of Capricorn 23½° S, Arctic Circle 66½° N, Antarctic Circle 66½° S.

Longitude

Longitude is the angular distance of a place east or west of the Prime Meridian, measured in degrees.

• The Prime Meridian (0°) passes through Greenwich in London. It divides the Earth into the eastern and western hemispheres.
• Lines of longitude are called meridians. They run north–south and meet at the poles.
• Longitude goes up to 180° east and west. The 180° meridian is roughly where the International Date Line runs.

Giving a location

Always write latitude first, then longitude, with directions. Kampala is at about 0° 19′ N, 32° 35′ E. Uganda lies roughly between latitudes 4° N and 1½° S, and longitudes 29½° E and 35° E, so the equator passes through the country.

Calculating time using longitude

The Earth turns through 360° in 24 hours. So:

• 15° of longitude = 1 hour (360 ÷ 24)
• 1° of longitude = 4 minutes (60 ÷ 15)

Because the Earth rotates from west to east, places to the east see the sun first, so their time is ahead. Places to the west are behind.

Steps

1. Find the difference in longitude between the two places. If both are on the same side of Greenwich (both E or both W), subtract. If one is east and one is west, add.
2. Convert the difference into time: multiply degrees by 4 minutes, or divide by 15 for hours.
3. Add the time if the place you want is to the east; subtract if it is to the west.

Standard time and time zones

If every town used its own sun time, timetables would be confusing. Countries use standard time based on one meridian. East Africa Time (Uganda, Kenya, Tanzania) is 3 hours ahead of Greenwich (UTC+3), based on the 45° E meridian. So when it is 9:00 am in London (in winter), it is 12:00 noon in Kampala.

Summary

• The Earth rotates west to east every 24 hours, causing day and night and time differences.
• It revolves around the sun every 365¼ days. With its axis tilted at 23½°, this causes seasons and the apparent movement of the overhead sun between the tropics.
• Vertical rays near the equator are concentrated and pass through less atmosphere, so the tropics are hot; slanting rays make temperate and polar regions cooler.
• Latitude measures distance north or south of the equator; longitude measures distance east or west of the Prime Meridian.
• 15° = 1 hour; 1° = 4 minutes. East is ahead, west is behind.

Practice questions

1. Distinguish between rotation and revolution of the Earth and give two effects of each.
2. Explain why places near the equator are hotter than places near the poles. Use a labelled diagram.
3. Name the latitudes of the Tropic of Cancer, the Tropic of Capricorn and the Arctic Circle.
4. Give the approximate latitude and longitude of Kampala, and explain why the equator is important to Uganda.
5. When it is 10:00 am at Greenwich, what is the local sun time at a place on 60° E?
6. A football match starts at 4:00 pm in Kampala (use 30° E for local sun time). What is the local sun time at a place on 15° W?
7. Activity of integration. Your school’s ICT club is planning live video calls between Ugandan schools and partner schools in Ghana (0°) and in Japan (135° E). Using local sun time and Kampala at 30° E, advise the club on a good time for each call so that both schools are in lessons, and explain your calculation.

Answers: 5) 60 × 4 = 240 min = 4 h, so 2:00 pm. 6) 30 + 15 = 45° = 3 h, so 1:00 pm.